World Soil Resources Report
AGRO-ECOLOGICAL ZONES PROJECT
METHODOLOGY AND RESULTS
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Also issued in this series:
1. Report of the First Meeting of the Advisory Panel on the Soil Map of the World.
Rome, 19-23 June 1961
2. Report of the First Meeting on Soil Survey, Correlation and Interpretation for Latin
America, Rio de Janeiro, Brazil, 28-31 May 1962.
3. Report of the First Soil Correlation Seminar for Europe, Moscow, LU S.S.R. 16-28
4. Report of the First Soil Correlation Seminar for South and Central Asia. Tashkent,
Uzbekistan, U.S.S.R., 14 September- 2 October 1962.
5. Report of the Fourth Session of the Working Party on Soil Classification and
Survey (Subcommission on Land and Water Use of the European Commission on
Agriculture), Lisbon, Portugal, 6-10 March 1963.
6. Report of the Second Meeting of the Advisory Panel on the Soil Mao of the World.
Rome, 9-11 July 1963.
7. Report of the Second Soil Correlation Seminar for Europe. Bucharest, Romania.
29 July 6 August 1963.
8. Report of the Third Meeting of the advisory Panel on the Soil Map of the World.
Paris, 3 January 1964.
9. Adequacy of Soil Studies in Paraguay, Bolivia and Peru, Novernber-December
10. Report on the Soils of Bolivia, January 1964.
11. Report on the Soils of Paraguay, January 1964.
12. Preliminary Definitions, Legend and Correlation Table for the Soil Map of the
World, Rome, August 1964.
13. Report of the Fourth Meeting of the Advisory Panel on the Soil Map of the World,
Rome, 18-21 May 1964.
14. Report of the Meeting on the Classification and Correlation of Soils from Volcanic
Ash, Tokyo, Japan, 11-27 June 1964.
15. Report of the First Session of the Working Party on Soil Classification, Survey
and Soil Resources (European Commission on Agriculture), Florence, Italy. 1-3
16. Detailed Legend for the Third Draft of the Soil Map of South America. June 1965.
17. Report of the First Meeting on Soil Correlation for North America. Mexico. 1-8
18. The Soil Resources of Latin America. October 1965.
19. Report of the Third Soil Correlation Seminar for Europe: Bulgara. Greece. Ro-
mania, Turkey, Yugoslavia, 29 August 22 September 1965.
20. Report of the Meeting of Rapporteurs, Soil Map of Europe (Scalle 1:1 000 000)
(Working Party on Soil Classification and Survey, European Commission on Agri-
culture), Bonn. Federal Republic of Germany, 29 November 3 December 1965.
21. Report of the Second Meeting on Soil Survey, Correlation and Interpretation for
Latin America, Rio de Janeiro, Brazil, 13-16 July 1965.
22. Report of the Soil Resources Expedition in Western and Central Brazil, 24 June -
Bibliography on Soils and Related Sciences for Latin America (1st edition), De-
Report on the Soils of Paraguay (2nd edition), August 1964.
Report of the Soil Correlation Study Tour in Uruguay, Brazil and Argentina, June-
Report of the Meeting on Soil Correlation and Soil Resources Appraisal in India,
New Delhi, India, 5-15 April 1965.
Report of the Sixth Session of the Working Party on Soil Classification and Survey
of the European Commission on Agriculture, Montpellier, France, 7-11 March
Report of the Second Meeting on Soil Correlation for North America, Winni-
peg-Vancouver, Canada, 25 July 5 August 1966.
Report of the Fifth Meeting of the Advisory Panel on the Soil Map of the World,
Moscow, U.S.S.R., 20-28 August 1966.
Report of the Meeting of the Soil Correlation Committee for South America, Bue-
nos Aires, Argentina, 12-19 December 1966.
Trace Element Problems in Relation to Soil Units in Europe (Working Party on
Soil Classification and Survey of the European Commission on Agriculture),
Approaches to Soil Classification, 1968.
Definitions of Soil Units for the Soil Map of the World, April 1968.
Soil Map of South America 1:5 000 000, Draft Explanatory Text, November 1968.
Report of a Soil Correlation Study Tour in Sweden and Poland, 27 September -
14 October 1968.
Meeting of Rapporteurs, Soil Map of Europe (Scale 1:1 000 000), Working Party on
Soil Classification and Survey, European Commission on Agriculture, Poitiers,
France, 21-23 June 1967.
Supplement to Definition of Soil Units for the Soil Map of the World, July 1969.
Seventh Session of the Working Party on Soil Classification and Survey, European
Commission on Agriculture, Varna, Bulgaria, 11-13 September 1969.
A Correlation Study of Red and Yellow Soils in Areas with a Mediterranean Cli-
Report of the Regional Seminar on the Evaluation of Soil Resources in West Africa,
Kumasi, Ghana, 14-19 December 1970.
Soil Survey and Soil Fertility Research in Asia and the Far East, New Delhi, 15-20
Report of the Eighth Session of the Working Party on Soil Classification and
Survey, European Commission on Agriculture, Helsinki, Finland, 5-7 July 1971.
Report of the Ninth Session of the Working Party on Soil Classification and Survey,
European Commission on Agriculture, Ghent, Belgium, 28-31 August 1973.
First Meeting of the West African Sub-Committee on Soil Correlation for Soil
Evaluation and Management, Accra, Ghana, 12-19 June 1972.
Report on the Ad Hoc Expert Consultation of Land Evaluation, Rome, Italy, 6-8
First Meeting of the Eastern African Sub-Committee-for Soil Correlation and Land
Evaluation, Nairobi, Kenya, 11-16 March 1974.
Second Meeting of the Eastern African Sub-Committee for Soil Correlation and
Land Evaluation, Addis Ababa, Ethiopia, 25-30 October 1976.
WORLD SOIL RESOURCES REPORT
Report on the
RESULTS FOR AFRICA
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
The copyright in this book is vested in the Food and Agriculture Orga-
nization of the United Nations. The book may not be reproduced, in whole
or in part, by any method or process, without written permission from
the copyright holder. Applications for such permission, with a statement
of the purpose and extent of the reproduction desired, should be addressed
to the Director, Publications Division, Food and Agriculture Organization
of the United Nations, Via, delle Terme di Caracalla, 00100 Rome, Italy.
The designations employed and the presentation
of material in this publication do not imply the
expression of any opinion whatsoever on the
part of the Food and Agriculture Organization
of the United Nations concerning the legal
status of any country, territory, city or area or
of its authorities, or concerning the delimitation
of its frontiers or boundaries.
Agricultural Services Division
Land and Water Development Division
H. Van Velthuizen
Management Services Division
Office of the Assistant Director-General
Policy Analysis Division
Plant Production and Protection Division
Project Coordinator G.M. Higgins
- iv -
Appreciation is expressed and acknowledgements paid to the following
persons for advice and work during the reported phase of project activities:
- For technical advice and discussions:
External to FAO -
M. Chartier (Director, Station for Bioc1.-iatological Research, C.N.R.A.,
Versailles), J.P. Cooper (Direc+"-, Welsh Plant Breeding Station, Aberystwyth),
D.H. Jennings (Department or Ecology and Plant Physiology, University of
Liverpool), M.G. Long (Director, C.E.P.E., Montpellier), M.P. Roche (Director,
G.E.R.D.A.T., Montpellier), B. Seguin (C.N.R.A., Montfavet, Avignon), E. Servat
(Professor, E.N.S.A.M., Montpellier).
Within FAO -
H. Al Jibouri (Senior Officer, Field Food Crops, AGP), A. Amati (Chief,
Computer Services Branch, AFM), A. Bozzini (Chief, Crop and Grassland
Production Service, AGP), H. Braun (Technical Officer, Soil Fertility, AGLS),
N.R. Carpenter (Chief, Farm Management Unit, AGS), W.C. James (Agriculturalist,
Plant Protection Service, AGP), F.I. Massoud (Technical Officer, Soil Reclam-
ation, AGLS), M. Mathieu (Chief, Fertilizer and Plant Nutrition Service, AGL)
R.D. Narain (Director, Statistics Division, ESS), M. Obradovich (Climatologist,
PHI 74/003), C. Poulton (Senior Officer, Grassland and Pasture Crops Group,
AGP), M.F. Purnell (Technical Officer, Land Classification, AGLS), S. Risopoulos
(Pasture Improvement Specialist, AGPC), F. Riveros (Tropical Pasture Improvement
Specialist, AGPC), H.C. Ruck (Horticulturalist, AGPC) N. Stalbrand (Technical
Officer, Fertilizer and Plant Nutrition Service, AGL), M. Smith (Associate
Expert, Water Resources, Development and Management Service, AGL).
- For measurement of the areas of the agro-ecological zones:
S. Allara, M. Ferrari, C. Fortuna, U. Galeano. G. Innocenti,
H. Kallinger-Federici, G. Livoti, A. Usai, M. Zanetti
- For climatic data collection:
P. Raymakers, F. van den Steen van Ommeren, M. Obradovich
- For present land use data analysis:
F. Barbieri, N. Schiavone-Nicollier
- For typing and preparation of tables;
H. Kallinger-Fedirici, F.L. Schiller
For cartographic preparation:
D. Mazzei, M. Zolotarioff-Innocenti
Projections reveal that to sustain the likely world population in
the year 2000, an increase of 60 percent in agricultural production will
be required. "Is there sufficient land to do this?" becomes the over-
riding question, but little precise information exists on which to base
a reliable answer.
Previous appraisals of the global extents of arable lands, to
support present and future human populations, vary from 3 to 7 thousand
million hectares. Estimates of the populations these lands can support,
vary from 7.5 to 40 thousand million.
These estimates however, do not take into account differences in
production potential when it is calculated for a) different crops (with
widely differing climatic and soil requirements, e.g. millet and potato)
and b) different levels of inputs and technology (e.g. subsistence
cultivation and commercial production). Such factors must be taken into
account to arrive at realistic estimates of the agricultural production
potential of the various lands of the world.
Equally important, in planning optimum use of the world's land
resources, is the fact that these resources are immobile and unevenly
distributed. Accordingly, not all crops can be grown in all areas; and
expansion of production, through increased inputs and investment, will
need to be planned and achieved in the context of a sound inventory of
land and its production potential for various types of land uses.
Recognizing these facts FAO initiated, in September 1976, a study
of potential land use by agro-ecological zones to obtain a first approx-
imation of the production potential of the world's land resources, and
so provide the physical data base necessary for planning future agric-
ultural development. Initially the project deals with rainfed production
potential, at two levels of inputs, for eleven crops in developing
The present volume reports results for Africa. Part A gives
general and technical accounts of the overall methodology employed in
the assessment. Part B provides simple information on the rainfed
agricultural production potential of the continent's various land
resources. The former part is intended for national staff wishing to
apply the developed techniques to detailed individual country studies,
to which the methodology is equally applicable.
PART A METHODOLOGY
CHAPTER 1 INTRODUCTION
1.1 Principles 1
1.2 Procedures 2
1.3 Proposals, Basic Data, Assumptions, and Scope 2
1.4 Alternative Uses Considered 3
1.5 Climatic and Soil Requirements of Crops 4
1.6 Inventory of Land and Mapping Units 5
1.7 Matching of Crop Requirements to the Land Inventory 7
1.8 The Land Suitability Classification 8
CHAPTER 1 ALTERNATIVE LAND USES CONSIDERED
2.1 World Crop Production 11
2.2 Selected Crops 11
2.3 Land Utilization Types 15
CHAPTER 3 CLIMATIC ADAPTABILITY OF CROPS
3.1 Introduction 19
3.2 Photosynthesis and Phenological Characteristics 20
3.3 Crop Adaptability Inventory 21
CHAPTER 4 SOIL REqJIREMENTS OF CROPS
4.1 Introduction 27
4.2 Basic Soil Requirements of Selected Crops 27
CHAPTER 5 CLIMATIC INVENTORY
5.1 Introduction 31
5.2 Concepts of the Climatic Inventory 32
5.3 Major Climatic Divisions 33
5.4 Growing Period 33
5.5 Results for Africa 39
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CHAPTER 6 SOIL INVENTORY
6.1 Introduction 43
6.2 Sources of Information 43
6.3 The Legend 44
6.4 Computerization of Distribution of Soil Units 56
CHAPTER 7 MATCHING: NET BIOMASS PRODUCTION AND YIELD OF CROPS
7.1 Introduction 61
7.2 Biomass Production 62
7.3 Crop Yield 70
7.4 Agro-climatic Constraints 71
7.5 Net Biomass and Yield of Crops for Africa 75
CHAPTER 8 MATCHING: SOIL REQUIREMENTS OF CROPS WITH SOIL
8.1 Introduction 81
8.2 Soil Unit Ratings 81
8.3 Phase Modifications 86
8.4 Slope Modifications 88
8.5 Texture Modifications 89
CHAPTER 9 THE SUITABILITY CLASSIFICATION
9.1 Overview 91
9.2 The Agro-climatic Constraints 92
9.3 Crop Yields with Agro-climatic Constraints
and the Agro-climatic Suitability Assessment 94
9.4 Computation of the Land Suitability 103
9.5 Presentation of the Land Suitability
Assessment Results 106
PART B RESULTS
CHAPTER 10 RESULTS
10.1 Pearl Millet
10.2 Sorghum 116
10.3 Maize 120
10.4 Wheat 124
10.5 Soybean 128
10.6 Phaseolus Bean 132
10.7 Sweet Potato
10.8 White Potato
LIST OF TABLES
2.1 Main Crops by Regions in Decreasing Order of Importance with regard
to: Area, Production and Value
3.1 Crop Adaptability Inventory Group I
3.2 Crop Adaptability Inventory Group II
3.3 Crop Adaptability Inventory Group III
3.4 Crop Adaptability Inventory Group IV
3.5 Crop Adaptability Inventory Group V
4.1 Soil and Land Suitability Requirements of Crops
5.1 Characteristics of Major Climates in Africa
5.2 Extents of Growing Period Zones by Major Climates
6.1 Diagnostic Horizons and Properties of the Soil Units
6.2 Relative Distribution of Dominant Soil, Associated Soil(s) and
Inclusion(s) Expressed in Percentage of the Area of the Mapping
6.3 Distribution of Soil Units by Lengths of Growing Period
7.1 Average Photosynthesis Response of Four Groups of Crops to
Radiation and Temperature
7.2 The Photosynthetically Active Radiation on Very Clear Days and
the Daily Gross Photosynthesis Rate of Crop Canopies on Very
Clear and Overcast Days (from de Wit)
7.3 Harvest Index of High Yielding Cultivars of Field Crops under
7.4 Crop Characteristics Considered in the Potential Net Biomass
and Yield Calculations for Groups II and III Crops
7.5 Potential Net Biomass and Yield of Groups II and III Crops for
Tropical and Subtropical (Summer Rainfall) Areas
7.6 Crop Characteristics Considered in the Potential Net Biomass
and Yield Calculations for Groups I and IV Crops
7.7 Potential Net Biomass and Yield of Groups I and IV Crops for
Tropical, Subtropical (Summer Rainfall) and Subtropical
(Winter Rainfall) Areas
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8.1 Soil Unit/Crop Ratings 82
9.1 Agro-climatic Constraints by Crops, Groups II and III Crops
in Tropical and Subtropical (Summer Rainfall) Areas 95
9.2 Agro-climatic Constraints by Crops, Groups I and IV Crops
in Tropical and Subtropical Areas 96
9.3 Agro-climatic Suitability Classification and Yield (with
Constraints) of Crops by Lengths of Growing Period, Groups
II and III Crops in Tropical and Subtropical Areas 98
9.4 Agro-climatic Suitability Classification and Yield (with
Constraints) of Crops by Lengths of Growing Period, Groups
I and IV Crops in Tropical and Subtropical Areas 100
10.1 Land Suitability Assessment Pearl .Millet 109
10.2 Land Suitability Assessment Sorghum 113
10.3 Land Suitability Assessment Maize 117
10.4 Land Suitability Assessment Wheat 121
10.5 Land Suitability Assessment Soybean 125
10.6 Land Suitability Assessment Phaseolus Bean 129
10.7 Land Suitability Assessment Sweet Potato 133
10.8 Land Suitability Assessment White Potato 137
10.9 Land Suitability Assessment Cassava 141
10.10 Land Suitability Assessment Cotton 145
10.11 Land Suitability Assessment Rice 148
LIST OF FIGURES
3.1 Average Relationships between Maximum Photosynthesis Rate
and Temperature for Crop Groups I, II, III and IV 22
3.2 Relationship between Leaf Photosynthesis Rate at Optimum
Temperature and Photosynthetically Active Radiation for
Crop Groups I, II, III and IV 23
5.1 Examples of Four Types of Growing Period 36
5.2 Generalized Climatic Inventory Africa
Major Climatic Divisions and Lengths of Growing Period Zones 40
7.1 Typical Cumulative Crop Growth Curve Showing the Point of
Inflection during the Period of Maximum Growth
7.2 The Normal Shape of the Curve of Crop Growth Rate Plotted 64
against Time Showing Average Crop Growth Rate 64
7.3 Relationship between Leaf Area Inde3 and Maximum Growth Rate 68
9.1 Agro-ecological Data Base Land Suitability Assessment
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Generalized Agro-climatic Suitability Assessment for Rainfed Production of:
10.1 Pearl Millet 108
10.2 Sorghum 112
10.3 Maize 116
10.4 Wheat 120
10.5 Soybean 124
10.6 Phaseolus Bean 128
10.7 Sweet Potato 132
10.8 White Potato 136
10.9 Cassava 140
10.10 Cotton 144
PART A METHODOLOGY
The methodology, developed to assess the potential agricultural use
of the world's land resources, applies five basic principles fundamental to any
sound evaluation of land, namely:
i. land suitability is meaningful only in relation to a specified use,
e.g. land suited to the cultivation of peirl millet is not necessarily
suited to the cultivation of paddy rice;
ii. suitability refers to use on a sustained basis, e.g. the specified use
of land must not result in its degradation through processes such as
wind erosion, water erosion or salinization;
iii. evaluation involves comparison of more than one kind of envisaged use,
e.g. suitability for pearl millet or sorghum or maize and not just for
iv. different kinds of land use are compared on a simple economic basis,
i.e. suitability for each use is assessed by comparing the value of
the goods produced to the cost of production;
v. a multidisciplinary approach is adopted, i.e. inputs from crop ecologists,
agronomists, economists and climatologists are required, in addition to
those from pedologists, to make a sound assessment of land suitability for
a specified use.
These principles have been formulated over the past seven years through
international cooperation to develop a methodology for land evaluation. This
methodology, described in A Framework for Land Evaluation, recognizes a sixth
principle: that the evaluation is made in terms relevant to the physical,
economic and social context of the area concerned.
The present assessment, concentrates on physical and some simple
economic land aspects only, to provide the essential base for sound physical
planning of agricultural developments, upon which more detailed economic and social
considerations may be later superimposed.
While the present study is confined to the rainfed production potential
of a limited number of major world crops, the methodology is suited for adaptation,
refinement and expansion to detailed country studies covering all crops at regional
and project levels.
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The overall methodology of the assessment is also in accordance with the
agreed land evaluation procedures and, in summary, comprises the following
i. review and refinement of the proposals of the evaluation in conjunction
with identification of the basic data and assumption to be used;
ii. selection of alternative land uses (crops, levels of inputs, etc.) for
iii. determination of the climatic and soil requirements of the selected
alternative land uses;
iv. compilation of an inventory of the land (climate and soil) and mapping
units (agro-ecological zones) with particular respect to (iii);
v. matching of the requirements (iii) with the land inventory (iv) and
calculation of anticipated production potential in the different
agro-ecological zones recognized;
vi. estimation of production costs, and identification of the various
suitability classes to be employed and their differentiating
vii. classification of the land into various suitability classes for the
selected alternative land uses, and presentation of results.
The various main activities are described in the subsequent chapters;
the present chapter only briefly outlines the concepts, principles and procedures
1.3 PROPOSALS, BASIC DATA, ASSUMPTIONS AND SCOPE
The basic intent of the project was to make, from existing information, a
first approximation of the present and potential use of the world's land
resources and so provide the physical data base necessary for planning future
Refinement of the study proposals revealed that available quantitative data
on surface and groundwater resources was insufficient to include an assessment of
irrigation potential in the study. While such water resource data do exist for a
number of individual countries, they are not available on a regional basis and the
present study is therefore confined to rainfed potential only. When the available
water resources of regions are quantified with development costs, then the study can
be extended to include irrigated production potential.
