like other crops, requires the full and balanced complement of nutrients. These include Nitrogen (N), Potassium (K), Phosphorous (P), Sulfur (S), Magnesium (Mg), Manganese (Mn), Zinc (Zn), Copper (Cu) and Iron (Fe). However, tea has a particular need for sulfur, over and above its function as a major nutrient for rapid healthy growth and development.
Sulfur is important as a soil amendment chemical to generate and maintain a favored low pH for the acid-loving tea bush. The most important commercial function lies in its close association with specific tea chemicals that give the liquor its sought after taste, color, brightness, strength and body. It seems strange that the quality of a fine aroma beverage like tea should depend heavily on an unpleasant smelling chemical like sulfur. But there is a lot more to sulfur than meets the eye.
Sulfur is one of the oldest known elements and referred to in antiquity as “biblical brimstone.” It is widely abundant in the earth’s crust and closely associated with volcanic activity and hot water springs. As a yellow solid that burns with a pungent smell, sulfur has long been associated with all things evil. But there is an aesthetic and health-giving side to sulfur that comes through strongly in the cupped infusion of tea.
Sulfur has long been used in ointments to cure skin complaints and featured strongly in some the very first drugs (sulfa drugs) used as antibiotics. Indeed, tea was first consumed as a medicine in the Yangtze Kiang Valley of China around 2700 BC. And today after water, tea is the most heavily consumed drink in the world because of its health giving, dietetic and even therapeutic properties.
For many years farmers and growers were indulged with free sulfur as non-costed “rider” impurities in NPK fertilizers. As byproducts of industry, it was spewed out and spread over the land by coal-fired power stations and factory chimneys.
But forward-thinking governments backed using modern technology to clean up and “green-up” the environment. And with it came the progressive “de-greening” of crops when sulfur deficiency started to show as leaf yellowing or chlorosis.
The early signs in many crops were confusing because the leaf yellowing of sulfur deficiency symptoms is not unlike that for nitrogen. Nitrogen deficiency shows first in older leaves, while sulfur deficits affect the youngest leaves. Eventually scientists began to look at the role of sulfur in the growth, development and quality of crops, and none more so than Camellia Sinensis - the tea plant.
Sulfur Requirements for Tea
Sulfur requirement for tea is very high at 16 to 26 kg/hectare/year. But this should come as no surprise since tea grows naturally on soils of volcanic origin, created by the very same geological processes that create elemental sulfur in large amounts.
The sulfur content of fresh green tea leaves should be maintained at 8% to 2% on a dry matter basis to achieve maximum yields of fresh leaves which can be processed into high quality tea products. In order to produce one ton of finished tea, the factory requires five tons of freshly picked leaves that will have extracted no less than 10 kg of sulfur from the soil. And tea production by its very nature, with regular plucking of the youngest and most nutrient-rich foliage, will exhaust soil nutrients including sulfur if fertilizer applications are not made.
When averaged out over the whole tea bush, the sulfur content of leaves is way down in the list of nutrients. But when nutrient analysis is targeted on the third youngest unfurled leaf, sulfur shoots up to rank equal third with phosphorus.
These figures are clearly important given the selection of new shoots during picking for processing terminal buds (golden tips) and young leaves into the finest grade teas. Plucking the terminal bud plus three leaves gives the highest yield of quality shoots some 25% more than bud plus two leaves. And finished tea products based on shoots picked to include this third leaf are premium teas, with high concentrations of two important tea chemicals, polyphenols (tannins) and theine (tea caffeine).
Up to 40% of the sulfur absorbed by tea roots will be re-cycled into the soil as fallen mature leaves and pruning waste, but there is still a considerable shortfall. A tea harvest weighing in at 3000 kg will have removed some six to nine kg of sulfur/hectare/year from the soil, say researchers at the internationally famous Tocklai Experimental Station in Jorhat India, home of the famous Assam teas. Field trials showed soils in the tea-growing areas of North Eastern India become seriously depleted of sulfur through continuous picking, unless suitable fertilizer application programs are in place.
Sulfur shortfalls on tea estates are nothing new. Indeed sulfur deficiency symptoms in tea bushes, called “sulfur yellows” or “yellow disease,” have long been recognized in East and Central Africa. Sulfur is central to plant nutrition. It is needed for synthesis of sulfur containing essential amino acids like cysteine and methionine and for the manufacture of chlorophyll pigments. When in short supply, chloroplasts break down and with it the normal dark green color of tea foliage.
Young leaves are the hardest hit by an unhealthy looking pale yellow color, and general yellowing of the inter-veinal areas. New shoots are smaller and internodes (distance between leaves) shorter due to a slow down in growth that is followed by general shoot necrosis if sulfur deficiency persists.
Consequences for yield and tea quality are severe; new shoots comprising bud and two to three leaves are selected for the finest grade tea. To make matters worse, leaf yellowing symptoms are exacerbated by lower temperatures, the very conditions experienced at night on the hillsides where tea is typically grown. Indeed sulfur is known to enhance frost resistance of tea bushes.
