M. Latham and P.M. Ahn
International Board for Soil Research and Management (IBSRAM)
PO Box 9-109, Bangkhen
Bangkok 10900, Thailand
Soil management and network concepts
Major Vertisol management problems
Development of the MOVUSAC network
The IBSRAM Vertisols network in Africa, Management of Vertisols under Semi-Arid Conditions (MOVUSAC), aims by collaborative research, to study Vertisols management in a range of semi-arid environments, and, by transferring experience between similar areas, to evolve locally adapted management techniques and cropping systems. Because of their difficult physical properties Vertisols are generally underutilised at present. Agreed priorities for Vertisol management research include soil water management and tillage problems. The aim is to develop techniques for using as much as possible of the water received while also providing surface drainage to avoid waterlogging. Man-made microrelief patterns to improve surface drainage include cambered beds, ridges, narrow beds and furrows, and broadbeds and furrows. In very dry areas, water harvesting techniques involve planting crops in the furrows rather than on the beds and ridges.
Since its inception in September 1983 at the close of an international workshop on soils (ACIAR, 1984), the International Board for Soil Research and Management (IBSRAM) has worked to establish soil management networks in the tropics. IBSRAM is establishing regional soil management networks around three major target areas: the management of Vertisols, the management of acid tropical soil, and tropical land development for sustainable agriculture. The African Vertisols network-Management of Vertisols under Semi-Arid Conditions (MOVUSAC) - was created at a regional seminar held in Nairobi in December 1986 (IBSRAM, 1987). Thirteen countries participated in the seminar and most of them have presented project proposals to form a collaborative research network.
This presentation explains the concepts on which soil management and networks are based, the major problems of Vertisol management under semiarid conditions and the current development of the MOVUSAC network.
Soil management is a broad concept which encompasses soils, crops and the farmers who use them. It must embrace such aspects as fertilizer and lime application, transfer of technologies on well classified soils, soil and water management and adapted cropping systems, in order to provide comprehensive technologies which can easily be applied by farmers. Soil management research thus requires a multidisciplinary approach.
A collaborative research network, as envisaged by IBSRAM, involves cooperators in a range of countries who work on similar problems. In the case of the African Vertisol network MOVUSAC the common problems relate to the management of these very distinctive cracking clays under a range of semi-arid environments. The properties of these soils set them apart from non-cracking soils, and pose management problems which, up to now, have made these soils relatively underutilised.
The properties of Vertisols are very well defined, and as these soils are widespread throughout the tropics, there are obvious possibilities for transferring aspects of successful Vertisol management experience from one area to another. With appropriate management, their productivity can be greatly increased, often with little extra cost. ICRISAT's (International Crops Research Institute for the Semi-Arid Tropics) experience in semi-arid peninsular India has shown that in some cases appropriate management can increase yields several fold, at little cost, and has demonstrated to farmers that Vertisols have a much greater potential productivity under rainfed conditions than was thought.
As the IBSRAM African Vertisols Network takes shape and research proposals are discussed, it is becoming clear that if they can all be implemented, the result will be a Pan-African effort, with many countries conducting coordinated research on soils which are essentially similar, but which require somewhat different management according to rainfall amount and distribution differences. IBSRAM's role in this effort has many characteristics, reflecting its view of how a network should function. Initially IBSRAM, in collaboration with the Network Coordinating Committee, assists cooperators in presenting their proposals to potential donors and in helping to secure the necessary funding. Since a network requires a methodology which has been agreed upon and is well understood by network cooperators, IBSRAM also organises training courses for "front-line" researchers. During experimental work, IBSRAM assists by providing information relevant to Vertisol management from its own data bank and from external sources.
In addition to its own staff, IBSRAM draws on the services of specialised consultants to help solve specific problems. Meetings of network cooperators will be held to discuss problems, results and future work, so that their experiences are shared. In this way, cooperators are not working on soil management in isolation, each in his own country, but should feel themselves part of a Pan-African team, supported by IBSRAM and by each other as well as by donor organisations. Thus the concept of a "network" moves from being a goal to a reality.
To be efficient, a network must focus on a limited number of objectives. From the report of the inaugural workshop on management of Vertisols for improved agricultural production (IBSRAM, 1985) and from the report of the participants in the Nairobi seminar (IBSRAM, 1987) major constraints to production as seen by African cooperators can be ranked: soil water management, tillage, cropping systems, and nutrient management.