The study is also confined to assessing the rainfed production potential for
the main crops of the world only. To comply with the principles of sound land
evaluation, a separate assessment is required for each crop and level of input.
In the present study, it has only been possible to deal fully with 11 major crops,
each at two input levels. Perennial tree crops such as rubber, tea, coffee, oil-
palm and other fruit crops are not included, although economically important on a
- 3 -
No major land improvements, such as large scale drainage schemes, are
considered. The study takes into account only on-farm improvements and
Refinement of the proposal also revealed that the study could only, in the
time available, deal directly with assessing the developing world's land resources.
Information on the potential of the developed world's land resources has been
requested directly from appropriate countries and will, when available, be added
to the results of the present study to complete a global picture.
The assessment of present land use by agro-ecological zones has so far been
confined to ten test-case countries. Continuing work on these countries will enable
formulation of a suitable questionnaire for the supply of present land use data from
other countries. Once available, on an agro-ecological zone basis, it will provide
an opportunity to assess the magnitude of the potential that is underused and under-
As originally conceived, the project was to have used existing climatic,
crop requirement and economic data for the assessment. This was impossible
because the following information was not available:
a. a global quantitative climatic classification
for rainfed agriculture;
b. a crop agro-climatic adaptability classification,
in a form suitable for matching crops with climatic
and soil resources;
c. crop production cost data by soil and climatic zones.
All such data had therefore to be generated.
The project assesses the potential use of the developing world's land
resources for the rainfed production of a limited number of main crops by agro-
ecological zones. The 1:5 000 000 FAO/Unesco Soil Map of the World, and generated
climatic and crop adaptability inventories, form the basis of the study. The
results are presented as extents of land diversely suited to the production of the
various crops, within a climatic, framework. Four classes of potential suitability
1.4 ALTERNATIVE USES CONSIDERED
Selection of the crops to be considered in the assessment was based on
computer calculations of the twenty most important crops in the world, according
to a) area occupied, b) total production and c) value. The lists were also compiled
by main regions and are presented in entirety in Chapter 2.
In view of the facts that crop values fluctuate considerably and that
production is very dependent on the area cultivated, the final selection was made
from the list of the world's twenty most important crops, with regard to total
area occupied. These, in decreasing order of importance, are: wheat, rice, maize,
barley, pearl millet, sorghum, soybean, cotton, oat, phaseolus bean, white potato,
groundnut, rye, sweet potato, sugarcane, cassava, pea, chickpea, grape and rape-
- 4 -
To avoid inequality in regional distribution, e.g. grapes are not important
in Central America, Asia, the Far East, the Caribbean, tropical Africa and China,
crops not common to the majority of regions were omitted. This left the 12 crops
underlined above, which are widely distributed throughout the world. They comprise:
5 basic foodgrain crops (wheat, rice, maize, pearl millet, sorghum)
3 root crops (white potato, sweet potato, cassava)
2 leguminous food crops (phaseolue bean, soybean) and
2 cash crops (sugarcane, cotton).
Although some information on sugarcane is included in the report, this crop
was omitted from the final assessment. This omission proved necessary because an
evaluation for totally rainfed, commercial sugar production potential must take
into account factory management requirements. These were considered to be outside
the scope of the present assessment.
The 11 crops used in the assessment therefore, areas wheat, rice, maize, pearl
millet, sorghum, white potato, sweet potato, cassava, phaseolus bean, soybean and
Each of the crops is considered at two input levels: low inputs and high
inputs as detailed in Chapter 2. The former approximates to a low technological
level and involves hand cultivation. It can be compared to traditional systems
of shifting cultivation or bush fallow rotation. The high input level involves
mechanical cultivation under capital intensive management practices. Thus a
total of 22 alternative land uses are considered in the assessment, i.e. 2 for
each of the 11 crops, corresponding to low and high input levels. Details of
these land use alternatives are given in Chapter 2.
1.5 CLIMATIC AND SOIL REQUIREMENTS OF CROPS
Definition of the climatic and soil requirements of the crops used in the
assessment is probably the most important single facet of the whole study. This
work was split between two working groups, one dealing with climatic requirements
and crop adaptability, the other covering soil requirements.
Previous attempts to quantify the climatic requirements of crops have
adequately recognized the importance of rainfall and soil moisture. Less specific
information is available for the important parameter of temperature. Equal
emphasis has been placed on both parameters in the present study.
Of similar significance is the nature of the photosynthetic pathway of the crop
itself which, when related to temperature and radiation, determines productivity
if the phonological requirements are met during the period when water is available
Accordingly an inventory of crops was prepared, based on their climatic
requirements for both photosynthesis and phenology. Five main groups of crops are
recognized, as detailed in Chapter 3, e.g. Group II: photosynthesis pathway C3,
comprising rice, cotton, phaseolus bean, soybean, sweet potato and cassava;
Group III: photosynthesis pathway C4, comprising pearl millet, sorghum, maize
and sugarcane. Cultivars of maize and sorghum specially adapted to highland/cool
temperature conditions have been taken into account in this grouping (e.g. Group
This inventory gives, among other information, ranges of temperature require-
ments for both the aspects of growth and, hence, forms the basis of the crops'
climatic requirements. These are subsequently matched to existing climatic conditions,
e.g. Group III: optimum temperature response to photosynthesis from 30 to 350 C, operative
temperature range from 15 to 450 ; climatic region suitable for rainfed production:
Once the photosynthetic and phonological requirements are met, the genetic yield
potential of a crop, under constraint-free conditions, is climatically governed by the
number of days to maturity. This, in turn, is determined by the length of growing period.
Constraint-free yields were calculated for all crops and all lengths of growing period,
e.g. sorghum in a 75-89 day growing period, anticipated yield (without constraints)
0.2-1.1 t/ha; 90-119 day growing period, 1.2-2.5 t/ha; 120-149 day growing period,
2.5-3.8 t/ha. Such data were used as the basis of the agro-climatic suitability assessment.
Details of crop adaptability and climatic requirements, lengths of growing
period, and constraint-free yield calculation are presented in Chapters 3, 5, and 7
respectively. A list of reported C3- and C4-species, including grasses and legumes,
is in preparation and will be published in a separate volume.
Soil requirements of crops were assessed as follows. For each crop, all
available data on soil characteristics considered meaningful for agricultural production
were first listed, e.g. soil depth, soil texture, salinity, stoniness, etc. For each
crop, each property was then quantitatively subdivided into those for optimum condit-
ions and for range of conditions, e.g. pearl millet: optimum soil pH: 5.5-7.5, range
of soil pH: 5.2-8.2. When a property fell outside the defined range, e.g. in the
case of millet, a soil pH of 8.8, the soil was considered as currently not suitable
for the production of that crop.
The information for each subdivision, for each property, was then averaged
and reviewed by a 10 man working group. With the exception of slope, there was a
remarkable agreement on results, which are presented in Chapter 4.
This listing of optimal and minimal values of soil properties for each crop
formed the basis of the subsequent rating of the soil units (according to the
FAO/Unesco Soil Map of the World) for the production of the individual crops.
1.6 INVENTORY OF LAND AND MAPPING UNITS
Having identified the climatic and soil requirements for the crops of the
assessment, the next stage of the evaluation is an inventory of the land resources
with particular emphasis on the attributes of land relevant to the identified
climatic and soil requirements.
In the case of climate, water availability and temperature are key factors
in determining crop suitability for rainfed agriculture. Climate was inventoried
by computer calculations of the period in days when available water and temperature
regime permit crop growth, i.e. the growing period. This concept had to be developed
because of the lack of any climatic classification suited to quantitative assessment
of suitability for rainfed agriculture. The growing period is the continuous period
during the year, from the time when rainfall exceeds half potential evapotranspirat-
ion (calcuate-da by-the Penman method) until the time when rainfall falls below full
potential evapotranspiration, plus a number of days required to evaporate an assumed
100 mm of soil moisture reserve when available. Consequently a normal growing period
has a humid phase, i.e. a period in which rainfall is greater than potential evapo-
transpiration. Additionally it excludes any period when crop growth is not
possible because of low temperatures (e.g. 24hr-mean temperature less than 6.50; in
the case of winter wheat).
- 6 -
Length of growing period data (730 stations for Africa) were calculated and
plotted on 1:5 000 000 scale maps. Zones with similar lengths of growing periods
were delineated by constructing isolines at intervals of thirty days, e.g. areas
with growing periods of 90 119 days, 120 149 days, 150 179 days etc. Zones
with a humid phase were designated as normal zones. Those without a humid phase,
and consequently unable to meet full crop water requirement from rainfall, were
designated as intermediate zones. An additional isoline for a growing period of
75 days was also included, to cover the possibilities of production of short
duration pearl millet cultivars in drier areas.
For each zone thus delineated by the length of growing period isolines,
average values of major climatic elements (radiation, day and night temperatures,
etc.), characterizing the climate during the growing period, were calculated to form
the basis for subsequent crop matching and constraint-free biomass and yield calculations.
Concomitantly with this activity, and particularly to cater for the purely temperat-
ure adaptability requirements for crops, major climatic divisions were recognized,
e.g. tropical, subtropical, etc. Each of these was subdivided according to marked
changes in temperature regimes of importance to the 11 major crops, e.g. mean
temperature daring the growing period more than 2000 (tropical lowlands, less than
1 500 metres altitude); mean temperature during the growing period less than 200C
(tropical highlands, 1 500 to 3 000 metres). For the African continent a total
of eight main climatic divisions was recognized, each farther subdivided by
lengths of growing periods.
Details of the climatic inventory are given in Chapter 5 together with a
generalized reduction of the compiled 1:5 000 000 climatic base map of the African
The soil inventory was based entirely on the 1:5 000 000 FAO/Unesco Soil Map
of the World, which comprises 19 map sheets covering the world's land area. The
continent of Africa is covered by three sheets. Each set of maps, covering a
continent or large region, is accompanied by an explanatory volume giving the extent
of each soil mapping unit by countries. A total of 106 soil units are recognized
and are shown on the maps, in various combinations, as mapping units or soil
associations. In the majority of the mapping units, the texture of the dominant
soil is defined (coarse, medium or heavy) together with the dominant slope in the
unit (level, rolling or mountainous).
Each mapping unit may consist of up to 8 individual soil units and the
percentage of each, within a mapping unit, has been estimated. The different soil
untts vary greatly in characteristics from sandy soils low in organic matter,
e.g. Arenosols, to water saturated soils with thick horizons of fresh organic matter,
e g. Histosols.
Important land characteristics, not reflected by the soil units themselves,
are shown on the maps as overprints of various phases, such as stony phase or saline
phase. Areas of dunes or shifting sands, salt flats and rock debris are also shown
as overprints. All information on the extent of the various soil units, phases,
slope and texture classes, including associated soils and inclusions, by countries,
Details of the soil inventory are given in Chapter 6.
On completion of the climatic inventory, the various main climatic divisions,
and the isolines delineating various lengths of growing periods, were superimposed on
the appropriate Soil Map of the World sheets. The resultant map output is original
and creates the agro-ecological zones of the project, wherein areas with similar
soils and climates are delineated. Area measurements of the resultant unique soil/
climatic zones were effected through a 2 mm (100 km2) grid count. The results were
corrected for reported total areas of countries' land masses and refed into the
computer. The subsequent computer programme printed out the total areas of soil
units, subdivided by phases, slope classes and texture classes, by major climatic
divisions and lengths of growing periods, on a country and regional basis.
This soil/climate inventory forms the physical basis of the assessment.
Country-specific present land use data, currently available only by
administrative units, are being recompiled on the basis of the above inventory.
This will be accomplished by superimposition of the soil/climate inventory on to
administrative area maps and allocation of the appropriate percentage of admin-
istrative area to each major climatic zone and length of growing period.
1.7 MATCHING OF CROP REQUIREMENTS TO THE LAND INVENTORY
Comparison of the climatic and soil requirements of the crops, with the
climatic and soil conditions of the agro-ecological zones, is the basis of the
suitability assessment. If the requirements of a particular crop are well met,
the zone is physically well suited to its production. If the requirements are not
met, e.g. insufficient moisture or inadequate soil depth, the zone is obviously
not fully suited to the production of that particular crop. Each crop is
considered individually in the matching exercise which is undertaken separately
for the two levels of inputs considered, e.g. stony soil phases may be suitable for
low input level cultivation (hand tools) but are definitely not suited to high
input level cultivation (mechanized).
The initial step in the matching process is comparison of the temperature
requirements of the major groups of crops recognized with the identified main
climatic divisions. This step indicates the crops which should be considered, from
a temperature viewpoint, in each main climatic division, e.g. the growing period
temperature conditions in subtropical winter rainfall areas do not permit the
growth of Group II crops (cassava, groundnut, soybean, etc.).
Provided a crop is suited to a main climatic division, the next step in the
matching exercise is to ascertain its constraint-free yield in the various lengths of
growing periods inventoried in that main climatic division.
Computation of constraint-free yields of the different crops, in appr-
opriate main climatic divisions by lengths of growing periods, has been
a major activity in the present assessment. It has been achieved by first
calculating the net biomass production (total plant dry matter), taking into
account a) the gross biomass production capacity of the crops as influenced by the
response of photosynthesis to radiation and temperature, and b) the respiration
losses as influenced by temperature.. Subsequently, by using the appropriate harvest
index value (fraction of the net biomass that is economically useful), constraint-
free yield data have been derived from the net biomass. Yield reducing agro-
climatic constraints (e.g. rainfall variability, wet harvesting conditions) and
soil constraints (e.g. shallow depth) are not taken into account in these calcul-
ations but are dealt with in later stages of the suitability assessment.
Details of the calculations for constraint-free biomass and yield, with results
for the 11 crops of the assessment in Africa, are presented in Chapter 7. An example
of the method of calculation is also given.
This part of the matching exercise, i.e. comparison of crop climatic require-
ments with the different climates of the agro-ecological zones, thus provides
quantified data on anticipated, constraint-free crop yields in appropriate main
climatic divisions and lengths of growing periods.
Such quantification was z.ot possible for the soil part of the matching
exercise because of a) the lack of an agreed model for calculating potential crop
yields from the FAO/Unesco soil map units, and b) the dearth of actual crop yield
data from positively identified soil 'inits. Consequently, a qualitative approach
for matching each crop's soil requirements with soil units was adopted. This
involved comparing the properties of the coil units with the crop's soil requirements
and judging to what extent the soil's attriiites met the crop's requirements.
If the soil unit largely met the crop's requirements, it was adjudged S1, i.e.
soil conditions would not affect the constraint-free yield. If the soil unit
only partly met the crop's requirements, it was adjudged S2, i.e. the soil would not
allow the full climatic yield potential of the crop to be attained. Failure to meet
the crop's minimum soil requirements resulted in a grading of N, meaning that the
soil could not support production of the crop. Fertility constraints are included
in the soil ratings. Similar rules were made for the various slope and texture
classes and soil phases, and programmed for computer application.
Details of the soil matching methodology are given in Chapter 8.
1.8 THE LAND SUITABILITY CLASSIFICATION
The previously described matching activities provide anticipated, constraint-
free crop yields by main climatic divisions and lengths of growing period, with
identification of the degree and extent of various soil limitations in these areas.
These anticipated yields, however, do not take into account yield reductions
due to climatic (rainfall) variability, moisture stress, excess moisture, and
losses due to pests, diseases and weeds. Such constraints need to be taken into
account to arrive at anticipated yields that are agronomically attainable, from the
recognized zones. Such calculations have been effected for Africa, the constraints
varying according to the zone, e.g. endemic pests, disease and weed losses are
relatively higher on short-term crops in longer growing period areas. Climatic
variability has been included as a constraint, particularly in short growing period
areas. The constraints have been rated as nil/slight, moderate or severe,
corresponding to anticipated yield reductions of 0, 25 or 50 percent respectively.
These yield reductions are applied to arrive at the agro-climatically attainable yields
e.g. moderate constraints to maize, in the growing period 270 299 days under
high inputs, due to attack by stem borers, leaf blight and streak virus, are
considered as causing a yield reduction of 25 percent to the constraint-free
yield. The resultant agro-climatically attainable yields so calculated are for high
input/ideal soil conditions only. Agro-climatically attainable low input yields have
been calculated in a similar manner, the constraint-free yield under low inputs being
25 percent of the constraint-free yield under high input conditions.
The agro-climatic suitability assessment for each crop at both input levels,
was achieved by considering the whole agronomically possible yield range and class-
ifying each individual growing period yield into one of four classes defined in
terms of a percentage range of the maximum attainable without constraints. Growing
period zones, capable of yielding 80 percent or more of the maximum yield attainable,
were classified as very suitable; zones yielding less than 80 percent to 40 percent
as suitable; zones yielding less than 40 percent to 20 percent as marginally suit-
able and zones yielding less than 20 percent as not suitable.
This activity resulted in an agro-climatic crop suitability assessment of
each major climatic division and each length of growing period zone.
For each crop and level of input, appropriate production costs are being
calculated. Results to date indicate that the division between 'not suitable' and
'marginally suitable' relates to the break-even point of the 'norm' value of the
produce in comparison to 'norm' production costs.
In the final assessment of potential land suitability, the soil assessment
was superimposed on the agro-climatic assessment. In the case of areas of soils
adjudged S1 for a particular crop, no change was made in the agro-climatic suit-
ability assessment. Areas of soils adjudged S2 had the agro-climatic suitability
assessment downgraded by one suitability class. Areas of soils adjudged as N
resulted in a final suitability assessment of that land as not suitable, the very
severe soil limitations overriding the climatic attributes.
The final land suitability assessment, reported by the computer printouts
in Chapter 10, gives areas of lands:
for the production of each crop, at each of the two levels of inputs considered.
The four classes are related to the anticipated yield as a percentage of
the maximum attainable under optimum agro-climatic and soil conditions, and so
provide the necessary data for calculation of the rainfed production potential
of any given area.
- 11 -
ALTERNATIVE LAND USES CONSIDERED
2.1 WORLD CROP PRODUCTION
Separate assessments of suitability are necessary for each crop and
each level of input; therefore, the crops dealt with in the study had to
be limited to a number which could be handled in the two year duration of the
project. As the study deals with global assessments, it was important that the
crops considered were of global significance. They were therefore selected from
data on world crop production to ensure that the most important were included
in the study.
Data on world crop production are available in the FAO Production Yearbook
by crops, regions and individual countries.
These data were recomputed to provide, by the ten major geographical regions
used in the FAO Production Yearbook, lists of the most important crops in terms of:
area occupied, total production and economic value. The results are presented in
Table 2.1, the crops being listed in decreasing order of importance with regard to
the above three factors, i.e. area, production and value.
Pertinent implications in the table are that a) area and production data
on cereals relate to crops harvested for grain only; b) many countries make no
distinction between millet and sorghum and, in such cases, both grains are reported
under millet; c) for a number of countries yam and sweet potato are recorded
together under sweet potato; d) potato refes to white or Irish potato; e) root
crops are assessed by fresh weight; f) dry bean refeisto the species of the genus
Phaseolus as defined in the Production Yearbook; g) wine is not included in the
lists, production being recorded under the original product, i.e. grape; h) prices
used generally represent averages of producer prices for the period 1961-65;
i) all area and production data used are for the period 1971-75. Further notes
on the data used in the lists are to be found in the FAO Production Yearbook.
2.2 SELECTED CROPS
In choosing the criteria for selection of crops to be evaluated, preference,
was given to importance with regard to area occupied by the crop. Importance with
regard to value was not considered because of wide annual variations in prices, and
total production was considered to be very much a function of area occupied,
AIN CROPS BY REGIONS IN DECREASING ORDER OF IMPORTANCE WITH REGARD TO: AREA, PRODUCTION
Central America South America
Table 2.1 M
Broad bean dry
N East Asia
Broad bean dry
Broad bean dry
Broad bean dry
Broad bean dry
Broad bean dry
- 14 -
The 11 crops selected are those with the widest regional distribution in the
lists and comprise:
1. Wheat (1)
2. Paddy rice (2
3. Maize (3
4. Pearl millet (5
5. Sorghum (6
6. Soybean (7
7. Cotton (8
8. Phaseolus bean (10
9. White potato (11
10. Sweet potato (14
11. Cassava (16
Figures in parenthesis indicate world ratings with regard to area occupied.
Sugarcane was originally included in the: above list but was later excluded
in the final assessment for the reasons given in Section 1.4.