“Sulfur yellows” are an ongoing problem in countries such as Kenya, Uganda, Malawi and Zambia to a degree that demands remedy with sulfur fertilizer. Originally there was no such thing as a dedicated sulfur fertilizer. The tea estates applied sulfur through nitrogen and potassium fertilizers such as ammonium sulfate and potassium sulfate that have sulfur in the molecule.
Now with universal need for sulfur for crops in general, the industry has responded with new and dedicated sulfur fertilizers. Products containing yellow elemental sulfur as pastilles or prills are widely used and especially on tea. The sulfur is degraded in the soil by Thiobacillus bacteria to form soluble sulfate that is absorbed by the root system of the tea bush.
But selective use of traditional fertilizer with a sulfur component is still important. For instance, Tocklai researchers report increased problems with sulfur deficiency following replacement of ammonium sulfate (a sulfur containing nitrogen fertilizer) and single super phosphate (a sulfur containing phosphate fertilizer). These were substituted with urea and rock phosphate, respectively, neither, of which have sulfur in the molecule.
Sulfur fertilizers are additionally used as soil amendment products, to increase soil acidity in tea estate rehabilitation. Trials by the Tea Research Foundation of Kenya on the Kapchorua Tea Estate using a water degradable sulfur pastille called “Brimstone 90” (90% elemental sulfur) showed real benefits. This dedicated sulfur fertilizer reduced soil pH from 6.3 to 5.5, a feat not achieved by the normally used aluminum sulfate. The tea bush demands an acid soil with a pH between 4.5 and 5.5, and performs poorly in soils that are too compact or alkaline.
Sulfur and Tea Quality
Everyone knows that correctly dosed and balanced fertilizer improves crop. The benefits are generally difficult to pin down, but tea is different. The fresh green leaves are put through a series of exhaustive processes, including withering, rolling, fermentation and firing. This generates and consolidates a group of all-important chemicals that combine to determine tea quality through flavor and aroma, body, strength, color and brightness of the liquor. Tea processing ends with the infusion to leach out the chemicals into hot water for assessment by expert tasters. And results from this ultimate test can be used to pin-point the effect of growing conditions including soil nutrient status.
This is exactly what has been done at Tocklai, where tea agronomists, food scientists and tasters have identified the significance and exact role of sulfur in tea quality. Field trials over a six-year period using a variety of sulfur sources, including gypsum (calcium sulfate), ammonium sulfate and micronized elemental sulfur gave positive responses but only up to a certain level. Tea yields increased with application of sulfur up to 40kg hectare/year, with 20 kg/hectare/year being the most cost-effective treatment.
The color, brightness, strength, body, taste and flavor of the tea liquor are adversely affected by sulfur-deficiency. But the Tocklai trials went farther and related changes in these parameters to levels of specific chemicals in leaves.
The tea was produced by CTC (crushing, tearing and curling), the most commonly used method of rolling in the Indian tea industry. Key tea chemicals were measured using biochemical analysis and HPLC (High Performance Liquid Chromatography) of black tea liquors, and tasters from J. Thomas & Co Limited in Calcutta were used to assess organoleptic quality.
Tasters’ scores at 74.0 for tea produced with 20kg sulfur/hectare were by far the highest, 13.7 higher than no sulfur and a full 10.0 higher than 40kg sulfur/hectare. Objective quality factors showed the same trend with brightness and total color responding positively to sulfur fertilizer.
Two groups of chemicals called theaflavins and thearubigins are responsible for body, strength, taste, odor and the bright amber/red color of quality liquor were key to the quality equation. They were found in higher concentrations following sulfur application, with 20kg sulfur/hectare giving the best overall result (Table 3). Marginal increases in the flavonol glycosides such as rutin and quercetin are thought to have contributed to brighter color and enhanced flavor.
The contribution of sulfur to the bright, amber-red color of quality black tea liquor is particularly interesting. Sulfur in its common solid state is a yellow powder. On heating its melts to give a bright amber liquid which crystallizes on cooling to produce amber crystals with a color and shine uncannily similar to that of quality black tea liquor.
These findings can be related to original concentrations of polyphenols in fresh leaves. Because theaflavins and thearubigins are produced by enzyme controlled oxidative reactions on polyphenols during fermentation. Polyphenols are heavily concentrated in the youngest growth, terminal bud (28%), smallest (first) leaf (28%), second leaf (21%) and third leaf (18%).
In summary, sulfur deficiency hits hard at young growth (terminal buds and first three unfurled leaves) selectively plucked for the fine grade teas. These have the highest concentration of polyphenols converted during fermentation by polyphenol oxidase enzymes into the all important tea chemicals responsible for quality liquor.
For more information, contact Dr. Marcus Ross, Tel: (49)561-9301-2417, Fax: (49)561-9301-1416, E-mail: firstname.lastname@example.org.