Management of soil water is both the most difficult and the most important aspect of Vertisol management in the semi-arid tropics. The crops, particularly at the seedling stage, can suffer from extremes of drought, or from excess water, leading to waterlogging. In the semi-arid regions of the Vertisol Network, where rainfall averages between less than 500 mm and 1200 mm a year, the first priority is to make full use of as much as possible of the water received. In practice, farmers often make use of only part of the available water, usually by planting too late for various reasons. This is true for many soil types in semi-arid areas, but is particularly a problem with Vertisols because the moisture range at which tillage can take place is narrower than for most soils, and the difficulties of tillage outside this range are greater, due to the rather extreme consistency properties shown by Vertisols. When dry, they are extremely hard, and when wet, extremely sticky (Willcocks, 1987). Attempts to cultivate when too wet can lead to soil sticking to implements and the formation of large clods. Tillage has to take place at an intermediate moisture content, and waiting for this intermediate moisture content may cause delays.
Tillage of the dry soil before the rains, while feasible on light textured soils, is not normally practiced by African small farmers on Vertisols. ICRISAT's deep Vertisol management technology in India includes pre-monsoon tillage to prepare a seedbed. An initial ploughing after the harvest leaves clods which, helped by occasional showers, crumble during the dry season. A further passage with an ox-drawn cultivator in the few days before the predicted onset of the rains prepares a loose seedbed some 15 cm deep in which seeds and fertilisers are placed while the soil is still dry. The advantage is that seeds germinate with the first rain heavy enough (25 mm or more) to moisten the soil sufficiently to the seed depth.
Most traditional Indian farmers do not try to get on to Vertisols during the first few weeks of heavy rains; they do not start ploughing until the rains have subsided and the soil moisture has fallen to a level allowing tillage. Thus most of the extensive Vertisols of peninsular India are fallow during a good part of the rains, so that crops planted on them make use of only the late wet season showers and residual soil moisture. In many areas of Africa, farmers delay moving on to at least the poorer-drained Vertisols for similar reasons. The poor internal drainage of Vertisols and their extremely slow hydraulic conductivities, leading to waterlogging, thus delay planting, and a part of the wet season is lost, whereas other lighter soils in the same area may be planted promptly at the onset of the rains.
In the case of a Vertisol which has formed wide deep cracks in the dry season, initial entry of rainfall when it comes is easy, since water moves freely into the cracks. If the first rains of the wet season are heavy and large quantities of water enter the cracks before the soil swells and the cracks close up, the soil profile may be well charged to the depth of cracking (which, by definition, is at least 50 cm in a Vertisol). Much less favourable is an initial fall of light rain, which closes the cracks before much water has entered. Once the cracks are closed most Vertisols are notoriously impermeable. Infiltration rapidly falls to very low rates, and water may simply form puddles on the soil surface, much of it to be lost by evaporation. Important management differences are related to the degree to which a grumusolic self-mulching topsoil is present, and hence to differences in rainfall acceptance by the wet soil.
In addition to the need to get as much of the rain as possible into the soil for use by the crop, there is the need to provide adequate surface drainage to avoid plant injury or slow growth from waterlogging once the cracks have closed and infiltration rates are very slow. A traditional and relatively early method was the cambered bed, also useful on many clay soils other than Vertisols. A cambered bed can be formed by ploughing up and down so that the soil is turned inwards to the centre. Cambered beds have been used successfully in many areas of Africa, including the Accra plains of Ghana and the northwestern cotton growing areas of Tanzania. Other man-made microrelief patterns designed to improve surface drainage include various beds and mounds made with hoes and other implements, and a range of ridges, narrow beds and broadbeds, often made by animal-drawn implements, and all separated by a furrow whose essential role is to provide surface drainage in about the upper 10-15 cm of the ridge or bed and to lead away the drainage water gently, on a low gradient, and dispose of it safely.
In very dry areas, waterlogging is less likely to be a problem, and tillage is important to get every drop of water in the soil, and minimise runoff and evaporative losses. The roles of ridges and furrows are reversed: water is designed to run off the ridge, sometimes suitably broadened, which then becomes a water harvesting device designed to lead runoff into the furrow in which the crop is planted. To block the water movement in the furrow, the ridges may be 'tied' at intervals with a cross ridge, although in occasional unusually wet years the ridges may be untied. The feet that Vertisols, once wetted, have very slow infiltration rates so that water runs off easily is in this ease an advantage for water harvesting in the ridges.