With the exception of sugarcane, the crops omitted from the top 20 were
those with uneven geographical distribution, e.g. grape, although the ninteenth most
important crop on a world basis, is of little importance in the African, Central
American and Far Eastern Regions. Evaluation of these 11 crops also covers the most
important crops in each of the major ecological regions recognized in the FAO
State of Food and Agriculture (SOFA) report, as shown below:
as in type Grain crop Root crop Industrial crop
SOFA report ty
Temperate Wheat White potato --
Mediterranean Wheat -
Arid and semi-arid Sorghum Sweet potato Cotton
Sub-humid tropical Maize Soybean
Humid tropical Rice Cassava -
Highland Phaseolus bean White potato -
Thus in only one region, the Mediterranean, is the evaluation restricted to one
crop, i.e. wheat.
Pertinent in the definition of the crops of the evaluation are the facts
a. the cereal and leguminous grain crops are produced for dry grain only
(e.g. they do not include green cob production of maize, production of soybean
and phaseolus bean as vegetables);
b. the rice considered is lowland rainfed paddy rice (grown under uncontrolled,
but bunded, flood water regimes) and does not include upland or hill
- 15 -
c. sorghums with white or yellow grains are considered in the evaluation;
sorghums with red or brown bitter grains are mainly used for brewing beer and
are not explicitly considered in the assessment;
d. wheat refers to both bread and durum wheat;
e. phaseolus bean comprises P. vulgaris, P. lunatus, P. aureus, P. radiatus,
P. mungo and P. angularis;
f. the crops comprise readily available local cultivars (low inputs) and high
yielding cultivars (high inputs) of the following durations:
Phaseolus bean 90-120 days
Soybean 90-120 days
Paddy rice 100-130 days
Cotton 170-180 days
Sweet potato 120-150 days
Cassava 180-330 days
Pearl millet 70- 90 days
Sorghum 90-120 days mean temperature > 200C /
> 120 days mean temperature < 2000
Maize 90-120 days mean temperature: 20 C
> 120 days (mean temperature <-200C)
Wheat 100-130 days (spring cultivars)
> 120 days (winter cultivars)
White potato 90-150 days.
The selection of the crops to be used in the study was endorsed by the
Perspective Study on World Agricultural Development (PSWAD) steering committee
on 31 January 1977.
2.3 LAND UTILIZATION TYPES
Having selected the crops to be considered in an assessment, it is necessary
to define the conditions under which they are to be grown. Without such a definition
the evaluation is not valid, because suitability for a crop varies very considerably
according to the circumstances under which it is produced. Two simple examples
serve to illustrate this point: a) lands with slopes of more than 14 percent,
or of a very stony nature, are not normally suited to mechanical cultivation, but
can be cultivated with hand tools and, b) very heavy soils, such as Vertisols, cannot
be cultivated with hand tools, but are suited to mechanized cultivation. Thus, a
description of the circumstances of cultivation is vital to any sound evaluation of
land. Combined descriptions of inputs, produce, technical know-how etc. serve as
the land utilization types of the evaluation.
Detailed definition of land utilization types applicable to a global land
evaluation is difficult, because of the very wide variation in economic, social and
management factors across regions. However, for an adequate description of any land
utilization type, information on the following items is desirable:
Produce, including goods (e.g. crops, livestock, timber), services
(e.g. recreational facilities) or other benefits (e.g. wildlife conservation).
Market orientation, including whether towards subsistence or commercial
1/ See footnote 5, Table 7.4.
- 16 -
Power sources (e.g. hand labour, draught animal, fuel using
Technical knowledge and attitudes of land users.
Technology employed (e.g. implements and machinery, fertilizers,
livestock breeds, farm transport, methods of timber felling).
Infrastructure requirements (e.g. sawmills, tea factories,
agricultural advisory services).
Size and configuration of land holdings, including whether
consolidated or fragmented.
Land tenure, the legal or customary manner in which rights to
land are held, by individuals or groups.
Income levels, expressed per capital, per unit of production
(e.g. farm) or per unit area.
It is not possible to collate this data for a global or even a regional
assessment, variations being too large. Accordingly, generalized major land
utilization types have been considered in the assessment, approximating to cond-
itions of low inputs and high inputs.
The following assumptions are used for the two input levels (land
utilization types), related to the items listed above.
Produce and production
The 11 rainfed crops described on p. 13;
sole cropping of single crops only, no
multiple (sequential or mixed) cropping.
High, including uncosted
Manual labour with hand
Local cultivars. No (or
insufficient) fertilizer, no
chemical pest and disease
control, fallow periods. No
irrigation or major water
Market accessibility not
Low, family labour costed,
including harvesting operations.
High yielding cultivars.
Adequate fertilizer application,
chemical pest, disease and weed
control, no fallow periods. No
irrigation or major water control
Communications and market
accessibility essential, high
level of advisory services
Small, sometimes fragmented Large, consolidated
Income levels Low
- 17 -
These attributes are the definition of the major land utilization types
employed in the assessment.
Storage facilities are not considered as an on-farm production input in the
study; consequently, neither are post harvest storage losses.
The two input levels can be visualized as representing two points on a
production/input curve, corresponding to no (or few)on-farm capital inputs and a high
level of capital inputs. It should be emphasized that:
i. better adapted land utilization types can be more productive under specific
environmental conditions, e.g. a land utilization type combining appropriate
mechanization with hand labour in the wet or humid tropics;
ii. individual country production systems occupy different positions on the
production/input curve reflecting the level of inputs currently being used.
Specific to rice are the assumptions that: a) fields are bunded to a height
of some 10 cm and, b) that the crop fits into the category of 'water conserved
rice' as opposed to 'irrigated rice with complete water control' and 'upland rain-
- 19 -
CLIMATIC ADAPTABILITY OF CROPS
This chapter deals with the interrelationships between climatic factors and
crop characteristics, in terms of climatic adaptability of crops and productivity
Photosynthesis produces the source of assimilates which plants use for growth,
and the rate of photosynthesis is influenced by both radiation and temperature.
However, plants also have an obligatory developmental pattern in time (and space)
which must be met if the photosynthetic assimilates are to be converted into
economically useful yields of satisfactory quantity and quality. The developmental
sequence of crop growth in relation to the calender (i.e. crop phenology) is
influenced by climatic factors.
In general, temperature determines the rate of growth and development; but
in some crops temperature may also determine whether a particular developmental
process will begin or not (e.g. chilling requirement for initiating flower buds in
pyrethrum), the time when it will begin, the subsequent rate of development, and
the time when the process stops. In some crops both day-length and temperature may
determine the time when the plant will flower, e.g. photoperiodic winter cereals
which require vernalization. In other crops, at a given temperature, day-length
alone may determine the time of flowering (e.g. photoperiodic tropical cereals),
while low temperatures can cause problems through delay in flowering and maturation
and poor seed set (e.g. maize and sorghum in high-altitude areas in the tropics).
In some perennial crops, when there is no 'genetic' check on growth, unimpeded
growth and development can be harmful to yield. Therefore, a temperature and/or
dry season check on excessive growth is required to avoid overbearing and crop
exhaustion (e.g. arabica coffee) so that subsequent yields are obtained on a
regularly sustained basis. Further, in some crops the quality of the economically
useful yield is influenced by temperature (e.g. pineapple, tea), while in other
crops (e.g. rainfed cotton, sorghum and wheat) yield formation must take place
when the appropriate climatic conditions accord with those required for obtaining
yields of acceptable quality. Wet conditions during the yield formation period and
maturation lead to poor quality lint in cotton, attack from sucking bugs and grain
mould in sorghum, grain germination in the panicle in wheat, in addition to other
agronomic problems associated with crop harvest and storage in such conditions.
Accordingly, in the study, consideration has been given to the specific climatic
requirements for yield quality in addition to the climatic requirements for photo-
synthesis and phenology in order to define the climatic adaptability of crops.
- 20 -
3.2 PHOTOSYNTHESIS AND PHENOLOGICAL CHARACTERISTICS
Within any suitable length of growing period, the temperature regime
(and photoperiodic regime when the available cultivars are photosensitive to day-
length) governs which crop can be grown. When the climatic phenological require-
ments are met, then both the temperature and radiation regimes set a limit to crop
productivity. This is because natural selection and breeding has forced the physio-
logical processes of crops to operate at optimum rates only within a certain range
in temperature. In particular, the evolutionary changes that have occurred in the
biochemical and physical characteristics of photosynthesis have led to a large
variation between crops both in the optimum temperature requirement for photosynthesis
and the response of photosynthesis to changes in temperature and radiation. Conse-
quently, when the climate of a region is phenologically suitable for a given crop,
it is possible to relate the temperature and radiation regimes to crop productivity
from knowledge of the temperature and radiation response of photosynthesis, and the
effect of temperature on respiration. Because the temperature and radiation
responses of photosynthesis depend on the nature of the photosynthesis pathway, it
is possible to group crop species with similar assimilation pathways and of more
or less equal photosynthesis ability.
For a rainfed crop to be successful, it is necessary that its growth cycle
(from germination to crop maturity in annuals, or the period of new growth in any
year in established perennials) should be of such a length that it is comfortably
contained within the growing period. In some crops, climatic factors may control
when a particular developmental process begins, while in other crops climatic
factors have to be agronomically utilized to stop certain developmental processes.
Once these 'requirements' have been met, then whatever the climate may have to offer
during the growing period, different crops are obliged to follow a certain develop-
mental pattern in time, and accumulate yield at different stages in their life cycle.
The rate at which plant parts are formed in time and space, although influenced by
temperature, imposes a limitation on the use of time available during the growing
period for growth and yield forming activities. Therefore, if a crop is to produce
satisfactory yields, it must be allowed to meet its phenological obligation
of proceeding unimpeded through the various developmental events in time.
Consequently, when the length of the growing period is limited, then the days to
maturity of crops must match the growing period accordingly. Failure to do so does
not completely exclude cultivation of the crop but does result in reductions of yield
and quality because the time available for yield forming activities is curtailed.
Further, although a crop species may offer cultivars of different life-span,
the agronomically optimum length of growth cycle (for a growing period of a given
length) is when the developmental time-table is of such a length that it allows
-the crop to produce sufficient vegetative growth to support concurrently (e.g.
grain legume crops, root and tuber crops) or subsequently (e.g. cereal crops),
the necessary yield forming activities within the ecologically determined start and
end of the growing period. In practice it is often necessary to locate the yield
forming activities of some crops during a particular part of the growing period in
order to meet the conditions necessary for harvest, storage and social use.
Consequently, the optimum physiological length of growth cycle of a crop that
is also agronomically optimum, and the location of the crop's life-span within the
growing period, is influenced by location-specific constraints in addition to pheno-
- 21 -
3.3 CROP ADAPTABILITY INVENTORY
To assess crop species for their climatic suitability, it is convenient to
prepare an inventory of crops based an their climatic requirements for both photo-
synthesis and phenology which bear a relationship to yield in quantity, and where
necessary to yield in quality. The rate of crop photosynthesis, growth and yield
are directly related to the assimilation pathway and its response to temperature
and radiation. However, the phenological climatic requirements, which must be met,
are not specific to a photosynthesis pathway.
Consequently, crop species have been classified into 5 groups (Tables 3.1 to
3.5 in back pocket) according to their fairly distinct photosynthesis character-
istics. Further, because the time required to form yield depends on both the
phenological constraints on the use of time available in the growing season and the
location of yield (e.g. seed, leaf, stem, root), this information is also given so
that additional characteristics of crops may be used to match crops and their cult-
ivars to prevailing climatic conditions of regions in question. It should be noted
that the primary aim in preparing this crop adaptability inventory has not been to
include all cultivated species, but to include a sufficiently large number of impor-
tant crop species (including the crops of the study) so as to highlight the variab-
ility in crop characteristics between species and its consequences in relation to
It should be further noted that crops vary greatly in their climatic require-
ments to produce economically useful yields. This is a reflection of the climatic
adaptability characteristics that have been carried over through natural and human
selection from the various ecological centres of genetic diversity, and what breeders
have managed to achieve with the help of the available germ-plasm of our cultivated
genetic resources in either removing or introducing specific or wide climatic
adaptability characteristics to suit agronomic goals for higher yields.
Because the overall objective of the study is to assess the present and potent-
ial land use for 11 major crops only(commencing with Africa), the inventory of crops in
Tables 3.1 to 3.5 must be looked at in its proper perspective, i.e. it is an attempt
to develop an approach to assess climatic adaptability of crops, which may be
expanded to meet further requirements for assessment of land for crops other than
the 11 crops presently under consideration.
3.3.1 Characteristics Related to Photosynthesis
The division of crops into five groups is based on the differences between
crop species in their photosynthesis pathways and the response of photosynthesis to
temperature and radiation, because these differences determine productivity when
climatic phenological requirements are met.
The two major pathways of photosynthesis are the 03 pathway and the
04 pathway. In the former, the first product of photosynthesis is a 3-carbon
organic acid (3-phosphoglyceric acid) while in the latter the first products are
4-carbon organic acids (malate and aspartate). In general the C3 assimilation
pathway is adapted to operate at optimum rates under conditions of low temperature
(15-2000), and C -species have relatively lower rates of CO2 exchange at a given
radiation level than C4-species which are adapted to operate at optimum rates under
conditions of higher temperature (30-350C) and have comparatively higher rates of CO2
exchange (Figs. 3.1 and 3.2). 1/ Further, C4-species have maximum rates of photo-
synthesis in the range 70-100 mg CO2 dm-2h' with light saturation at 1.0-1.4 cal
cm-2 min-1 total radiation, while C -species have maximum rates of photosynthesis in
the range 15-30 mg CO2 dm-2 h-1 with light saturation at 0.2-0.6 cal cm-2 min-1
(Fig. 3.2). These C3- and C4-species constitute Groups I (Table 3.1 ) and III (Table
1/ A comprehensive list of reported C and C -species is in preparation, and
will be published in a separate volume.
Average relationship between maximum leaf photosynthesis rate and temperature for crop
Groups I, II, III and IV
- 23 -
I I I I I OQ
0.1 0.2 0.3 0.4 0.5 0.6
Ar (col cm-2 min-1 )
Relationship between leaf photosynthesis rate
at optimum temperature and photosynthetically
radiation (Ar) for crop Groups I, II, III and
Pm is the maximum leaf photosynthesis rate at
- 24 -
Breeding and selection, through both natural and human agencies, has changed
the temperature response of photosynthesis in some C3- and C4-species. Consequently,
there are C3-species (e.g. cotton, groundnut) whose optimum temperature is in a medium
to high range (25-300C), and there are C4-species (e.g. maize, sorghum), where, for
temperate and tropical highland cultivars, the optimum temperature is in a low to medium
range (20-3000) (Figs. 3.1 and 3.2). These C3-species and the specially adapted
cultivars of C4-species constitute groups II (Table 3.2) and IV (Table 3.4) respectively.
One further group of species has evolved and adapted to operate under
xerophytic conditions. These species have the Crassulacean Acid Metabolism (CAM).
Although the biochemistry of photosynthesis in the CAM-species has several features
in common with C4-species, particularly the synthesis of 4-carbon organic acids,
CAM-species have some unique features which are not observed in C3- and C -species.
These include capturing the light energy during the day-time and fixing CO2 during
the night-time, with consequently very high water use efficiencies. There are two
CAM-species of agricultural importance (i.e. pineapple and sisal), and these cons-
titute group V (Table 3.5).
3.3.2 Characteristics Related to Phenology and Yield
In the sequence of events in the developmental pattern of crop growth, yield
formation and senescence, organs are differentiated in which assimilates for growth
are made and accumulated. Some of these organs become the yield forming organs
where the harvestable assimilates (e.g. starch or other carbohydrates, protein, oil,
fibres) are stored.
Botanically the growth habit of a crop may be determinate (where no more
leaves are formed once the apical bud of the vegetative shoot has become reproductive,
and fruits and seeds are borne on terminal inflorescences) or indeterminate 1/
(where the apical bud remains vegetative and fruits and seeds are borne on lateral
inflorescences). Further, whether a crop's natural life-span is annual, biennial or
perennial, the time required to form yield, and the cultivated life-span, depend on
the portion of the plant that is economically useful.
When it is a reproductive part (e.g. fruit, seed), yield may be formed either
in the last phase of the crop's life in terminal inflorescence of annual crops (e.g.
cereals) and annual shoots of perennial crops (e.g. banana), or during a greater or
smaller fraction of the crop's life in fruits and seeds borne on early or late-formed
lateral inflorescences of annual crops (e.g. leguminous pulses and oil seeds, tomato)
and perennial crops (e.g. fruit tree crops).
When the economically useful portion is a vegetative part (e.g. leaf, root),
yield may be formed throughout much or all the period in which growth is possible
in a sufficiently long-lived annual (e.g. tobacco, potato), biennial (e.g. sugarbeet)
or perennial (e.g. yam, sugarcane) crop.
In some biennials (e.g. sugarbeet, onion) and perennials (e.g. sugarcane),
because yield is formed in vegetative parts, flowering lowers the yield. Consequently,
the former crops are grown as annuals under conditions which will not favour flowering
(or bolting). In the case of sugarcane, it is grown either as a biennial when non-
flowering cultivars are available, or on an annual basis (i.e. plant and ratoon crop
or plant crop only) when non-flowering cultivars are not available.
1/ Crops with sympodial branching habit (e.g. castor bean) or with monopodial and
sympodial branching habit (e.g. cotton) are regarded here as having an indeter-
minate growth habit. In such crops the apical meristem on sympodial branches
produces an inflorescence, and axillary buds below it grow into branches which
in turn produce apical inflorescences and sub-terminal branches.
- 25 -
Li Tables 3.1 to 3.5, information is given on days to maturity for crops
cultivated as annuals, growing period required for biennial and perennial crops,
and the main products obtained from them. In respect of their life-span
and the proportion of their growth cycle which can be used to form yield, information
is also given on growth habit, natural and cultivated life-span, location of yield,
and approximate yield formation period.
Generally, annual and biennial crops have a range of cultivars of different
life-spans, and in perennial crops the period of new growth in any year also varies
between cultivars and regions due to both genetic and ecological reasons. However,
the genetic yield potential of cultivars is related to the number of days to
maturity, or the period of new growth. This is because when the developmental
time-table is severely telescoped and phenological events occur too rapidly (as in
very early maturing cultivars of annual crops, or perennials crops with a very
short period of new growth), there is generally a drop in yield potential even when
the crop is allowed to reach normal maturity. Similarly, there can often be a drop
in yield potential when the developmental time-table is severely stretched and
phenological events occur too slowly. This is because the continuous or concurrent
growth of non-economically useful parts may compete for assimilates with the growth
of economically useful parts and thereby interfering with the yield forming
activities, or the continuous growth may exhaust the crop in the case of some
perennials so that subsequent yields are lower. There is, therefore, an optimum
physiological length of growth in a crop species related to the highest genetic
yield potential the species can offer. However, within the environmentally
determined length of growing period, the particular cultivar of a crop species,
found to be most suitable agronomically, may not necessarily be the one with the
highest genetic yield potential but may still allow satisfactory yields to be
obtained within a particular set of agronomic circumstances (land utilization type).
An indication is also given in Tables 3.1 to 3.5 of any specific
climatic requirements for crops, and the major climatic division within which the
most suitable areas of adaptability for rainfed production may be found when the
agronomic consequences of the climatic requirements for photosynthesis, phenology,
yield formation and quality are considered along with other production factors in
matching rainfed crops to a particular set of climatic conditions prevailing in a
specific area of interest.
- 27 -
SOIL REQUIREMENTS OF CROPS
Crop plants, like any plants, have certain specific site preferences for
optimal production. Some crops are very particular and site-specific, others are
less demanding and more versatile or adaptable in this regard.
Site specificity, or the optimal conditions for growing a particular
plant species or crop, is due to specific aerial (climatic) requirements on
one hand, and specific soil requirements on the 'other. For each particular
site, the integrated effect of climatic and soil condition determines the
production from a given crop, cultivated under a specified management system.
The effect of climate, through temperature and moisture supply, is overriding in
determining biomass and crop production. The effects of soils and land management
are modifiers. In general the modifying effects of soils and land management
become greater, the more the climatic conditions of a given area deviate from the
optimum for the growth of a crop.
Under ideal climatic conditions, for a given crop, the 'crop grows by itself'
on a variety of soils. As climatic conditions become more adverse, soil and
management conditions become more and more important in determining production.
