Sulphur is an important nutrient and soil amendment product in the cultivation and growth of the tea bush, says Dr Terry Mabbett.
THE TEA CROP’S requirement for sulphur is very high at 16 to 26 kg/hectare (ha) per annum, but not surprising since tea grows naturally on volcanic soils created by the same geological processes that generate elemental sulphur. The sulphur content of fresh green tea leaves should be maintained at 0.08% to 0.20% of total dry matter to achieve maximum yields for processing into high quality tea products. 1.0t (tonne) of a finished tea product requires 5t of freshly picked leaves that will have extracted no less than 10kg of sulphur from the soil. Tea production, by its very nature, with regular plucking of the youngest and most nutrient-rich foliage, will deplete soil nutrients including sulphur without fertiliser application.
When averaged out over the entire tea bush, sulphur content of leaves at 0.08% to 0.20% is way down in the list of nutrients, but when the nutrient analysis is targeted on the third-youngest unfurled leaf, sulphur at 0.50% shoots up to rank equal third with phosphorus. These latter figures are clearly important given that tea pickers select new shoots comprising terminal buds (golden
tips) and young leaves for processing into finest grade teas. Plucking the terminal bud, plus three leaves gives the highest yield of quality shoots and about 25% more than the bud plus two leaves. Shoots which include this third leaf process into premium teas with high concentrations of two important tea chemicals, polyphenols (tannins) and theine (tea caffeine).
Up to 40% of the sulphur absorbed by tea roots is recycled into the soil as fallen leaves and pruning waste, but there remains a considerable shortfall.
Tea harvests weighing in at 3,000kg will have removed some 6-9kg of sulphur/ha/year from the soil, say researchers at the Tocklai Experimental Station in Jorhat India and home of the famous Assam teas. Field trials in this tea-growing area of North Eastern India showed soils could become seriously depleted of sulphur through continuous picking unless appropriate fertiliser application was in place.
Sulphur shortfalls on tea estates are nothing new. Sulphur deficiency symptoms in tea bushes, commonly called ‘sulphur yellows’ or ‘yellow disease’, have long been recognised in tea growing areas around the world. Sulphur is central to plant nutrition. The nutrient is needed for the synthesis of sulphur-containing essential amino acids like cysteine and methionine and for the manufacture of chlorophyll pigments. When sulphur is in short supply chloroplasts break down and with it, the normal dark-green colour of healthy tea foliage.
Young leaves are the hardest hit by sulphur shortfalls. They develop an unhealthy-looking pale yellow colour and a general yellowing of interveinal areas.
New shoots are smaller and internodes (distance between leaves) shorter due to a slow down in growth, which may be followed by general shoot necrosis when sulphur deficiency persists. Consequences for yield and quality of tea are severe, especially for new shoots comprising of a bud and 2-3 leaves selected and picked to manufacture the finest grade tea.
To make matters worse, leaf yellowing symptoms are exacerbated by lower temperatures. These are the very conditions experienced at night on the hillsides where tea is typically grown. Sulphur is known to enhance frost resistance of tea bushes.
‘Sulphur yellows’ has proved to be an ongoing problem in a number of major African tea-growing countries, including Kenya, to the degree that it demands dedicated sulphur fertiliser. Sulphur was traditionally provided incidentally via nitrogen and potassium fertilisers, such as ammonium sulphate and potassium sulphate with the Sulphur element in the molecule. With the need for sulphur recognised in the fertiliser industry, it has responded with the design and development of sulphur fertiliser based on pure sulphur being formulated as pastilles and prills. The sulphur is degraded in the soil by Thiobacillus bacteria to form soluble sulphate that is absorbed by the root system of the tea bush.
However, the selective use of traditional fertiliser with a sulphur component may still be important. Researchers at the Tocklai Experimental Station reported how increased problems with sulphur deficiency could arise. And following replacement of ammonium sulphate (a sulphur-containing nitrogen fertiliser) and single super phosphate (a sulphur-containing phosphate fertiliser), when substituted with urea and rock phosphate, respectively, neither of which have sulphur in the molecule.
The extra benefit from applying sulphur fertiliser comes from sulphur’s additional role as a soil amendment product to increase soil acidity during tea estate rehabilitation. The tea bush requires an acid soil with a pH between 4.5 and 5.5 and will underperform in soils that are too compact or alkaline.
Sulphur and tea quality
Everyone knows that correctly dosed and balanced fertiliser improves crop yield and quality. 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 flavour and aroma, body, strength, colour and brightness of the liquor. Tea processing ends with the infusion to leach out the chemicals into hot water for assessment by expert tasters. 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 Experimental Station, where tea agronomists, food scientists and tasters have identified the significance and exact role of sulphur in tea quality.
Field trials over a six-year period using a variety of sulphur sources, including gypsum (calcium sulphate), ammonium sulphate and micronised elemental sulphur gave positive responses, but only up to a certain level. Sulphur up to 40kg ha/year increased tea yield. About 20kg sulphur/ha/year was the most cost-effective treatment.
Colour, brightness, strength, body, taste and flavour of the tea liquor are adversely affected by sulphur-deficiency, but the Tocklai trials went further and related changes in these parameters to levels of specific chemicals in the 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 sulphur/ha/year were by far the highest, 13.7 higher than no sulphur and a full 10.0 higher than 40kg sulphur/ha/year.
Other objective quality factors showed the same trend with brightness and total colour, responding positively to the application of sulphur fertiliser. Two group chemicals, called theaflavins and thearubigins, responsible for body, strength, taste, odour and the bright amber/red colour of quality liquor, were identified as keys to the quality status of tea. They were found in higher concentrations following sulphur application with 20kg sulphur/ha giving the best overall result. And there were marginal increases in the flavonol glycosides, such as rutin and quercetin and believed to have contributed to a brighter colour and enhanced flavour.
The contribution of sulphur to the bright, amber-red colour of quality black tea liquor is particularly interesting. Sulphur, in its common solid-state, is a yellow powder. On heating it melts to give
bright, amber liquid which crystallises on cooling to produce amber crystals with colour and ‘shine’ and uncannily similar to that of the 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, sulphur deficiency hits hardest at young growth (terminal buds and first three unfurled leaves) selectively plucked for finest 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.
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