Water movement into cracking clays illustrates a further fundamental difference between cracking and non-Braking soil. When rain falls on a non-cracking soil, such as an Alfisol, Ultisol or Oxisol, the wetting front moves downwards from the surface fairly evenly, particularly if the surface is without marked microrelief and has good rainfall acceptance properties. Although infiltration rates may decrease as the soil surface horizons approach field capacity, there are no sudden changes affecting the pattern of water entry and movement.
In contrast, the initial entry of water into Vertisols through cracks may be rapid; the heavier the downpour the more precipitation will enter the subsoil. Water in the cracks then diffuses laterally and relatively slowly to areas between cracks, resulting in very uneven water distribution patterns. Similar patterns may form in relation to uneven entry related to gilgai microrelief.
The lower the rainfall, the more critical is detailed study of differential water movement in Vertisols, which should be used is an adapted cropping system. Detailed water studies of this type, if applied to the water-harvesting ridge and furrow system being tried in southeast Zimbabwe, can be expected to show to what extent water is channeled into the furrows and where it is stored in relation to the crop root system. These studies could indicate what proportion of rain is exploited by the crop, and whether there is enough moisture in the soil at harvest to have supplied a crop with a longer growing season, or an additional short-season sequential crop. In this way studies of soil physics, soil management, and crop needs can be used to adopt cropping systems better adapted to soils and rainfall, and to raise yields, particularly in years of low rainfall.
Cropping systems are a major aspect of Vertisol management, and different strategies have to be adopted according to the amount and distribution of the rain (Willey, 1987). For example, intercropping and relay cropping may increase yields several-fold, as found by ICRISAT in India (Kanwar and Virmani, 1987). The success of these systems depends on a good understanding of the soil moisture regime and a better use of available water.
Fertility problems may also become a constraint, particularly in sustained, high-input systems. Under these conditions N. P and micronutrients (particularly Zn and Fe) may limit crop production (Le Mare, 1987; Katyal et al, 1987). In most African countries, however, at the present stage of rainfed Vertisol use in semi-arid areas, nutrients are less important than soil water management, as indicated by the fact that yields are more often raised by improved surface drainage or increased available water than by the use of fertilisers. For these reasons, a strategy giving priority to soil water management has emerged as the main focus for the Vertisols Network. Related to this is the development of improved techniques and an examination of the advantages of post-harvest tillage over rainy season tillage.
The ICRISAT system of post-harvest ploughing followed later by seedbed preparation and dry seeding before the rains illustrates what can be done when the onset of the rains is predictable. However, in many African areas of low and unreliable rainfall, both the amount of rain and the onset of the wet season vary widely from year to year. Forecasting is more difficult, even with the sophisticated techniques for climatic analysis now available.
An acceptable strategy under these conditions is to secure a higher minimum yield in dry years, and thus reduce the risk of crop failure, while recognising the need for surface drainage to minimise waterlogging effects in wet years. However, the farmer who plays safe on part of his land may also wish to devote additional space to a higher-risk crop or a longer growing season variety which, although it will fail in a bad year, will give higher or more valuable yields in a good year. This is the case where a relatively safe sorghum crop is combined with a riskier but more appreciated maize crop, or where a higher yielding long-season maize is planted in addition to safer but lower-yielding short-season maize.
These considerations suggest the main emphasis of the MOVUSAC Network. The properties of Vertisols which distinguish them from the more extensive non-cracking soils of the tropics are fundamentally similar wherever these soils occur. The variations which affect soil management are of two types: edaphic and climatic. Variations in the properties of the Vertisols, and variations in climate (amount, distribution, and reliability of rainfall), together ensure that management and cropping systems are site-specific. This means that we cannot plan to transfer a cropping system from one locality to another without modification. What we can do is to look at basic principles and patterns of adaptation to local conditions in relation to changes in soil and climatic parameters relevant to management.
Vertisols on the African continent are found in a range of rainfall regimes, from relatively wet in Ethiopia, to semi-arid in Zimbabwe and the Sudan. Individual network cooperators working in a single country will study local soil management, and their results will be shared with other parts of Africa, so that the management research as a whole is not national but continental. It is hoped that this work will eventually produce a gradation of management practices adapted to rainfall, soils, and socioeconomic factors.
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