However, many soils are a result of climatic action and, as a result,
climate and soil in many instances have a mutual enhancing effect on crop
productivity. The close interrelation of climate and (zonal) soil and natural
plant community, to socie extent, simplifies assessment of land suitability.
4.2 BAIUC WOIL REQUIHWWMNTh OF SELECTED CROPS
The basic soil requirements of crop plants may be summarized under the
following headings, related to internal and external soil properties:
a. Internal requirements:
the soil temperature regime, as a function of the heat balance of
soils as related to annual or seasonal and/or daily temperature
the soil moisture regime, as a function of the water balance of soils
as related to the soil's capacity to store, retain, transport and
release moisture for crop growth, and/or to the soil's permeability
and drainage characteristics;
the soil aeration regime, as a function of the soil air balance as
related to its capacity to supply and transport oxygen to the root
zone and to remove carbon dioxide;
- 28 -
the natural soil fertility regime, as related to the soil's capacity
to store, retain and release plant nutrients in such kinds and
proportions as required by crops during growth;
the effective soil depth available for root development and foothold
of the crop;
soil texture and stoniness, at the surface and within the whole depth
of soil required for normal crop development;
the absence of soil salinity and of specific toxic substances or
ions deleterious to crop growth;
other specific properties, e.g. soil tilth as required for germination
and early growth.
b. External requirements: in addition to the above internal soil requirements
of crops, a number of external soil requirements are of importance, e.g.:
soil slope, topography and characteristics determined by micro-
and macro-relief of the soil;
occurrence of flooding as related to crop susceptibility to flooding
during the growing period (e.g. potato, maize very susceptible), or,
inversely, to flooding requirements (e.g. rice). The incidence,
regularity (irregularity) and depth of flooding are possibly the most
important factors determining the potential use of extensive river
flood plain soils in the world;
soil accessibility and trafficability under certain management
From the basic soil requirements of crops, a number of crop response related
soil characteristics can be derived at least for the most important crops and
existing cultivars. One of these characteristics is, for instance, soil pH. For
most crops and cultivars, optimal soil pH is known and can be quantified by a range
within which it is not limiting to growth. Outside the optimal range, there is a
critical range within which the crop can be grown successfully but with diminished
yield. Beyond the critical range, the crop cannot be expected to yield satisfactorily
unless special precautionary management measures are taken.
The sane holds for other soil requirements of plants related to soil
characteristics. Many soil characteristics can be defined in a range that is optimal
for a given crop, a range that is critical or marginal, and a range that is
unsuitable under present technology.
Table 4.1 presents, for each of the crops of the assessment, optimal and
critical ranges of the following soil characteristics: soil slope, soil depth,
soil drainage, texture and clay type, natural fertility, salinity, pH, free calcium
carbonate content and gypsum content. Seven drainage classes and 14 texture classes,
subdivided by clay type, are used in the ranges. (Table 4.1 in back pocket.)
- 29 -
The characteristics were selected on the basis of: a) importance in
assessing soils for crop growth and b) availability in the FAO/Unesco Soil
Map of the World. The ratings of the characteristics were arrived at by
literature review and assessment of the compiled findings by FAO pedologists
and crop specialists.
Difficulty was experienced in rating crop requirements with regard to
inherent (natural) soil fertility, this attribute being a combination of many
individual soil characteristics. To a large extent, however, inherent fertility
is related to the cation exchange capacity of the soil and the relative degree
of saturation of the organic and mineral exchange complex. As both of these
characteristics, i.e. CEC and base saturation, are reflected or implied in the
FAO/Unesco World Soil Map Legend, they were used as the basis for estimating
crop fertility requirements as low, medium or high.
Many of the soil characteristics listed above are at least partly
intrinsically related to the soil units as defined in the Legend of the
FAO/Unesco Soil Map of the World. This relationship (Table 6.1) has guided
the definition of optimal and marginal ranges of the various soil characteristics
and so simplified the subsequent matching of the different soil units with the
inventoried soil requirements of crops, as described in Chapter 8.
- 31 -
Agricultural productivity from a given land utilization type can be analysed
as an input/output system (see Chapter 2 for descriptions of land utilization types
and input levels under consideration). To achieve such an analysis on a quantitative
basis, it is necessary to quantify the relationship between the biological (e.g.
crop) and physical (e.g. climate, soil) components for the land utilization types
under consideration. Such an approach, in assessing crop/climate relationships,
provides a rational basis for land resource assessment and for planning efficient
exploitation of climatic resources for agriculture.
While present knowledge does not allow full quantification of all the agronomic
consequences of climate in relation to crop adaptability and production, a number of
important crop/climate relationships can be quantified in order to allow:
i. an assessment of the influence of climate on the regional
distribution (space and time related) of the crops of the
ii. within a region, the production that can be attained under
conditions that are free from soil and agro-climatic constraints 1/
(see Chapter 7).
Temperature and water are the major climatic factors that govern the climatic
adaptability and distribution (both in space and time) of crops. In combination
with solar radiation, these climatic factors condition the net photosynthesis and
allow the plant to accumulate dry matter (and accomplish its successive developmental
stages) according to the rates and patterns which are specific to the main groups
of cultivated plants as described in Chapter 3.
In different parts of the globe, temperature and water availability from rain-
fall act in different proportions as constraints in relation to year-round rainfed
crop production. In warm tropical regions, the major constraint limiting the time
available for rainfed crop production is availability of water. In subtropical
regions with winter rainfall, low temperature and radiation during the winter period
may limit crop growth although water may be available; during the summer period in
such areas water availability may limit crop growth despite a favourable temperature
and radiation climate.
1/ An agro-climatic constraint has its origin in the prevailing climate and can
have an effect on land suitability depending on the land utilization type. In
the present assessment yield losses due- to agronomic consequences of the
effect of climate are dealt with through a system of ratings as described in
Chapters 7 and 9.
- 32 -
Further, when water is available, temperature conditions in one region may
permit growth of a crop from Groups II and III only, while the temperate conditions
in another region may only permit growth of a crop from Groups I and IV. These
situations occur particularly when temperature decreases are spatial in nature
(e.g. decrease in temperature due to increase in altitude and/or latitude in the
tropics and subtropics with summer rainfall). However, when the water availability
period is long and temperature changes are seasonal in nature, part of the water
availability period may be suitable for growth of a crop from Groups II and III only,
while another part of the water available period may be suitable for growth of a
crop from Groups I and IV (e.g. in the tropics in areas away from the equator and in
5.2 CONCEPTS OF THE CLIMATIC INVENTORY
The growing period, has been used as the basis of the assessment of climatic
resources. This is defined as the period in which water availability and temperature
permit crop growth. Because the present study deals with an assessment of rainfed
production only, an inventory of climatic resources was prepared to allow:
i. a differentiation of the continents into major climatic regions (divisions)
reflecting changes in the geographical and seasonal distribution of the
crops of the assessment (as described in Section 5-3);
ii. a quantification of the length of time available during which rainfed prod-
uction is possible (as described in Section 5.3);
iii. a calculation of yield of crops that can be attained under conditions free
from constraints (as described in Chapter 7);
iv. a semi-quantification of agro-climatic constraint in terms of ratings to take
into account yield losses likely to occur due to such agro-climatic constraints
(as described in Chapter 9).
No global inventory of lengths of growing periods and their temperature and
radiation characteristics exists. It has therefore been necessary to synthesize such
an inventory, using monthly average values of rainfall and potential evapotranspir-
ation, the latter calculated from radiation, maximum and minimum screen temperatures,
humidity and wind speed. The inventory was also used to identify major climatic
divisions, through considerations of mean temperatures during the growing period.
The effects of rainfall variability and water stress (drought) are taken
into account as an agro-climatic constraint (together with other agro-climatic
constraints) in the calculation of anticipated yields in Chapter 9. Agro-climatic
constraints such as hazards of damage from hail and hurricane have not been taken
into account. When frost hazards determine the limits of distribution of a crop,
this has been taken into account based on average temperature conditions only.
The methodology used for preparing the climatic inventory is described in
the following sections.
- 33 -
5.3 MAJOR CLIMATIC DIVISIONS
To take into account crop temperature requirements that limit the distribution
of the crops of the study, on a global scale, prevailing temperature regimes have
been inventoried by identification of major divisions. Further,to identify these
major climates, temperature criteria corresponding approximately to the requirements
of the crops of the study were established. Mean temperatures of more than 20 C
during the growing period were regarded as being suitable for consideration for crop
production of crops from Groups II and III, while mean temperatures less than 20 C
were regarded as suitable for consideration of crops from Groups I and IV.
To arrive at the major climatic divisions for Africa, first the effect of
latitude, in space and time, on mean temperature was taken into account. To inventory
this, monthly mean temperatures were reduced to sea level temperatures, and areas
with mean temperatures of all months greater than 1800 were separated from those with
a period with monthly mean temperatures less than 18C. These two areas were des-
ignated the tropics and the subtropics respectively. The subtropics were separated
into areas where the rainfall was in the cooler part of the year (i.e. subtropics with
winter rainfall) and where it was in the warmer part of the year (i.e. subtropics
with summer rainfall).
To take into account the effect of altitude on mean temperature during the
growing period, the tropics and the subtropics with summer rainfall were each
divided into three major climates, while the subtropics with winter rainfall
divided into two. This led to recognition of eight major climates for Africa.
The characteristics of the major climatic divisions inventoried for Africa,
and their relationships to the crop adaptability groups formulated in Chapter 3,
are presented in Table 5.1.
5.4 GROWING PERIOD
To enable quantitative assessment of the growing period, the following working
definition is used. The growing period is the period (in days) during a year when
precipitation exceeds half the potential evapotranspiration, plus a period required
to evapotranspire an assumed 100 mm of water from excess precipitation (or less
if not available) stored in the soil profile. A normal growing period must exhibit
a humid period, i.e. a period with an excess of precipitation over potential evapo-
transpiration. Additionally, any time interval during the period when water is
available, but with temperatures too low for crop growth, is excluded.
The calculation of the growing period is based on a simple water balance
model, comparing precipitation (P) with potential evapotranspiration (PET).
The data utilized for the calculation of the water balance, and for further
climate-related calculations, have been obtained from meteorological stations where
extended records of rainfall, maximum and minimum temperatures, vapour pressure,
wind speed and sunshine duration were available on a monthly and yearly basis.
Where data was incomplete, interpolation from other observed or estimated climatic
elements has been carried out and compared with data obtained from neighboring
stations. In all, 730 stations were used in this exercise for Africa.
CHARACTERISTICS OF MAJOR CLIMATES IN AFRICA
Major climates Suitable for 24hr-mean Total
consideration temperature extent
SDescriptive name during the regime over the (000 ha)
growing period growing period
for crop group
1 Warm tropics or tropical lowlands II and III > 20C 2 029 975
2 Cool tropics or tropical highlands I and IV < 2000C 96 604
3 Cold tropics or tropical mountains not suitable 6.5/100C 2 903
4 Warm sub-tropics (summer rainfall) II and III 20 C 291 894
5 Cool sub-tropics (summer rainfall) I and IV 2000 39 900
6 Cold sub-tropics (summer rainfall) not suitable 6.5/1000C 193
7 Cool sub-tropics (winter rainfall I = 6.500 543 198
8 Cold sub-tropics (winter rainfall) not suitable 6.5C 6 663
- 35 -
5.4.1 Growing Period Detertnired by Water Availability
For the assessment of the growing period as determined by water availability,
the following concepts, definitions and methodologies are employed (also see Fig. 5.1):
i. Beginning of growing period
The determination of the beginning of the growing period is based
on the start of the rainy season.
The first rains fall on soil which is generally dry at the surface
and which has a large soil moisture deficit in the soil profile. In the
absence of soil moisture reserves, seedbed preparation, seed germination and
the initial growth of crops are therefore entirely dependent on the amount
and frequency distribution of these early rains. Work in East and West Africa
indicates that the reliability of precipitation (in terms of frequency and
amount) of these early rains increases considerably, once the monthly prec-
ipitation is equal to, or exceeds half of the monthly potential evapotrans-
piration. Thus, a 'false start of rains' can be largely eliminated if the
beginning of the growing period (and start of the rainy season) is taken as
the time when precipitation equals half potential evapotranspiration (P=0.5 PET)-.
Additionally, this premise takes into account the fact that the amount of
moisture required to sustain growth of germinating crops is much below the full
rate of evapotranspiration and during crop emergence it approximates to about
0.5 PET. Therefore, the amount of precipitation that is equal to (or greater
than) 0.5 PET has been considered as being sufficient to meet the water
requirements of establishing crops. Accordingly, in the model, the time
when P = 0.5 PET is taken as the beginning of the growing period.
ii. Humid period
A normal growing period is defined as one with a period when there is
an excess of precipitation over potential evapotranspiration, i.e. a humid
period. Such a period not only meets the full evapotranspiration demands of
crops at a complete or maximum canopy cover, but also replenishes the moisture
deficit of the soil profile.
J/ For the estimation of PET the Penman 1948 formula has been used.
Where measured radiation was not available, in the calculation of the total
radiation, the expression (0.18 + 0.55 n/N) was replaced by (0.25 + 0.45 n/N) for
dry climates and by (0.29 + 0.42 n/N) for humid climates, where n = the actual number
of hours of bright sunshine and N = the maximum possible number of sunshine hours.
In the aerodynamic term of the formula, the expression (1.00 + 0.54u) has been
used, where u= wind speed expressed in m/sec at 2 m height. Underestimation of PET
in continental climates, due to advection, has been taken into account. For locations
with average minimum temperatures above 5C, and differences between the average maximum
and minimum temperatures of more than 1200, the expression is modified as follows:
120 >(T max T mini -S13C (1.00 + 0.61u
1300 -(T max T min -14 1.00 + 0.68u
140 =:(T max T min i1500 (1.00 + 0.75u)
150UC (T max T min) =160C (1.00 + 0.82u)
or (T max T min) > 1600 (1.00 + 0.89u)
a bI b2 c d
3. All year round humid
S, 0.5 PET
4. All year round dry
S 0.5 PET
Fig. 5.1: Examples of four types of growing period.
a Beginning of rains and growing period
b and b2 Start and end of humid period respectively
c End of rains and rainy season
d End of growing period
PET Potential evapotranspiration
- 36 -
- 37 -
iii. End of rains and rainy season
During the post humid period, precipitation is again less than potential
evapotranspiration and crops begin to draw upon water stored in the soil.
Subsequently, the frequency and amount of precipitation decreases sharply and
rainfall deficit increases. This results in a marked alteration of the environ-
ment and triggers pronounced changes in physiological responses of crops.
Under such conditions, and in the absence of soil moisture reserves, crops
are forced to mature when precipitation is equal to or less than 0.5 PET.
The time, when P = 0.5 PET in the post humid period is taken as the end of
rains and rainy season.
iv. End of growing period
The growing period for most crops continues beyond the rainy season
and, to a greater or lesser extent, crops often mature on moisture reserves
stored in the soil profile. Soil moisture storage must be therefore considered
in defining the length of the growing period. However, the amount of soil
moisture stored in the soil profile, and available to a crop, varies with the
depth of the profile, the soil's physical characteristics, the rooting pattern
of the crop and other factors. Furthermore, changes in soil moisture reserve
lead to changes in the actual evapotranspiration rate.
In the model, a general figure of 100 mm storage water has been assumed
as being available to crops. Accordingly, the time taken to evapotranspire
this 100 mm of storage water (or less if 100 mm excess precipitation is not
available in the humid period) has been added to the duration of the rainy
season, to set the end of the growing period. The choice of 100 mm is based
on experimental evidence from East and West Africa which indicates that the
crops of the study can utilize stored soil moisture in the range of 75 125 mm,
by the time of harvest. Where storage water is likely to be less than 100 mm
due to soil characteristics (e.g. shallow soil depth), this is taken into
account in the computation of the land suitability in Chapter 9.
5.4.2 Different Types of Growing Periods Determined by Water Availability
In addition to normal growing periods (example 1, Fig. 5.1), as defined in
the preceding section, three other types of water available growing periods have been
recognized in Africa, namely:
Intermediate growing period (example 2, Fig. 5.1)
Throughout the year, the average monthly precipitation does not exceed
the full rate of the average monthly potential evapotranspiration,
but it does exceed half the potential evapotranspiration. The beginning
and the end of such an intermediate growing period are defined as
the points where the precipitation curve crosses the 0.5 PET curve. All
areas with intermediate growing periods have been inventoried separately.
All year round humid growing period (example 3, Fig. 5.1)
The average monthly precipitation, for every month of the year, exceeds the
full rate of the average monthly potential evapotranspiration. Areas with all
year round humid growing periods have been included and inventoried as areas
with a normal growing period of 365 days.
All year round dry period (example 4, Fig. 5.1)
The average monthly precipitation for every month of the year is lower than
half the average monthly potential evapotranspiration. Areas with all year
round dry periods have been inventoried separately as areas with a growing
period of 0 days.
- 38 -
5.4.3 Growing Period Determined by Water Availability and Temperature
The first set of calculations, effected in the climatic inventory, quantify
the periods when water is available for crop grwoth. Subsequently, in appropriate
areas, these values are reduced by the period of time when crop growth is limited
In locations where such periods of time are equal to (or greater than) the
water available periods the areas are designated as having no growing period (i.e.
major climatic divisions 3, 6 and 8, Section 5.3).
In locations where the low temperature periods only partly restrict growth,
appropriate reductions are applied to the calculated water available periods to
arrive at the growing periods.
This situation applies particularly to the Atlas mountain areas of North
Africa. In this region (cool subtropics with winter rainfall, climatic division 7)
the most important crop of the study is winter wheat. Evidence from Africa indicates
that the growth of this grop is severely restricted when the 24-hour (daily) mean
temperature is below 6.5 C.
Accordingly, in Africa, the period during which temperatures lower than
6.500 occur, has been subtracted from the calculated water availability period,
to arrive at a growing period when both water and temperature permit crop growth.
As illustrated below, this period can show a discontinuity.
150 days water availability period = 150 days
30 ds period with mean temperatures
0 I below 6.50 = 30 days
30 days 90 days growing period = 120 days
Although the winter wheat crop in the above example occupies the land for a
total of 150 days, the actual growth period is restricted to the calculated growing
period of 120 days with an intermediate dormant period of 30 days.
5.4.4 Two Growing Periods in One Year
Areas characterized by a bimodal rainfall pattern, which may result in one or
more growing periods, have been provisionally treated in the following manner.
When the rainfall deficit is less than 50 mm during the drier period between
the two rainfall peaks, the water availability period is considered as being a
continuous growing period. When the deficit is more than 50 mm, two possible growing
periods are considered and the longer of the two growing periods is inventoried. The
latter circumstance does not exclude survival and subsequent growth of certain long
duration crops such as cassava.
- 39 -
5.5 RESULTS FOR AFRICA
5.5.1 Climatic Data Output
The growing periods for more than 700 stations in Africa were computed and
for each station, the following outputs were obtained:
dates of the beginning and the end of growing period
length of the growing period (in days)
definition of the type of growing period (see Sections 5.4.1 and 5.4.2)
average values for 24hr-mean, maximum and minimum temperatures,
and average values of day time and night time temperatures (oC)
during the growing period
average radiation (cal cm-2 day ) during the growing period
annual potential evapotranspiration (mm)
potential evapotranspiration (mm) during the growing period
annual precipitation (mm)
precipitation during the growing period (mm)
excess precipitation (mm)
post humid moisture storage (mm)
'surplus water (mm) in excess of storage.
5.5.2 Area Inventory
The area inventory of lengths and types of growing periods, by the major
climatic divisions recognized, was undertaken on the 1:5 million FAO/Unesco Soil
Map of the World by:
i. plotting the individual station values of the lengths of growing periods;
ii. constructing isolines of growing periods with values of 0, 75, 90, 120, 150,
180, 210, 240, 270, 300, 330 and 365 days, delineating length of growing
period zones of 0 74 days, 75 89 days, 90 119 days, etc.
Areas with 'normal' growing periods were delineated by continuous lines,
while those with 'intermediate' growing periods were distinguished with broken
lines. Where station data were limited, rainfall information and land use maps
provided guidance for the isoline construction.
