Sulphur (S) - Macronutrients

Sulphur is present in the soil in inorganic and organic forms. In most soils organically bounds is the major S reservoir. The inorganic forms of S in soil consist mainly of SO^2-₄. In arid regions, soil mav accumulate high amounts of sulphur salts such as CaSO₄, MgSO₄, and Na₂SO₄. Under humid conditions however, SO^2-₄ is present either in soil solution or is adsorbed on soil colloids. The organic S of the soil is made available to plants by microbial activity. In this process of mineralisation H₂S is formed which under aerobic conditions readily undergoes auto-oxidation and forms SO^2-₄ In anaerobic media, however, H₂S is oxidised ,to elemental S by chemotrophic sulphur bacteria such as Beggiatoa, and Thiothrix. Further oxidation of S results in the formation of H₂SO₄. AS a result increase in soil acidity can occur. , Plants mainly absorb S in the form of SO^2-₄ .

It is mainly translocated in an upward (acropetal) direction. Downward (basipetal) movement of S is relatively poor. There is now a considerable evidence to show that plants can utilise sulphur dioxide also. The most important sulphur containing compounds are cysteine, methionine, lipoic acid, coenzyme A, biotin, thiamin and ferredoxin (an electron carrier, a type of non-heme iron-sulphur protein). Sulphur forms disulphide bridges in polypeptides. In field crops sulphur deficiency and nitrogen deficiency are sometimes difficult to distinguish. In plants suffering from S deficiency the rate of plant growth is reduced.

Generally, the growth of the shoots is more affected than root. In contrast to N deficiency, chlorotic symptoms occur first in the younger, the most recently formed leaves. Although the content of S incrops is similar to P content, S application does not play an important role as P fertilisation. This is because SO^2-₄ is not strongly bound to soil particles as phosphate and is thus more available to plants. In addition, substantial amounts of S can come from the atmosphere or from fertilisers which contain S along with other major nutrients being applied e.g. ammonium sulphate or potassium sulphate. The most important sulphur containing fertilisers are gypsum, super- phosphate, ammonium sulphate and potassium sulphate.