This 1:5 million inventory was initially reduced to 1:10 million, and
subsequently to the generalized (page size) map which accompanies this report
GENERALIZED CLIMATIC INVENTORY AFRICA
MAJOR CLIMATIC DIVISIONS AND LENGTHS OF GROWING PERIOD ZONES
..**'" TROPICS / SUBTROPICS
..'" SUMMER / WINTER RAINFALL
HIGH ALTITUDES / COLD TEMPERATURES
HIGH ALTITUDES / COOL TEMPERATURES
- 41 -
It must be emphasized that, for reasons of cartographic presentation, the
locations of some of the isolines on the generalized (page size) map do not
completely correspond to those on the original 1:5 million base map. Publication
of the original material at a 1:10 million scale will be undertaken at a later
Area measurement of the various growing period zones, by the major climatic
divisions, was achieved by undertaking a 2 mm (100 km2) grid count on the original
1:5 million base-map.
The results for Africa are presented in Table 5.2, showing the extents of the
various lengths of growing period zones, by major climatic divisions, in thousands
EXTENTS (000 HA) OF GROWING PERIOD ZONES BY MAJOR CLIMATES
Major Climate 1 2 3 4 5 6 7 8 Total
Lengths of growing
nil 2 903 193 6 663 9 759
(A) 365 N 147 381 2 234 149 615
(B) 330-364 N 75 472 2 404 77 876
(C) 300-329 N 73 665 3 243 76 908
(D) 270-299 N 127 840 11 063 549 229 139 681
(E) 240-269 N .134 048 16 057 438 502 1 046 152 091
(F) 210-239 N 130 509 14 647 868 570 8 231 154 825
(G) 180-209 N 226 096 13 419 3 793 4 636 7 857 255 801
(H) 150-179 N 175 018 6 508 1 756 723 6 358 190 363
(I) 120-149 N 114 454 5 921 6 527 126 902
(J) 90-119 N 72 180 6 993 7 489 86 662
(K) 75- 89 N 58 238 3 863 777 62 878
(L) 74 N 369 467 2 190 4 601 376 258
(M) 0 N 183 660 4 078 237 744 6 547 442 929 874 958
(N) 74 I 66 933 3 690 26 510 11 825 44 496 153 454
(P) 75- 89 I 24 279 179 2 020 873 8 748 36 117
(Q) 90-119 I 50 273 97 7 203 1 356 4 139 63 068
(R) 120-149 I 446 6 905 2 463 9 814
(S) 150-179 I 16 4 108 10 176 14 300
Totals 2 029 975 96 604 2 903 291 894 39 900 193 543 198 6 663 3 011 330
Letters in parenthesis refer
N normal growing period
to computer coding.
I intermediate growing period
- 43 -
The FAO/Jnesco Soil Map of the World, at the 1:5 million scale, was used as
the basic document for the appraisal of the world's soil resources for this study.
It was considered that the scale was suitable for presenting a comprehensive picture
of the world soil resources, taking into account the limited duration of the project.
The compilation of the world soil map is now complete and the soil information
is available on a global basis. The map comprises 19 map sheets of 80 x 110 cm,
and the agro-ecological zones project used the available printed sheets and some hand
coloured drafts for its studies. The map is accompanied by 10 explanatory volumes.
Volume I Legend explains the methodology and the definition of soil units used
for the compilation of the map. The other volumes are related to 9 groups of map
sheets covering the major regions of the world (continent or subcontinent).
6.2 SOURCES OF INFORMATION
The Soil Map of the World was compiled using all available information from
soil reports of field surveys and a collection of maps of soil, geology, topography,
vegetation and climate (some 11 000 maps) assembled over a period of 15 years.
Intensive use was also made of first-hand information supplied by FAO field staff.
Such materials vary widely in reliability, precision, level of detail, scale,
methodologies and approaches to soil classification, thus necessitating tedious
Where the soil map is based on systematic soil surveys, the boundaries and
the characteristics of the mapping units are plotted from direct observations and
the map is reliable.
Where the map is compiled from soil reconnaissance, the boundaries are based
to a. large extent on compilations of topography, geology, vegetation and climate.
Information regarding the composition of soil associations is derived from field
observations, and the density of these is not sufficient for a systematic field
inspection of the soil boundaries.
For those parts of the soil map compiled from general information, both the
boundaries of the mapping units and the composition of the soil associations are
largely based on the interpretation of data on land form, geology, vegetation and
climate. In such parts, only occasional field observations have been made and these
are insufficient to supply detailed information on the distribution of the different
soils throughout the area.
These three broad degrees of reliability are indicated on each sheet of the
Soil Map of the World by means of a small scale inset map, which specifies whether
the information was derived from systematic soil surveys, reconnaissance surveys or
general information. The map reliability has to be considered in the appraisal of
land resources for a specific area. Additionally (at the world scale) some
differences exist, between contributors to the map, in the interpretation of the
definition of some soil units, particularly with respect to Fluvisols and Lithosols.
It has not been possible to take into account these differences in interpretation in
the present assessment which, of necessity, is based on the information supplied for
compilation of the Soil Map of the World.
6.3 THE LEGEND
On a small scale map, such as the Soil Map of the World, the mapping units con-
sist generally of associations of individual soil units occurring within the limits of
a mappable physiographic entity. The mapping units reflect as precisely as possible
the soil pattern of large regions.
6.3.1 The Soil Units and their Characteristics
The legend of the Soil Map of the World comprises 106 different soil units
(26 major soil units). The soil units adopted were selected on the basis of present
knowledge on the formation, characteristics and distribution of the soils covering
the earth's surface, their importance as resources for agricultural production and
their significance as factors of the environment.
The soil units have been defined in terms of measurable and observable
properties of the soil itself and are combined into 'diagnostic horizons', similar
to the corresponding diagnostic horizons of the U.S.D.A. Taxonomy.
Many of the properties are relevant to soil use and production potential and
therefore have a practical application value. Consequently, the soil units which
have been distinguished on the Soil Map of the World have value for predicting
possible optimum uses of soils.
The complete definitions of the soil units are given in Volume I (Legend)
of the FAO/Unesco Soil Map of the World. The symbols used to represent the soil
units are reproduced below from the cited Legend.
J FLUVISOLS Q ARENOSOLS Z SOLONCHAKS K KASTANOZEMS
Je Eutric Fluvisols Qc Cambic Arenosols Zo Orthic Solonchaks Kh Haplic Kastanozems
Jc Calcaric Fluvisols Ql Luvic Arenosols Zm Mollic Solonchaks Kk Calcic Kastanozems
Jd Dystric Fluvisols Qf Ferralic Arenosols Zt Takyric Solonchaks KI Luvic Kastanozems
Jt Thionic Fluvisols Qa Albic Arenosols Zg Gleyic Solonchaks
G GLEYSOLS E RENDZINAS S SOLONETZ
Ch Haplic Chernozems
Ge Eutric Gleysols So Orthic Solonetz Ck Calcic Chernozems
Gc Calcaric Gleysols Sm Mollic Solonetz Cl Luvic Chernozems
Gd Dystric Gleysols Sg Gleyic Solonetz Cg Glossic Chernozems
Gm Mollic Gleysols
Gh Humic Gleysols
Gp Plinthic Gleysols
Gx Gelic Gleysols
- 45 -
Vp Pellic Vertisols
Vc Chromic Vertisols
De Eutric Podzoluvisols
Dd Dystric Podzoluvisols
Dg Gleyic Podzoluvisols
Ao Orthic Acrisols
Af Ferric Acrisols
Ah Humic Acrisols
Ap Plinthic Acrisols
Ag Gleyic Acrisols
Haplic Phaeozem s
Mo Orthic Greyzems
Mg Gleyic Greyzems
Oe Eutric Histosols
Od Dystric Histosols
Ox Gelic Histosols
Bk Calcic Cambisols
Bc Chromic Cambisols
By Vertic Cambisols
Bf Ferralic Cambisols
We Eutric Planosols
Wd Dystric Planosols
Wm Mollic Planosols
Wh Humic Planosols
Ws Solodic Planosols
Wx Gelic Planosols
Fo Orthic Ferralsols
Fx Xanthic Ferralsols
Fr Rhodic Ferralsols
Fh Humic Ferralsols
Fa Acric Ferralsols
Fp Plinthic Ferralsols
- 46 -
The main characteristics of these soil units, in terms of presence of
diagnostic horizons and other important agricultural properties, which are
included or implied in their definitions, are presented in Table 6.1. Complete
definitions of diagnostic horizons and properties of the soil units are given in
Volume I (Legend) of the Soil Map of the World. For ease of interpretation of
Table 6.1, these characteristics are briefly described below.
Histic H horizon:
Mollic A horizon:
Umbric A horizon:
Cambic B horizon:
Spodic B horizon:
Oxic B horizon:
Albic E horizon:
Aridic moisture regime:
or very high:
CEC very low:
surface layer of organic material more than 20 cm thick.
surface horizon with dark colour, medium to high humus
content, high base saturation.
surface horiz' n with dark colour, medium to high humus
content, low base saturation.
surface horizon with light colour, low humus content.
subsoil horizon with accumulation of illuvial clay.
subsoil horizon with accumulation of illuvial clay and
high exchangeable sodium.
subsoil horizon with a structure and/or colour different
from overlying and underlying horizons.
subsoil horizon with accumulation of iron and/or humus.
subsoil horizon with residual accumulation of sesquioxides
and low CEC.
horizon of accumulation of calcium carbonate.
horizon of accumulation of calcium sulphate.
horizon with strong acidity and prominent mottling.
eluvial horizon from which clay and free iron oxide have
been removed, light colour.
no available water in soil for most of the year.
calcium carbonate present at least between 20 and 50 cm
from the surface.
exchange complex dominated by allophane or montmorillonite.
exchange complex dominated by kaolinite (CEC less than
24 meq/100 g clay).
less than 1.5 meq/100 g clay.
formation of deep and wide cracks upon drying.
layer which is permanently frozen.
mottled subsoil layer which irreversibly hardens upon
exposure to repeated wetting and drying.
electrical conductivity (EC) higher than 15 mmhos/cm.
electrical conductivity (EC) between 4 and 15 mmhos/cm.
saturation with exchangeable sodium of more than 15 percent.
saturation with exchangeable sodium of 6 to 15 percent.
subsoil layer with very firm or hard consistence, but can
still be penetrated by spade or auger.
extremely hard continuous subsoil layer which cannot be
penetrated by spade or auger.
- 4t -
Abrupt textural change:
less than 18 percent clay and more than 65 percent sand.
more than 35 percent clay.
considerable increase in clay content within a very short
deep and irregular penetration of an albic E horizon into
an argillic B horizon.
6.3.2 The Mapping Units or Soil Associations
The mapping units are associations of soil units and are usually composed of
a dominant soil and of associated soils. Each of the latter occupies at least
20 percent of the area of a mapping unit; important soils which cover less than
20 percent of the area of a mapping unit are added as inclusions.
The soil associations have been registered on the map by the symbol of the
dominant soil unit followed by a figure which refers to the list of soil associa-
tions printed on the reverse of the map. This list of soil associations also
gives the full composition (associated soils and inclusions) of each association.
The textural class of the dominant soil and the slope class are also given
for each association. Three textural classes are differentiated and labelled, in
the mapping unit symbol, by the numerals 1, 2 or 3. The textural classes correspond
to the three divisions shown in the texture diagram below.
1. Coarse textured
2. Medium textured
3. Heavy (fine) textured
eVrN V V
4. 0 %E
Doo do 00 0 1?> o 0 10,
wu o NNNN o Co M H A Q R i p Q Q s-4C
0 BO o 0 a +a o 0 d 0 S 0g -) o H 9o 0 P- g (D 1s0 CD 1 o CD Soil units and phases
Histic H horizon
Mollic A horizon
Umbric A horizon
Ochric A horizon
Argillic B horizon
Natric B horizon
Cambic B horizon
Spodic B horizon
Oxic B horizon
Albic E horizon or
Aridic moisture regime
CEC high or very high
CEC low (4 24 meq)
X M M
M MM M
H M M M x
M 8 8 M M
- 49 -
4. 4. F
; 4 o
0 .0 0 C 0 4
S S _O o 0 ed
2 C 0 o & 0
. 8. I v o s
0 0 40 4. a)+
Ol\ Lf\ 4. M > +. a)
00 oo 0 >
A ( r r-4 r-4 P- 0 -H.
>. r- 0 4 m
a ~ (i-^^ d k g
0-' 0~-. C .
0P 0w- C 0 0 P C)
*ri ir\ .,- ir te p MA o o P. O
4.. *. 0* r- 4H 0 H X
(d u-, o I p a
0 0 13 r4
A 0 V 0 .A *
-P -P .o 0 > m a
Cd a nz a C c *t H
S- C- P 4 0a)
c 0 A" t
0 2 C B --
m W ) 0 0
v 0 0 EH M
d Q a ..
4p 0 0 0 C
n! -p o 0 -p .rj
'3 4. C) + d S
M 0 H. 1 E-4 *a) 6-'
4 0 0
*H r-4 0
4 +4 0
*- .r 4
If #p +p
M M m
- 50 -
TABLE 6.1 (Cont.)
DIAGNOSTIC HORIZONS AND PROPERTIES OF THE SOIL UNITS
0 0 0 0
N NS N N
-H *H -H 1-H
0 0 0 0
0 0 0 0
N 04 ,0 $0
o o oh.ee >
0 0 0 NH ;1F.
SC o o o .C e o o
0 o o .- o nb
o o o 0 o
0 0 0
-4 0 0 0 ,f
cd p- 0 N n s3
m ma N C) 6 0 H
otaOH to N oco
x or Y
x or x
- 51 -
C) 0 W)
-H 10 0
'H '4 C
4, *H m
4 +> Cd
0 0) 4
C H C)
a oo -
0 0 0
8 x 8 8 x X
xx x x
Sx x x x x x 8 x x
8 X X X x X
x X X x x x
x x X x
x x x xx
Soil units and phases
Histic H horizon
Mollic A horizon
Umbric A horizon
Ochric A horizon
Argillic B horizon
Natric B horizon
Cambic B horizon
Spodic B horizon
Oxic B horizon
Albic E horizon or
Aridic moisture regime
CEC high or very high
CEC low (4 24 meq)
X X X
8 MM X MM X X
S X8 MX X M XX
x x X x X
X x M
xM X ,
x xx xx
o P3 r- D
Pc c cl- d-- i- c" 0 P- D 0
0 F 0 0 0
M M H H MM M M
3i) so be M) 0 M oiyg -d) w. -' o ]i
Base saturation>50 percent
(or pH > 5.5)
Base saturation <50 percent
(or pH <5.5)
Cracking clay (vertic and
Depth: very shallow (4 10 cm)
shallow (10 to 50 cm)
Drainage: poor or very poor
imperfect to moderate
Cemented hardpan (within 100 cm'
Abrupt textural change
status J/ :) low
a Soil units and phases
a z !z
- 54 -
Three slope classes are distinguished. They indicate the slope which dominates
the area of the soil association and are labelled, in the mapping unit symbol, by the
letters a, b or c. The three slope classes are as follows:
a. level to gently undulating; dominant slopes ranging
between 0 and 8 percent,
b. rolling to hilly; dominant slopes ranging between
8 and 30 percent,
c. steeply dissected to mountainous; dominant slopes are
greater than 30 percent.
The textural and slope classes are limited to three because of the small
scale of the map. It must be realized that for management purposes of specific
areas, texture and slope classes should be defined more precisely.
An example of a mapping unit symbol is shown below:
Dominant soil (Lc) < > Textural class (3)
Figure representing the <>--- Slope class (a)
composition of the soil association
(5) given on the back of the map sheet.
Explanatory volumes, accompanying the Soil Map of the World sheets, provide
information on the extents of each association and data on the climate, vegetation,
lithology and the country of occurrence.
The Soil Map of the World is comprised of an estimated 5 000 different mapping
Phases indicate land characteristics which are not reflected by the
composition of the soil association but are significant to the use or management
of the land. The 12 phases recognized on the Soil Map of the World are: stony,
lithic, petric, petrocalcic, petrogypsic, petroferric, phreatic, fragipan, duripan,
saline, sodic and cerrado. The phases are shown on the map by overprints and their
definitions are given in Volume I of the Soil Map of the World.
6.3.3 Composition of Mapping Units
The list of soil associations shows that widely different soil units may
be present in any mapping unit. For instance, deep plateau soils, shallow soils
on steep land and hydromorphic alluvial soils (which are regularly associated in
a physiographic unit), have been included in a single mapping unit where it was
not possible to show them separately because of the small scale of the map. The
plateau soils may be suitable for upland annual crops, such as sorghum, but not for
- 55 -
rainfed paddy rice and, vice versa, the alluvial soils would probably be suitable
for paddy rice but not necessarily for sorghum. In turn the steep soils may be
suitable for perennial tree crops, pasture or forest but not for annual crops
because of erosion hazard. This simple example illustrates that the suitability
of a soil association for a specific crop cannot be assessed without taking account
of each individual soil unit in the association.
Although the extent (000 ha) of each soil association is given in the
respective explanatory volumes of the map sheets, the areas covered by each soil
unit within the soil associations are not available. For the project, such areas
had to be calculated on the basis of the number of soil units in each association,
as shown in Table 6.2
Table 6.2 RELATIVE DISTRIBUTION OF DOMINANT SOIL, ASSOCIATED SOIL(S) AND
INCUWSION(S) EXPRESSED IN PERCENTAGE OF THE AREA OF ME MAPPING UNITS
Dominant soil Associated soil(s) Inclusion(s)
Percentage of Number of Percentage Number of Percentage
area soil units of area soil units of area
100 0 0 0 0
70 1 30 0 0
60 1 30 1 10
60 2 20 + 20 0 0
50 2 20 + 20 1 10
30 3 20 + 20 + 20 1 10
50 1 30 2 10 + 10
40 1 30 .3 10 + 10 + 10
50 1 30 4 5 + 5 + 5 + 5
40 2 20 + 20 2 10 + 10
30 2 20 + 20 3 1l + 10 + 10
30 3 20 + 20 + 20 2 5 + 5
25 3 20 + 20 + 20 3 5 + 5 + 5
24 3 20 + 20 + 20 4 4 + 4 + 4 + 4
The associations dominated by Lithosols
were allocated a slightly different
Lithosol + 1 associated soil :- Y2 + Y2 distribution of the area
Lithosol + 2 associated soils: Y3 + Y3 + Y3 distribution of the area
- 56 -
6.4 COMPUTERIZATION OF DISTRIBUTION OF SOIL UNITS
The initial stage of the computer programme recorded the extent and
composition of each soil mapping unit according to the information available
in the explanatory texts of the Soil Map of the World. This data was sub-
sequently computer listed by countries and presented in the form of a 'turn
around' document for inclusion of areas of each mapping unit by major climatic
divisions and lengths of growing periods. Such area calculations were effected
by superimposing the climatic inventory (Chapter 5) on the Soil Map of the
World and calculating (2 mm grid count) the extent of each mapping unit in each
major climatic division and each length of growing period zone.
The second stage of the computer programme converted this basic data input
into the extents of individual soil units (by phases, slope and texture classes)
in each major climatic division and length of growing period zone. This conversion,
from areas of mapping units to areas of individual soil units, was achieved
by applying the mapping unit composition rules given in Table 6.2 and the following
rules on texture, slope, phase and major undifferentiated soil unit distribution.
i. The texture class description (i.e. 1, 2 or 3), when present in the
mapping unit symbol, applies to the dominant soil of the mapping unit.
Where two texture classes are indicated they each apply to 50 percent
of the dominant soil unit.
Dominant soils of mapping units where texture is not described, and
all associated and included soils, are considered as medium textured
(i.e. 2), except:
(a) Qc, Ql, Qf, Qa, Po, Pf, Ph, Pp, Pg, Fx, which are invariably
classified as coarse textures (i.e. 1).
(b) Vp, Zc, Zt, Bo, Bv, Lv, Fr, which are invariably classified
as fine textured (i.e. 3).
ii. The slope class description (i.e. a, b or c) when present in the mapping
unit symbol, applies to the dominant soil of the mapping unit. Where
two slope classes are indicated, they each apply to 50 percent of the
dominant soil unit.
Dominant soils of mapping units where slope is not described, and all
associated and included soils, are allocated the following slope classes:
Slope class a: J, G, 0, W, Z, S, V.
Slope classes a/b: P, Y, X, K, C, H, M, L, D, F, Q.
Slope class b: R, E, B, A, N.
Slope classes b/c: T, U, I.
iii. Where only major soil units are designated in the mapping units, e.g.
undifferentiated Acrisols (A), it is assumed that the whole mapping
unit consists of the first individual soil unit listed under the major
unit heading (Section 6.3.1), i.e. in the case of Aorisole, the applic-
able soil unit would be Orthic Acrisols (Ao). A similar rule has been
applied in the colouring of undifferentiated major soil units in the
FAO/Lnesco Soil Map of the World.
iv. The phase designation, when present in the mapping unit symbol, applies to
the dominant soil in the mapping unit, all associated soils and inclusions
being considered unaffected.
A summary of the distribution of individual soil units (combined for all
slopes, textures and phases) by major climatic divisions and lengths of growing
periods in Africa is presented in Table 6.3. A complete break down of the extent
of individual soil units, by countries, by slope texture and phases, by major
climatic divisions and by lengths of growing period zones is available at FAO.
This physical data base of the agro-ecological zones is the foundation of the
- 58 -
DISTRIBUTION OF SOIL UNITS (000 HA) BY LENGTHS OF GROWING PERIOD (ALL MAJOR CLIMATIC DIVISIONS)
SOIL UNIT SYh AlL
LENGTHS OF GROWING PERIOD CODING
EXTENSION / / 6 / C / D / E / F / G / H / I / J / K / L / / N / P / I / R / /
489 5 71 195 23 7
5 43 71 61 8
46488 859 3360 6768 10140 2744 691
11 3517 4370 7765 3419 2029 815
2387 13 283 567 164
320 459 581
949 382 88 86 62 8
87 153 65 18
29560 2254 3646 3625 4261 701 355 243 4
4299 3735 3776 1262 509 464 5 367 49
12702 52 721 1594 1866 1327 244
709 2161 1878 1056 889 2C5
269 53 10
16 13 2 3 172
20297 48 24 1731 3580 1918 1092 129 794 366
43 13 544 2191 2222 2120 816 1435 786 445
5025 532 134 292 1170 339 204
134 331 255 808 430 396
28445 184 444 3128 3893 2947 822 231 846 418
835 403 2601 3436 2989 1606 503 1164 1604 391
16082 1085 3725 1258 1238 14
4276 2776 1269 441
1193 14 99 288 243 12
4 70 62 401
7524 608 313 757 1803 598 159 44
3 303 1010 608 481 357 403 61 16
26219 13 218 2123 2514 565 5362 651 4
546 14 31 1535 2081 2478 1281 6227 576
6510 2 34 514 818 353 526 309
2 58 263 1010 315 786 1268 252
2914 3 76 150 493 59 21 95
479 2 11 265 134 109 316 511 190
5915 634 1806 137 6
62 743 2521 6
837 304 200
5756 838 834 1132 386 11
813 862 823 57
176594 33236 22507 23315 16842 562 64 2
23782 31016 16323 7750 384 190 573 48
23993 2376 694 8542 823 97
22 618 2734 7484 525 78
33994 293 1604 10974 3315 921 142 240 86
1640 3755 8825 981 613 489 40 48 28
75406 38666 3532 7278 9084 106 33 36
3983 5538 5656 1386 67 21 18 2
33981 4788 1622 2858 5830 1104 1633 12 72
1718 3278 3068 3321 1969 1951 268 469
2651 3 1 37 211 125 62 587 4
1 5 118 119 101 1245 32
38083 18087 2673 4462 1912 634
2798 4874 1307 891 445
26481 312 37 711 8521 2916 1067 198 350
44 132 1336 5044 1092 1458 495 2768
17448 6763 2452 698 1058 688 54 36
2680 549 1407 389 332 334 8
3787 5 1104 71 86 462 4
5 1935 61 22 32
7992 2142 133 640 1822 522 68
33 253 1153 495 369 51 311
87 20 27
397667 1413 2452 10434 28164 14378 6933 191289 4728 1609
3201 2355 7638 12173 20972 10066 44292 28351 5135 2084
o 9330 772 299 612 995 21 12 3327 64
56 311 340 771 520 27 856 347
43474 7 2 41 610 801 1365 24956 601
64 2 10 562 511 1612 9849 1927 554
9952 2264 674 1302 1112 86 5
3 816 1895 1184 541 70
34944 22 177 2055 6215 3345 1015 1581 321
5 117 889 2503 7288 2570 3461 1407 1973
3690 77 498 241 792 92 29 159
506 264 239 540 18 235
62 11 1
37 5 8
287 6 82 46
48 88 17
1642 250 218 2
112 505 515 18 22
811 64 206 46
46 199 145 88 17
5398 176 1155 802 12 101 139
252 172 189 1578 93 38 226 289 176
56752 58 2262 7573 3602 531 2819 2745 2686
875 58 1394 3390 8250 1987 1084 9031 5280 3127
138535 84 1574 10812 35114 19985 2297 124 753 872
289 6251 13584 27576 4545 9268 595 4017 791
25560 70 1567 9396 2458 72 24
46 455 1882 8843 341 273 133
3760 26 61 8 89 602 42
44 2 689 1732 367 98
- 59 -
Table 6.3 (Continued) DISTRIBUTION OF SOIL UNITS (000 HA) BY LENGTHS OF GROWING PERIOD (ALL MAJOR CLIMATIC DIVISIONS)
LENGTHS OF GROWING PERIOD CODING
SOIL UNIT SYMBOL EXTENSION / A / 8 / C / D / E / F / G / F / I / J / K / L / N / P / / / S /
S8035 442 2700 437 200 82 46
O 8035 10 1023 715 558 162 381 138 721
LP 18077 204 2971 6195 714 8 68
LP 140 52 563 3455 3451 21 215 20
LV 302 62 189 51
N 3284 75 4 241 1766
4 146 663 385
ND 56822 10588 5765 6349 7264 541 4
6163 9800 5348 4871 129
NE 30937 270 1675 4126 5316 1310 780 720
755 1752 4693 3797 2118 1018 2293 15 245 54
NH 7492 1672 1011 1195 325 134 16
551 1558 734 184 112
C 1095 26 45 222 132 155 13
23 108 181 113 64 1 12
00 6465 2982 318 227 818 332 23
325 188 548 355 18 108 223
OE 4715 92 1916 14 55 17
457 2039 28 97
p 6694 434 255 1762 216 255
435 1104 294 892 13 1034
PG 521 363 150
PH 3981 194 260 1146 815
97 25 4 1101 339
pp 145 2 32 21
C 6979 407 680 216 214 569 196
668 1498 92 815 352 1272
CA 26812 336 1086 748 1143 356 7 3390 1944 65
922 2330 807 687 12 84 7988 4854 53
QC 141513 2996 464 3443 6752 6977 9466 12365 4869 1111
22 464 758 3716 5078 7655 39696 24118 11084 479
QF 74509 9638 7094 8451 10103 2596 331 32
7058 15700 8475 2877 730 245 153 1026
8L 79502 48 1250 1729 10636 10266 705
112 1431 2164 2131 11251 33151 2936 1692
R 25011 868 1311 2767 253 426 5003 885
37 868 3288 1534 744 202 1113 4195 1517
RC 43924 40 7 239 182 700 702 22899 533
184 6 398 195 938 14170 2731
RO 24207 1058 1223 3355 4963 1225 488 14
18 859 2538 3574 2387 569 1936
RE 171565 60 51 1101 4093 10712 5137 74571 162
239 15 555 1321 8385 5993 58531 564 75
S 1389 37 2 43 52 42
2 445 454 88 224
SG 2462 433 469 54 295
387 206 97 6 515
SM 589 24 348
SO 13647 78 335 572 435 769 1142 312
24 153 1687 926 3398 2197 1291 328
T 116 40 15 16
22 14 9
TH 1254 97 118 186 157 78 10
57 118 194 116 123
TM 1687 335 120 83 268 45 101
75 154 90 213 134 69
TO 1909 2 28 82 168 718 45
66 42 12 27 128 291 206 94
IV 458 49 43 4 48 12
6 58 10 4 96 99 29
U 861 34 24 150 128 29
29 24 158 150 135
V 15024 2 161 1914 774 74 1322 1139
1 71 606 1700 204 425 3993 2638
VC 47641 3 8 806 6143 3950 3343 225 504 905
268 940 12851 3898 9454 625 284 3434
VP 42302 108 296 2135 5864 6417 1950 4 493
487 296 1358 2485 9285 3207 6164 1070 683
W 484 76 94
36 274 4
kO 9 4
ME 8227 17 201 1202 441 150 300 466
9 80 544 693 200 131 1426 913 1454
M 32 6
65 7135 7 28 152 1236 614 101 160
8 55 29 749 1719 684 1182 105 306
X 12802 42 193 463 244 1638 771
724 82 426 549 1806 5442 422
XH 26789 366 1513 2472 899 331
1055 1783 16046 1547 777
XK 55539 206 1229 1499 18374 2562 10
29 22 556 2714 7023 18116 3199
XL 2310 63 105 225 83 8
8 454 1308 56
XY 3468 56 62 1786 74
28 50 63 1231 118
Y 214636 43 236 191720 392
178 170 19806 2091
YH 26919 32 40 1020 14722 63
110 939 9680 313
- '60 -
Table 6.3 (Continued) DISTRIBUTION OF SO0 UNITS (000 HA) BY LENGTHS OF GOVWING PERIOD (ALL MAJOR CLIMATIC DIVISIONS)
- --------- ----------- ------------
LENGTHS OF GROWING PERIOD CODING
SOIL UNIT SYMBOL EXTENSION / A / 8 / C / D / E / F / / / h / I / J / K / L / P / N / P / C / R/ S /
YK 79240 20 58 65329 1387
60 28 401 5044 6913
YL 2349 805 12
YT 2313 57
YY 48317 21 63 39991 385
28 4 147 5666 2004 8
z 19727 3 55 39 337 363 228 7108 910
62 56 98 191 551 281 3084 4294 2067
ZG 7531 59 139 398 392 473 637 1004 194
144 278 403 425 701 1395 870 19
ZM 298 46 38 60 4 24 9
38 23 4 52
ZO 16694 12 172 755 450 5861 485 15
8 68 234 354 6926 1173 173 8
ZT 7075 3 2 6C41 30
7 3 646 343
IA 171410 135818
IC 3471 3013 4
10 29884 28153
GRAND TOTALS 3011330 149615 76908 152091 255801 126902 62878 874958 36117 9814
9759 77876 139681 154825 190363 86662 376258 153454 63068 14300
1. THE SOIL UNITS ARE LISTED IN ALPHABETICAL RODER FOR EASE OF REFERENCE AND ARE DESIGNATED BY CAPITAL LETTERS
CORRESPONDING TO THE ABBREVIATIONS OF THE LIST OF SOIL UNITS IN SECTION 6.3.1.
2. THE LENGTHS OF GROWING PERIOD CODINGS REFER TO THE FOLLOWING LENGTHS OF GROWING PERIODS:
NORMAL GROWING PERIODS INTERMEDIATE GROWING PERIODS COLD AREAS (NO GROWING PERIOD)
A 365 N 1- 74
B 330-364 P 75- 89
C 300-329 0 90-119
D 270-299 R 120-149
E 240-269 S 150-179
1 75- 89
L 1- 74
- 61 -
MATCHING: NET BIOMASS PRODUCTION AND YIELD OF CROPS
This chapter deals with the calculation of net biomass production and yield
of crops as determined by climatic factors and crop adaptability characteristics.
The derivation of the equations for calculating biomass and yield is presented
together with the biomass and yield values calculated for the selected crops.
The net biomass production of yield of. crops is defined here as the total
plant dry matter and the dry matter of the economically useful portion respectively,
that can be produced by healthy crops when adequately supplied with water and
nutrients. The time period during which climate will permit rainfed crop production
is defined as the growing period, and for the major climatic divisions in Africa
growing periods are given in Chapter 5.
The procedure set out here, for calculating the net biomass production and
yield of crops, uses the information on the climatic factors of radiation and
temperatures within the growing periods together with the actual photosynthesis
capacity of crops and the fraction of the net biomass which crops can convert into
economically useful yield. When the climatic phenological requirements are met, the
computed values of net biomass production and yield of crops indicate what is actually
possible at the upper limit of crop performance, when agronomic constraints from
growth and yield reducing factors, including climatic (e.g. water stress, temperature),
edaphic (e.g. low soil fertility, salinity) and biotic factors (e.g. pests, diseases,
weeds) are minimal.
To the extent that the upper agronomic limit of crop performance can be
computed for a given area, the estimated values also reflect the present agronomic
potential of the constraint-free climatic resources for crop production for the
area and crop genetic resources under consideration. However, responses of
physiological processes (e.g. photosynthesis, respiration) of a crop to climatic
factors are a function of its genetic make-up, or the adaptability which genes
impart to the genotype. Although the upper limit of crop performance is set by the
climatic characteristics of radiation and temperature, the extent to which it can
be reached is determined by the quality of the genetic resources of the crop in
The agronomic potential referred to above is a level of crop performance
which is attainable when location specific constraints are minimal. These conditions
often prevail at experimental research stations where crop yields measured from spenif-
ically designed maximum yield trials are often in the range predicted by the method
set out here. For example, the actual yields of maize in the different zones in
West Africa, from uniform trials with optimum rates of fertilizer application, are
related to the predicted upper agronomic limits of maize yields in the respective
zones. Further, it has been shown that although the present concentration of maize
production is in the Forest areas, actual yields are greater in the Savanna areas,
with actual maximum yields and predicted maximum yields being obtained in the
Northern Guinea Savanna.
- 62 -
7.2 BIOMASS PRODUCTION
7.2.1 Gross and Net Biomass Production
To calculate the net biomass production (Bn) of a crop, an estimation of the
gross biomass production (Bg) and respiration loss (R) is required (equation 1)
B B -R (1)
The equation relating the rate of net biomass production (bn) to the rate of gross
biomass production (bg) and respiration rate (r) is
b = b r (2)
The maximum rate of net biomass production (bnm) is reached when the crop
fully covers the ground surface. The cumulative growth curve has a signoid shape
so that bnm is the point of inflection on the growth curve (Fig. 7.1), and is equal
to the first derivative of the net growth occurring during the period of maximum growth.
If the first derivative of growth (i.e. growth per unit time) is plotted against time,
the resulting curve has the shape of a normal distribution curve (Fig. 7.2). The model
used here assumes that the seasonal average rate of net biomass production (bna) is
half the maximum crop growth rate, i.e. 0.5 bnm. The net biomass production for a
crop of N days (Bn) is then
Bn 0.5 bnm x N (3)
Therefore, if bnm can be calculated, Bn can be computed from equation (3) using the
appropriate value of N. To calculate bum we need to know the maximum rate of gross
biomass production (bgm) and the respiration rate at that time (rm) (equation 2).
7.2.2 Maximum Rate of Gross Biomass Production (b )
To calculate bgm of a closed crop, the starting point is the photosynthesis
function of single leaves (Fig. 3.2). The function describes the relationship
between the photosynthetically active radiation in cal cm-2 min-1 and the CO2
exchange rate in mg dm-2 leaf1surface h-1, or the production of carbohydrates in
kg CH20 ha-1 leaf surface h The equation expressing the leaf photosynthesis
P = Ar (Ar + Pm x E)-1 P (4)
where P1 = the net rate of CO2 exchange of leaves (kg ha- h ),
A' the photosynthetically active radiation (cal cm-2 min- ),
Pm = the rate of 002 exchange at light saturation, or the
maximum net rate of CO2 exchange of leaves (kg ha h- ),
E = the efficiency of photosynthesis defined as Pm/A where
-1 1 -2 -1
A is the radiation at which P1 0.5 Pm(kg ha- h- /cal cm-2min-).
The value of E has been found to be relatively constant between species while
Pm varies considerably (Table 7.1, Fig 3.2).
- 63 -
B = Bn
B m Slope at the point of
Inflection is equal to
-------------dB/dt = brim
Typical cumulative crop growth curve showing the point of
inflection during the period of maximum growth when the
slope dB/dt is equivalent to the maximum rate of net biomass
production (b ).
- 64 -
bna = 0.5 bnm
The normal shape of the curve of crop growth rate plotted
against time showing average crop growth rate (b na) = 0.5 bm
AVERAGE PHOTOSYNTHESIS RESPONSE OF FOUR GROUPS OF CROPS TO RADIATION
Characteristics Crop adaptability group 1/
I II III IV
Photosynthesis C C C C
pathway 3 3 4 4
Rate of photosynthesis
at light saturation at: 20 30 40 50 70 100 70 100
(mg CO2 dm-2 h-1)
(O0) for maximum 15 20 25 30 30 35 20 30
Radiation intensity at
(cal cm-2min-1) 0.2 0.6 0.3 0.8 > 1.0 >1.0
Major crops of the study Wheat Phaseolus bean. Pearl millet Sorghum (temperate and
Potato (tropical cultivars Sorghum tropical high altitude
Phaseolus bean Soybean (tropical cultivars) cultivars)
(temperate and trop- Rice Maize Maize (temperate and
ical high altitude Gotton (tropical cultivars) tropical high altitude
cultivars) Sweet potato Sugarcane cultivars)
1_/ For further information on crop adaptability groups, see Tables 3.1 to 3.4 in Chapter 3
- 66 -
Under field conditions, the maximum net rate of CO exchange or the rate of
gross biomass production (bgm) of a closed crop is dependent on how the radiation is
distributed within the canopy. This is determined by the arrangement of leaves in
space and on the transmission and reflection properties of leaves. The distribution
of radiation within a crop can be calculated by estimating the radiation intercepted
in every layer of leaves for a given leaf distribution function. In general, as the
radiation penetrates the canopy, its intensity decreases exponentially with the
increase in leaf area.
The amount of photosynthetically active radiation (PAR) on a perfectly clear
day (A ) at different latitudes has been given by de Wit, and the values at the
equator, 10N, 200N, 30ON and 40N are shown in Table 7.2. PAR on a totally overcast
day is 20 percent of Ac, and PAR is 50 percent of the total short-wave global radiation
(R ). Therefore, the fraction of the day-time when the sky is overcast (F) is then:
F = (Ac 0.5 Rg) / 0.8 Ac (5)
The maximum rate of gross biomass production by a crop, characterized by a set
of standard variables at leaf area index (LAI) .1/ of 5, can be calculated for perfectly
clear days as well as for totally overcast days, (be and bo respectively). The
actual rate of maximum gross biomass production (bgm) is then:
b = F x b + (1 F) b (6)
gm o a
Therefore, if the total short-wave global radiation (R ) is known, the F factor can
be calculated from equation (5) and bgm from equation 16).
Values of b and bo for different latitudes are given by de Wit for
Pm = 20 kg ha-1 h- and the values at the equator, 100N, 200N, 30ON and 40oN are
shown in Table 7.2. However, the maximum rate of C02 exchange (Pm) is dependent on
both temperature and phosotynthetic pathway of species (Table 7.1, Fig. 3.1). It
can be shown that 'y' percent increase in Pm relative to Pm of 20 kg ha-lh-1 leads
to (y x 0.2) percent and (y x 0.5) percent increase in bo and be respectively;
inversely 'y' percent decrease in P relative to Pm of 20 kg ha-1 h-1 leads to
(y x 2.5) percent and (y x 1) percent decrease in bo and be respectively. Hence, bgm
as calculated from equation (6) must be increased by
xF xb+ x (1 -F) x b (7)
10 -1 -10
for values of P greater than 20 kg ha- h and decreased by
Sx F xb + -x (1 -F) x b (8)
100 100 c
for values of P smaller than 20 kg ha h .
It must be kept in mind that at a given Pm value the magnitude of b m is
determined by LAI (Fig. 7.3). The effect of LAI on bgn is small when LAI is larger
than 5 and a near complete ground cover or light interception is achieved by crop
canopies in the field. Fig. 7.3 was derived from Fig. 15 in de Wit on the assumption
that the effect of variation in leaf age with depth in a crop canopy on the canopy
photosynthesis rate is negligible or absent.
LAI is the area of green leaves per unit area of ground surface.
Table 7.2 THE PHOTOSYNTHETICALLY ACTIVE RADIATION ON VERY CLEAR DAYS (Ac) IN CAL CM-2DAY
AND THE DAILY GROSS PHOTOSYNTHESIS RATE OF CROP CANOPIES ON VERY CLEAR (be) AND
OVERCAST (bo) DAYS 3IN W HA-1DAY-1 for Pm 20 KG CH 20 HA-1H-1, (FROM IE WIT)
North 15 15 15 15 15 15 15 15 15 15 15 15
Lat. Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
0 A 343 360 369 364 349 337 342 357 368 365 349 337
bc 413 424 429 426 417 410 413 422 429 427 418- 410
b 219 226 230 228 221 216 218 225 230 228 222 216
100 Ao 299 332 359 375 377 374 375 377 369 345 311 291
bc 376 401 422 437 440 440 440 439 431 411 385 370
bo 197 212 225 234 236 235 236 235 230 218 203 193
20 Ac 249 293 337 375 394 400 399 386 357 313 264 238
bc 334 371 407 439 460 468 465 451 425 387 348 325
bo 170 193 215 235 246 250 249 242 226 203 178 164
300 Ae 191 245 303 363 400 417 411 384 333 270 210 179
bc 281 333 385 437 471 489 483 456 412 356 299 269
bo 137 168 200 232 251 261 258 243 216 182 148 130
400 Ac 131 190 260 339 396 422 413 369 298 220 151 118
bc 218 283 353 427 480 506 497 455 390 314 241 204
bo 99 137 178 223 253 268 263 239 200 155 112 91
- 68 -
1 2 3 4 5
Relationship between leaf area index (LAI) and maximum growth
rate as a ratio of the maximum growth rate at LAI of 5
- 69 -
7.2.3 Respiration Rate of the Time of Maximum Rate of Gross Biomass Production (r )
To calculate the maximum rate of net biomass production (bnm) from bgm, a
measure of the rate of respiration (rm) is required. Unlike photosynthesis which
occurs mainly in the leaves, and only during the day, respiration proceeds through-
out the plant, day and night. The constituents of the plant are constantly turning
over and hence the plant has to pay a cost in terms of respiration to support this
and the maintenance of the cellular organization. This fraction has been termed the
maintenance respiration. The remaining intermediates and energy can be used to
synthesize additional material i.e. produce new growth; and this cost of producing
new growth is called growth respiration.
It has been shown by McCree that the growth respiration is a linear function
of the rate of gross biomass production, and the maintenance respiration is a linear
function of the net biomass that has already been accumulated (B). When the rate of
gross biomass production is bgm, the respiration rate (rm) is
r = kb +cB kg ha day-1 (9)
where k and c are the proportionality constants for growth respiration and maintenance
respiration respectively, and Bm is the net biomass at the time of maximum rate of
net biomass production (i.e. at the point of inflection on the growth curve, Fig% 7.1).
In the study of McCree, k for a legume crop and a non-legume crop was 0.28, and was
independent of temperature. However, c was found to be dependent on both species
and temperature. At 30 C, c for a legume crop was 0.0283, and for a non-legume crop
it was 0.0108. The temperature dependence of c for both species was
ct = c30 (0.044 + 0.0019 T + 0.0010 T2) (10)
The difference in maintenance respiration between legume and non-legume species arises
because the exact value depends on the chemical composition of the biomass, particul-
arly the rate of turnover of protein. In other words, it is costlier in terms of
energy to synthesis and maintain biomass richer in protein.
If the net biomass Bm is known, rm can be calculated from equation (9). As
pointed out earlier, the rate of maximum gross biomass production (bgm) calculated
from equation (6) is the rate at the time of the point of inflection on the growth
curve plotted against time (Figs. 7.1 and 7.2). At this point in time the cumulative
net biomass (i.e. B,) of the crop is assumed, in this model, to be equal to half the
net biomass that would be accumulated at the end of the crop's life. Therefore,
B Bm 0.5 Bn; and from equation (3), Bm for a crop of N days is:
B =0.25b x N (11)
7.2.4 Maximum Rate of Net Biomass Production (b ]
By combining the respiration equation (9) with the de Wit model of the rate
a gross photosynthesis (equation 6), it is possible to compute the maximum rate of net
biomass production (bnm) or the rate of dry matter production at full cover. The
resulting equation for a crop of N days is
bnm 0.72 b / (1 + 0.25 ctN)
- 70 -
7.2.5 Net Biomass Production (B,)
Net biomass production for a crop of N days is then given by equation (3),
where 0.5 bnm is the seasonal average rate of net biomass production. The resulting
equation for net biomass production,when LAI at the time of maximum gross biomass
production rate is 5,is therefore
Bn = 0.36 b gm/ (1/ N + 0.25 ct' (13)
When LAI at the time of maximum gross biomass production rate is less than 5, b
will be smaller and can be calculated by multiplying the bgm at LAI of 5 with
the appropriate factor obtained from Fig. 7.3.
7.3 CROP YIELD
Yield biomass (B ) of a crop can be 'calculated from equation
B = B x H. (14)
y n 1
where H. is the harvest index, defined as the fraction of the net biomass of
the crop that is economically useful(e.g. grain in cereals, sugar in sugarcane,
lint in cotton), and H. = B / B For a crop, Hi value depends on a number of
factors including the genetic potential of the cultivar (high or low yielding),
water regime (i.e. rainfed or irrigated), cultural practice. However, in order
to calculate the upper agronomic limit of crop performance under conditions of
minimal agronomic constraints, H. values appropriate to high yielding cultivars must
be used. Under such conditions +he range in H. of each crop is relatively narrow
The following is an example of how to calculate net biomass and yield of a
crop in a given location.
i. Information on climate
Location : 110 11' N
Altitude : 635 m
Growing period : 187 days
Start of growing period : 1 May
End of growing period : 3 November
Average radiation over the growing period: 452 cal cm-2day-1
Average day-time temperature over the growing period: 26.90 C
Average 24hr-mean temperature over the growing period: 25.3 0.
ii. Information on crop
Crop : Maize
Days to maturity : 120 days
Leaf area index (LAI) at the inflection point on the growth curve or
at the time of maximum growth rate : 5
Harvest index : 0.4
Crop adaptability group : III (see Table 3.3)
- 71 -
iii. Calculation of rate of gross biomass production (n)
Photosynthesis rate (Pm) at 26.9C: 65 kg h-1 (from Fig. 7.4)
Percentage difference in Pm relative to Pm = 20 kg ha-1 h- 225 percent
Average amount of the photosynthetically active radiation on clear
days (Ac) over the growing period: 361 cal cm-2 day-1 (from Table 7.2)
Fraction of the day-time when the sky is overcast (F): 0.46
(from equation 5)
Average rate of the gross biomass production for perfectly clear days
at Pm = 20 kg ha-lh-1 (bc) over the growing period: 427 kg ha-lh-1
(from Table 7.2)
Average rate of the gross biomass production for totally over ast ays
at Pm=20 kg ha-1h-1 (bo) over the growing period: 228 kg ha- day
(from Table 7.2)
Rate of gross biomass production at Pm = 20 kg ha-" h at LAI of 5:
336 kg ha-1day"1 (from equation 6)
Rate of gross biomass production at Pm = 65 kg ha-lh-1 at LAI of 5 (bgm):
643 kg ha-1day-1 (from equation 6 and 7).
iv. Calculation of total net biomass production (Bn) and yield (By)
Maintenance respiration coefficient at 30 C: 0.0108 (for non-legume crop)
Maintenance respiration coefficient at 25.30C (ct): 0.0079 (from
Bn = 22.4 t/ha (from equation 13)
B = 9.0 t/ha (from equation 14).
In the above example, the assumed LAI at the point of maximum crop growth rate
was 5, and the assumed harvest index was 0.4. Had the assumed LAI been 4 and harvest
index 0.35, B at LAI 4 would have been 0.91 of the Bn at LAI 5 (i.e. 20.4 t/ha) and
By 7.1 t/ha. The factor 0.91 is obtained from Fig. 7.3.
The methodology described above aims at quantifying the constraint free
performance of individual crops during the growing period from generalized photo-
synthesis responses of different groups of crops to average climatic factors of
radiation (i.e. average daily radiation) and temperature (i.e. average day-time
temperature), and generalized response of respiration to temperature (i.e. average
24h-mean temperature). The predicted values therefore can be different depending on
the extent to which the generalized responses and average radiation and temperature
values deviate from actual. However, it should be noted that the method has been
developed to suit the present aim for a global assessment of potential land use on
a continental basis, but it can be applied to the degree of detail required to suit
specific site and crop requirements.
7.4 AGRO-CLIMATIC CONSTRAINTS
The methodology for calculating net biomass and yield of crops provides
quantification of yields which can be anticipated under conditions that are essentially
free from constraints (agro-climatic and soil) within the growing period.
To arrive at an agro-climatic suitability assessment, yield losses likely to
occur due to the agro-climatic constraints must be deducted from the constraint free
yields. These yield losses are described in the following section, and their
application is presented in Chapter 9.
- 72 -
HARVEST INDEX (H.) OF HIGH YIELDING CULTIVARS OF FIELD CROPS
UNIER RAINFED CONDITIONS I./
Urop Urop Product Hi
group Range Average
Wheat Grain 0.35 0.45 / 0.40
White potato Tuber 0.55 0.65 0.60
I Phaseolus bean (temperate
and tropical high altitude
cultivars) J/ Grain 0.25 0.35 0.30
Phaseolus bean tropicall
cultiva.'-) 6/ Grain 0.25 0.35 0.30
Soybean Grain 0.30 0.40 0.35
II Rice I/ Grain 0.25 0.35 .1/ 0.30
Cotton Lint 8/, 0.06 0.10 0.07
Sweet potato Tuber 2/ 0.50 0.60 0.55
Uassava Tuber 10/ 0.50 0.60 0.55
Pearl millet Grain 0.20 0.30 0.25
Sorghum (tropical cultivars) Grain 0.20 0.30 / 0.25
Maize (tropical cultivars) Grain 0.30 0.40 0.35
Sugarcane Sugar L_/ 0.20 0.30 0.25
Sorghum (temperate and tropical
high altitude cultivars) Grain 0.20 0.30 0.25
Maize (temperate and tropical
high altitude cultivars) Grain 0.30 0.40 0.35
1/ For low yielding cultivars H. is lower and yields of such cultivars may be calculated
using appropriate H. values.
2/ Refers to bred and durum wheat.
/ Higher H. under controlled irrigation.
4/ Fresh tuber at 32.5 percent dry weight.
/ Refers to P. vulgaris and P. lunatus cultivars as a group.
6/ Refers to P. vulgaris, P. lunatus, P. aureus, P. radiatus, P. mungo and P_ angularis
cultivars as a group.
j/ Refers to rainfed/flood water lowland paddy rice under incomplete control of water supply.
8/ Lint at ginning percentage of 35.
2/ Fresh tuber at 30 percent dry weight.
10/ Fresh tuber at 35 percent dry weight.
11/ Sugar at 10 12 percent of fresh cane,.
- 73 -
Yield losses in a rainfed crop due to agro-climatic constraints can be
considered to be governed by the following conditions.
i. How well the length of the normal growth cycle (from sowing to physiological
maturity) of the crop in question fits into the available length of the
When the growing period is shorter than the growth cycle of the crop,
there is loss in yield. This quantitative loss can be taken into account
using equations 13 and 14 provided appropriate adjustments are made for both
LAI and harvest index (Table 7.4). However, the loss in the marketable value
of the produce due to poor quality of the yield as influenced by incomplete
yield formation (e.g. incomplete grain filling in grain crops leading to
shrivelled grains or yield of a lower grade, incomplete bulking in root and
tuber crops leading to a poor grade of ware), cannot be directly taken into
account through equations 13 and 14. This loss must be considered as an
agro-climatic constraint in addition to the quantitative yield loss due to
curtailment of yield formation period.
Yield losses can also occur when the length of the growing period
is much longer than the length of the growth cycle. These losses operate
through yield and quality reducing effects of a) pests, diseases and weeds
(see condition iii below), b) climatic factors affecting yield components and
yield formation (see condition iv), and c) climatic conditions affecting the
efficiency with which the necessary farming operations can be conducted
(see condition v). With some crops, part of these losses can be reduced by
using long duration cultivars (e.g. photoperiodic sorghum and millet) but
these cultivars often create other agronomic problems with the result that
the final suitability of different growing periods to such crops remains
ii. The degree of water stress during the growing period
This can affect crop growth, yield formation and quality of produce.
The yield reducing effects of water stress vary from crop to crop both
within and between growing periods considered. The total effect of water
stress can be considered in terms of a) the effect on growth of the whole
crop and b) the effect on yield formation and quality of produce. The
latter effect in some crops can be more severe than the former, particularly
in crops where the yield is a reproductive part (e.g. in grain crops) and
yield formation depends on the sensitivity of floral parts and fruit set to
water stress (e.g. silk drying in maize).
iii. The yield and quality reducing factors of pests, diseases and weeds
These comprise factors that operate indirectly through climatic
conditions, and vary from crop to crop both within and between growing
periods considered. To assess the agro-climatic constraints of the pest,
disease and weed complex, it is convenient to separate the effects on
yield that operate through loss in crop growth potential (e.g. pests and
diseases affecting vegetative parts in grain crops) from the effects on
yield that operate directly on yield formation and produce quality (e.g.
cotton strainer affecting lint quality, grain mould in sorghum affecting both
yield and grain quality, quelea attack on sorghum and millet).
- 74 -
iv. The climatic factors, operating directly or indirectly, that reduce yield and
quality of produce mainly through their effects on yield components and their
These include problems of poor seed set and/or maturity in sorghum and
maize under cool or low temperature conditions, problems of seed germination
in the panicle due to wet conditions at the end of grain filling in wheat,
problems of poor quality lint due to wet conditions during the time of boll
opening period in cotton, problems of poor seed set in wet conditions at the
time of flowering in some grain crops, and problems of excessive vegetative
growth and poor harvest index due to high night-time temperature or low
diurnal range in temperature.
V. The climatic factors which affect the efficiency of farming operations and
the cost of production
These operations include those related to land preparation, sowing
cultivation and crop protection during crop growth, and harvesting (including
operations related to handling the produce during harvest and the effective-
ness of being able to dry the produce). These constraints are essentially
workability constraints which have their origin in the prevailing climatic
conditions. The workability constraints vary from crop to crop both within
and between growing periods considered (e.g. operational problems of mechanical
sowing and harvesting under wet conditions, problems of handling wet produce,
problems of effectively applying biocides to crops under wet conditions).
These agro-climatic constraints of workability can cause direct losses in
yield and quality of produce, and/or impart a degree of relative unsuitability
to an area for a given crop from the point of view of how effectively
operations related to cultural practices and produce handling can be conducted
at a given level of inputs.
All the above agro-climatic constraints, causing direct or indirect losses
in yield and quality of produce, can be rearranged into a set of four, as follows:
a. yield losses due to water stress constraints on crop growth potential;
b. yield losses due to the effects of pest, disease and weed constraints
on crop growth potential;
c. yield losses due to water stress, pests and diseases, and climatic
constraints on yield components, yield formation and quality of produce;
d. yield losses due to workability constraints.
The above mentioned agro-climatic constraints are complex and dynamic and
their interrelations make it extremely difficult to assess quantitatively their
role and effect.
- 75 -
7.5 NET BIOMASS AND YIELD OF CROPS FOR AFRICA
7.5.1 Groups II and III Crops in Warm Tropics and Warm Subtropics (summer rainfall)
(major climates 1 and 4 respectively).
Crop characteristics considered in the net biomass and yield calculation
for Groups II and III crops (see Table 7.1) for Africa are presented in Table 7.4.
The net biomass and yield of the crops for Africa are presented in Table 7.5.
In both tables, values related to crop characteristics and of biomass and
yield apply to the respective lengths of growing period under which they appear in
the tables. For a growing period interval as a whole, the values continue to apply
as stated until the largest growing period value in the interval is shorter than or
equal to the longest growth cycle or days to maturity considered. For any growing
period interval, the lower yield figure refers to the shortest growth cycle length
(or days to harvest) considered in the interval.
The tropical and subtropical (summer rainfall) areas considered for Groups
II and III crops are those where the average mean temperature for the growing period
is greater than 20.0C (i.e. major climates 1 and 4 respectively; see Chapter 5).
In the tropical areas in general, this 20.0C mean temperature value corresponds
to an altitude of about 1 500 m so that the area as a whole lies within the 0 1 500 m
altitude range. There is an increase in the mean temperature with a decrease in
altitude but the effect of latitude on the altitude at which the mean temperature
becomes less than 20.0C, is small.
In the subtropical (summer rainfall).areas in southern Africa, at a given
altitude, latitude has a significant effect on the mean temperature during the growing
period. Although the area as a whole lies within 0 1 500 m altitude range, the
actual altitude at which mean temperatures become less than 20.000 for a particular
location is dependent on its latitudinal position.
7.5.2 Groups I and IV Crops in Cool Tropics, Cool Subtropics (summer rainfall), and
Cool Subtropics (winter rainfall) (major climates 2. 5, and 7 respectively).
Crop characteristics considered in the net biomass and yield calculations for
Groups I and IV crops (see Table 7.1) for Africa are presented in Table 7.6 (in back
pocket). The net biomass and yield of the crops for Africa are presented in Table 7.7
(in back pocket). In both tables, values related to crop characteristics and of
biomass and yield should be read as explained in Section 7.5.1 above.
The values for winter wheat apply only to the subtropical (winter rainfall)
areas (i.e. major climate 7).
The values for the remaining crops apply only to the tropical and subtropical
(summer rainfall) areas where the mean temperature during the growing period is less
than 20.00C (i.e. major climates 2 and 5 respectively).
In the tropics the area lies above 1 500 m altitude. In the subtropical
(summer rainfall) areas in southern Africa the altitude at which mean temperatures
are below 20.000 may range from sea level and above, to 1 500 m and above depending
on the latitudinal position of the location.
CROP CHARACTERISTICS CONSIDERED IN THE POTENTIAL NET BIOMASS AND YIELD
CALCULATIONS FOR GROUPS II AND III CROPS
Growing period (days)
75 89 90 119 120 149 150 79 180 209 210 239 240 269 270 299 300 329 330 364 365
70 89 70 90 70 90 70 90 70 90 70 90 70 90 70 90 70 90 70 90 70 90
23 29 23 30 23 30 23 30 23 30 23 30 23 30 23 30 23 30 23 30 23 30
3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0 3.1 4.0
0.25-0.'25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25
90 90 119 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 12C
16 29 30 40 30 40 30 40 30 40 30 40 30 40 30 40 30 40 30 40 30 40
2.5 3.0 3.0 4.0 3.0 4.0 3.0 4.0 3-0 4.0 3-0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.C
0.12-0.24 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25 0.25-0.25
90 90 119 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 120
16 29 30 40 30 40 30 40 30 40 30 40 30 40 30 40 30 40 30 40 30 40
2.5 3.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.C
0.17-0.34 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35 0.35-0.35
90 90 120 90 120 90 120 90 120 90 120 90 120 9Q 120 90 120 90 120 90 120
30 44 45 60 45 60 45 60 45 60 45 60 45 60 45 60 45 60 45 60 45 60
12.5 3.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0
0.20-0.29 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30
90 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 120 90 120
30- 44 45 60 45 60 45 60 45 60 45 60 45 60 45 60 45 60 45 60 45 60
2.5 3.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0 3.0 4.0
0.20-0.29 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30
150 150 150 150 160 150 160 150 160 150 160 150 160 150 160o.150 160 150 160
0 14 15 44 45 74 75 80 75 80 75 80 75 80 75 80 75 80 75 80 75 80
1.4 1.8 1.8 2.4 2.4 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
0 -0.01 0.01-0.04 0.04-0.07 0.07-0.07 0.07-0.07 0.07-0.07 0.07-0.07 0.07-0.07 0.07-0.07 0.07-0.07 0.07-0.07
a., 100 130 100 130 100 130 100 130 100 130 100 130 100 130 100 130 100 130 100 130 100 130
tRice b. 33 43 33 43 33 43 33 43 33 43 33 43 33 43 33 43 33 43 33- 43 33 43
II c. 4.2 5.0 4.2 5.0 4.2 5.0 4.2 5.0 4.2 5.0 4.2 5.0 4.2 5.0 4.2 5.0 4.2 5.0 4.2 -5.0 4.2 5.
d. 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30 0.30-0.30
a. 120 120 120 149 120 150 i20 150 120 150 120 150 120 150 120 150 120 150 120 150
Sweet potato b. 35 49 50 79 80 109 80 110 80 110 80 110 80 110 80 110 80 110 80 110 80 110
II c. 2.2 2.7 2.7 3.6 3.6 4.5 3.6 4.5 3.6 4.5 3.6 4.5 3.6 4.5 3.6 4.5 3.6 4.5 3.6 4.5 3.6 4.5
d. 0.24-0.34 0.34-0.54 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55
e. 75 89 89 119 120 149 150 179 180 209 210 239 240 269 270 299 300 329 330 330 330
cassava b. 35 49 50 79 80 109 110 139 140 169 170 199 200 229 230 259 260 289 290 290 290
II c. 0.9 1.1 1.1 1.5 1.5 1.9 1.9 2.3 2.3 2.6 2.6 3.0 3.0 3.0 3.0 18.104.22.168 3.0.0 3.0 3.0 3.0 3.0
d. 0.14-0.19 0.20-0.31 0.31-0.43 0.43-0.54 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55 0.55-0.55
Notes a. Length of normal growth cycle (days)
b. Yield'formation period (days)
c. LAI at laximum growth rate
d. Harvest index (H.)
e. Days to harvest
j/ See Tables 3.2 and 3.3 in Chapter 3.
2/ Yield formation period has been assumed to be the end one-third of the growth cycle in cereals, the second-half of the
growth cycle in legumes, 40 days after planting to harvest in sweet potato and cassava.
3/ Within the growth cycle range considered, LAI of the shorter duration crop reduced proportionately in relation to the LAI
of the longer duration crop. When growth cycle is curtailed due to growing period being shorter (e.g. 90-day sorghum at
75 days growing period), LAI reduced proportionately in relation to the LAI for the normal length of growth cycle considered.
4/ When yield formation period curtailed due to growing'period being shorter, H. reduced proportionately in relation to the
H. for the normal yield formation period considered. .
1/ Local agro-climatic adaptation of sorghum, in growing periods greater than 140 days, depends on the use of photoperiodic
cultivars which are sown at the start of the growing period but flower as the rains are ending so as to produce good
quality grains. Photoperiodic sorghum cultivars may have a growth cycle of up to 280 days but these have a log-cubic growth
curve as opposed to a log-quadratic growth curve in sorghum cultivars of 120 days or less (Fig. 7.1). During the cubic part
of the growth curve, there is little production of biomass. Also where there is an increase in biomass, there is a decrease
in harvest index. The final yield values for 120-day cultivars therefore would apply to cultivars of longer duration.
6/ Applies to cultivars of P. vulgaris, P. lunatus, P. aureus, P. radiatus, P. mungo and P_ angularis as a group.
L/ Applies to normal leaf cotton. 150 160 days to maturity does not include the last 20 days of boll opening period because
under rainfed conditions there is little further increase in biomass during the very last 20 days.
POTENTIAL NET BIOMASS (Bn) AND YIELD (B ) IN T/HA DRY WEIGHT OF GROUPS II AND III
CROPS FOR TROPICAL AND SUBTROPICAL (SUMMER RAINFALL) AREAS
Growing period (days)
CROP 75-89 90-119 120-149 150-179 180-209 210-239 240-269 270-299 300-329 330-364 365
Soybean Bn 6.5- 8.1 8.3-11.5 8.2-11.2 8.1-11.2 8.2-11.1 7.8-10.9 7.8-11.0 7.6-10.8 7.5-10.5 7.4-10.4 7.5-10.4
B 1.3- 2.3 2.5- 3.4 2.6- 3.4 2.4- 3.4 2.4- 3.3 2.3- 3.3 2.3- 3.3 2.3- 3.2 2.2- 3.1 2.2- 3.1 2.2- 3.1
Rice Bn 13.3-17.4 13.4-17.6 13.2-17.4 13.0-17.2 12.8-16.9 12.5-16.5 12.5-16.5 12.3-16.2 12.2-16.0 11.9-15.7 12.1-15.6
B 4.2- 5.2 4.0- 5.3 4.0- 5.2 3.9- 5.2 3.8- 5.1 3.7- 4.9 3.7- 4.9 3.7- 4.9 3.7- 4.8 3.6- 4.7 3.6- 4.7
Cotton B 5.0- 6.9 6.9-11.1 11.1-15.4 15.3-15.9 15.0-15.8 14.6-15.4 14.6-15.4 14.3-15.0 14.2-14.9 13.9-14.6 14.1-14.8
B 0 0.07 0.07-0.44 0.44-1.08 1.07-1.11 1.05-1.11 1.02-1.08 1.02-1.08 1.00-1.05 0.99-1.04 0.97-1.02 0.99-1.0X
Sweet potato Bn 7.1- 9.5 9.6-14.4 14.3-18.6 14.0-18.3 13.8-18.1 13.5-17.7 13.6-17.7 13.2-17.3 13.1-17.1 12.9-16.8 13.0-17.0
B 1.7- 3.2 3.3- 7.8 7.9-10.2 7.7-10.1 7.6- 9.9 7.4- 9.7 7.4- 9.7 7.3- 9.5 7.2- 9.4 7.1- 9.2 7.1- 9.3
Cassava Bn 3.3- 4.2 4.7- 7.8 7.7-10.9 10.9-14.3 14.2-17.7 18.7-20.7 20.7-22.6 21.7-23.4 23.1-24.8 24.1 24.4
B 0.5- 0.8 0.9- 2.4 2.4- 4.7 4.7- 7.9 7.8- 9.7 10.3-11.4 11.4-12.4 11.9-12.9 12.7-13.6 13.3 13.4
Pearl millet Bn 12.0-16.7 11.8-16.6 11.6-16.2 11.4-15.7 10.9-15.3 10.9-15.3 10.8-15.3 10.5-14.8 10.4-14.6 10.4-14.6 10.3-14.5
B 3.0- 4.2 2.9- 4.1 2.9- 4.0 2.8- 3.9 2.7- 3.8 2.7- 3.8 2.7- 3.8 2.6- 3.7 2.6- 3.6 2.6- 3.6 2.6- 3.6
Sorghum Bn 11.3-14.5 14.7-21.0 14.3-20.6 13.9-20.3 13.7-20.1 13.3-19.5 13.4-19.5 13.1-18.9 12.7-18.7 12.7-18.7 12.6-18.5
B 1.3- 3.4 3.7- 5.2 3.6- 5.1 3.5- 5.1 3.4- 5.0 3.3- 4.9 3.3- 4.9 3.3- 4.7 3.2- 4.7 3.2- 4.7 3.1- 4.6
Maize B 11.3-14.5 14.7-21.0 14.3-20.6 13.9-20.3 13.7-20.1 13.3-19.5 13.4-19.5 13.1-18.9 12.7-18.7 12.7-18.7 12.6-18.5
B 1.9- 4.9 5.1- 7.3 5.0- 7.2 4.9- 7.1 4.8- 7.0 4.6- 6.8 4.7- 6.8 4.6- 6.6 4.4- 6.5 4.4- 6.5 4.4- 6.5
Bn 6*5- 8.1
B 1.3- 2.3
- 79 -
For winter wheat in the subtropical (winter rainfall) areas, altitudes above
1 500 m have been taken to correspond to temperatures too low for its production, so
that the values apply to the altitude range 0 1 500 m.
For spring wheat, potato and phaseolus bean in the tropics, altitudes above
3 000 m have been taken to correspond to a risk of frost damage too great for success-
ful cultivation of the crops. The altitude of 3 000 m in the tropics corresponds to
mean temperatures of 10 to 12.50C or below. In the subtropical (summer rainfall)
areas in southern Africa, the altitude at which these frost related temperatures are
reached depends on the latitude. However, much of the high altitude area in southern
Africa is between 280S and 300S latitude in Lesotho where temperatures below 12.500
are reached at altitudes of about 2 500 m and above.
For maize and sorghum in the tropical and subtropical (summer rainfall)
areas mean temperatures below 15.000C have been considered too low for normal prod-
uction because of the very severe problems with seed set and maturation. In the
tropics, mean temperatures below 15.00C are reached at altitudes of 2 500 m and
above. Consequently, the yield values of these crops apply only to the altitude range
1 500 2 500 m and/or temperature range 15.0 20.000.
7.5.3 Cold Tropics, Cold Subtropics (summer rainfall) and Cold Subtropics (Winter rainfall)
(major climates 3, 6 and 8 respectively)
These major climates were assessed as being unsuitable for the cultivation of
the crops of the assessment because of temperature limitations described above in
Section 7.5.2 and Chapter 5.
- 81 -
MATCHING: SOIL REQUIREMENTS OF CROPS WITH SOIL CHARACTERISTICS
The soil matching exercise was undertaken by comparing the soil requirements
of crops (Chapter 4) with the soil inventory (Chapter 6) and resulted in the rating
of all soil units of the FAO/Unesco Soil Map of the World, in relation to the soil
requirements of the individual crops at two levels of inputs. The results are
presented in Table 8.1. Modifications to the. soil unit ratings were made according
to any significant limitations imposed by slope, texture and phase conditions as
described in the present chapter.
8.2 SOIL UNIT RATINGS
The ratings are based on how far the soil conditions of a soil unit meet
crop requirements under a specified level of inputs. The appraisal was effected
in three basic classes for each crop and level of inputs, i.e. very suitable
or suitable (S1), marginally suitable (S2), and not suitable (N). A rating
of Si indicates that there are no, or only minor, limitations to production of the
crop, provided climatic conditions are suitable. The rating of S2 was given when
it was considered that soil limitations are such that they would markedly affect
production of the crop, yet not to the extent of making the land unsuitable. A
rating of N was given when the soil limitations appear to be so severe that crop
production is not possible or, at best, very limited. The N rating was initially
divided into Ni and N2, N1 being applied when the soil limitations are considered
ameliorable through major land improvements (including initial heavy fertilizer
applications), and N2 where limitations are considered to be of a permanent nature.
In the final assessment however a combined N rating was used in place of N1 and N2.
The ultimate three ratings (31, S2 and N) were used in the final land suitability
assessment (Chapter 9) to modify the agro-climatic potential according to soil
Some basic concepts in the soil matching (rating) exercise (using the
initial Ni and N2 subdivision of N), were:
i. the ratings are made on the assumption that there have been no major land
improvements. This assumption was necessary because the location of any such
improved areas is not inventoried. It is appreciated that in many instances
the ratings will have to be changed if major land improvements have already
ii. it is also assumed that major land improvements, which can be effected,
cannot be satisfactorily maintained under low input conditions. Soil units
with such appropriate limitations were therefore rated as N2 under low
levels of inputs (i.e. the limitation cannot permanently be removed) and
N1 under high levels of inputs;
- 82 -
SOIL UNIT/CROP RATINGS
Crop: Wheat Sorghum Pearl millet Phaseolus bean Maize Soybean
Level of Low High Low High Low High Low High Low High Low High
Ge N2 N1/12 N2
Gc N2 N1/12 N2
Gd N2 N1/N2 N2
G Gm N2 N1/12 N2
Gh N2 N1/N2 N2
Gp N2 N2 N2
Gx N2 N2 N2
Re S1 S1 S1
R Rc S1 Si S1
Rd S2 S1 S2
Rx N2 N2 N2
I N2 N2 N2
Qc N2 S2/N12 S2
Q Q1 N2 S2/N 2 S2
Qf N2 N2 S2/9N2
Qa N2 N2 N2
E S2/N2 S2/N2 S2
U N2 N2 N2
To S2 S1 S1
T Tm S1 S1 S1
Th S1/S2 Si S1
Tv N2 r 2 S2/N2
V Vp S2/N2 S S2/ 2
Vc S2/N2 S-1 S2/N2
Zo N2 N1/N2 N2
Z Zm N2 N1/N2 N2
Zt N2 N2 N2
Zg N2 N2 N2
So 12 S2/12 N2
S Sm S2 S2 S2
Sg N2 N1/N2 N2
N1/N2 N2 N1/N2
N1/1N2 N2 N1/K2
N1/N2 N2 N1/N2
N1412 N2 N1/142
N2 N2 N2
N2 N2 N2
S1 S1 S1
S1 81 S1
S1 S2 S1
N2 N2 N2
N2 N2 N2
S2 S2 S1
S2 S2 S1
S2/N2 S2 S1/N2
N2 S2/N2 S2/N2
S2 S2 S2
N2 N2 N2
S1 S1 S1
S1 S1 S1
S1 S1 S1
S2/N2 S2/12 S2/N2
S1 S2/N2 S2
S1 S2/N2 S2
N1/42 N2 N1/N2
N1/12 N2 N1/N2
N2 N2 N2
N2 N2 N2
S2/1N2 N2 S2/N2
S2 S2 32
N1/12 N2 N1/N2
N2 N11N2 N2 N1/N2 N2
N2 N1/N2 N2 N1/N2 N2
N2 N1/N2 N2 N1/N2 N2
N2 N1/42 K2 N1/12 N2
N2 N1/N2 N2 N1/K2 N2
N2 N2 N2 N2 N2
N2 K2 N2 N2 N2
S1 S1 S1 S1 S1
S2 S2 S2 S2 52
S2 S1 S2 S1 S2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
S2 S2 N2 S2 S2
S2 S2 N2 S2 S2
S2/N2 S2/N2 N2 S2/K2 S2/1N2
N2 N2 N2 N2 N2
S2/1N2 S2/K2 S2/K2 S2/1N2 S2/N2
N2 N2 N2 N2 N2
S2 S1 S2 S1 S2
S1 S1 S1 S1 S1
S1/S2 s1 51/S2 S1 S1/S2
N2 N2 N2 N2 N2
S2/N2 S1/S2 S2/N2 S1 S2/N2
S2/N2 S1/S2 S2/12 S1 S2/N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
K2 N2 N2 N2 N2
N2 N2 N2 N2 N2
Y Y1 n.a.
X Xk S2
K Kk S1
C Ok S1
H Hc S1
n.a. = not applicable
N 1/ 2
- 83 -
Cotton White potato Sweet potato Sugarcane Cassava Rice
2w High Low High Low High Low High Low High Low High
N1 2 N2 N1/N2 N2
N1/2 N2 N2 N2
N11/N2 N2 N1N2 N2
N11/12 N2 N1/N2 N2
S N1/1N2 N2 N1/N2 N2
S N2 N2 N2 N2
S N2 N2 N2 N2
I S1 Sl Si Si
I Sl S2/N2 S1/S2 Sl
? S1 S2 S1 S2
2 N2 N2 N2 N2
2 N2 N2 N-2 N2
2/N2 S2/N2 S2 s /S2 S2/N 2
2/N2 S2/N2 S2 s1/S2 S2/N2
2 N2 S212 S2/N 2 S 2/ 2
2 N2 N2 N2 N2
i2/N2 S2/N2 S2/12 S2/N2 S2/N2
12 N2 S2/N2 S2/N2 N2
32 S1 S2 SI S2
31 Sl Sl Sl SI
31/S2 S1 S1/S2 SI S1/S2
S2/N2 S2/N2 S2/N2 S2/N2 S2/N2
S2 Sl N2 S2 N2
82 S1 N2 S2 N2
N2 Ni N2 N2 N2
N2 NI N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
N2 N2 N2 N2 N2
S1 S1 S1 31
S1 S2/N2 S2/N2 Sl
S1 S1 Sl Sl
SI Sl S1 31
S1 S2/N2 S2/N2 Sl
S1 S1 S1 Sl
Sl S1 S1 S1
S1 S1 Sl S1
Si S2/12 2/1N2 Sl
S1 S1 S1 S1
N1/N2 N2 N1/12 N2
- 84 -
Table 8.1 (cont.)
SOIL UNIT/CROP RATINGS
S1 S1 S1 S1 S1 Si
52 S2 S2 S2 S2 S2
S1 S1 '
n.a. = not applicable
N1 S2 S2
- 85 -
Cotton White potato Sweet potato Sugarcane Cassava Rice
Low High Low High Low High Low High Low High Low High
S1 S1 S1
N2 N1/i2 N2
S1 S1 Sl S1 S1 Sl S1 81
32 Sl S2 S1 S2 S1 S2 S1
32 S1 S2 S1 S2 S1 S2 S1
12 N1/N2 N2 1/N2 N2 N1/N2 S2 S2
T2 N2 N2 N2 N2 N2 N2 N2
Si Sl S2/N2 S2/N2 S1 Sl Sl S1
S1 S1 S1 S1 Sl Sl Si Sl
S1/S2 S1 S2/92 S2 s2/N2 S2 S1/S2 Sl
S2 S1/S2 S2/N2 S2/N2 S2/N2 S2/N2 S2 S1/S2
SI S1 Sl Sl SI S1 S1 S1
Sl S1 S1 Sl Si Sl S1 Sl
Sl Sl S1/N2 S1/N2, Sl Sl Sl Sl
S1/S2 Sl S2/N2 S2 S2 S2 S1/S2 Sl
S2 S1/S2 S2/N2 S2/N2 S2/N2 S2/N2 S2 S1/S2
S2 S1 S2 S2 S2 S2 S2 S2
S2/12 S2/N2 S2/N2 S2/N2 S2/N2 S2/12 S2/N2 S2/N2
N2 NI/N2 N2 NI/N2 N2 N1/N2 S2/N2 S2/N1
S1 Sl Sl
Sl S2 S1
N1/N2 S2/N2 S2/N1
S2/N2 S2/N2 S2 S1/S2 N2 S2/N2 S2/N2 S2/N2
S2 S2 S2 S2/S1 S2/N2 S2N2 S2/N2 S2/N2
S2/N2 S2/N2 S2/N2 S2/N2 2/2 S2/N2 2 S2/N2 S2/1N2
S2 S2 S2 S1/S2 32 S1/S2 S2 S1/S2
N2 N2 N2 N2 N2 N2 N2 N2
N2 N1/N2 N2 N1/N2 N2 N1/N2 N2 N1/N2
Sl S1 S1 Sl
S2 S1 S2 S1
S2 S1 S2 S1
N2 N2 Sl Sl
N2 N2 N2 N2
S2 S2 S1/S2 S1/S2
S1 81 S1 Sl
S2/N2 S2 81 Si
S2/N2 2/52 S2/N2 S2/N2
S1 S1 Sl S1 -
S1 Sl S1 Sl
S2 S2 Sl S1
S2/N2 S2 S1 S1
S2/N2 S2/12 S2/N2 S2/N2
S2 S1 S2 S2
S2/N2 S2/N2 S2 S2
N2 N2 Sl Sl
S2 S1/S2 S2 S1/S2 31/32 S1/S2 S1/S2 S1 S2 S2
S2/N2 S2 S2/N2 2/N1 S21 2/N2 S2/N1 S2 S1/S2 S2/12 S2
S2 S1/S2 S2 S2 S2 S2 S1 Sl S2 S2
S2/N2 S2 S2/N2 S2/N1 S2/N2 S2/N1 S2 S1 S2 S2
S2/N2 S2/N2 N2 N1/N2 N2 N1/N2 S2/N2 S2/N1 N2 N2
N2 N2 N2 N2 N2 N2 N2 N2 N2 N2
S2 S1 S2 S1/S2 S2 S1 32 Sl S1 S1 81 31
S2/N2 S2 S2/N2 S12/N2 S2/N2 S2/N2 S2/N2 S2 S2/N2 S2/N2 S2/N2 32/N2
S2 S1 S2 Sl S2 S1 S2 S1 S1 S1 S2 S1
S2/12 S2/N2 N2 N2 92/N2 S2/N2 N2 N2 S2/N2 S2/N2 S2 S2
N2 N1/N2 N2 N1/N2 N2 N1/N2 S2/N2 S2/N1 N2 N2 Sl S1
Sl Sl S1 Sl S1 S1 Sl S1 S1 S1 S1 Sl
S2 Sl S2 Sl S2 81 S2 Sl S2 S1 S2 Sl
S2 S1 S2 S1 S2 S1 S2 S1 S2 S1 S2 S1
S2 S2 S2 S2 S2 si/S2 S2 S2 S2 Si 32 S2
S2/N2 S2/N2 S2/N2 S2/1N2 S2/1N2 S2/N2 S2/1N2 S2/N2 S2 S2 N2 N2
S2 S1 S2 S1/S2 S2 S1- S2 Sl S2 Sl S1/S2 S1/S2
S2 S1 32 S1/S2 S2 Si/S2 S2 Sl S2 Sl S1/S2 S1/S2
N2 S2/N1 1T2 N2 N2 S2/1 N2 N1/S2 S2/N2 S2/N1 N2 N2
S2/N2 S2/1N2 N2 N2 S2/N2 S2/ N2 S2/N2 S2/2 S2/1N2 S2/1N2 S2 S2/N2
N2 N2 N2 NI/N2 N2 N1/N2 N2 N1/N2 N2 N1/N2 S2/N2 32/N2
N2 N2 N2 N/l2 N2 N1/ 2 N2 N1/N2 N2 N1/N2 N2 N2
N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2