Sulphur Content of Soils, Forms and Functions of Sulphur in Plants

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Soil sulphur occurs in both organic and inorganic forms. About 50 – 70% of the total S in top soil is in organic matter implying high S in soils with high organic matter content.

Total S content in soils is about 0.005 to 0.1% depending on the soil type. Organic S occurs as organic sulphates and carbon sulphides. It could occur in absorbable sulphates and as elemental sulphur, S, in some soils.

Most S in soils comes from parent materials, the S containing minerals such as FeS, CuS, NiS which could be oxidized by Micro-organisms to form water soluble sulphates: FeS + 202 S-oxidizing Fe3+ + SO4.

Forms and Functions of Sulphur

Like phosphate, sulphur is taken up by plants as the sulphate form, SO4 Sulphur concentration in plant tissue is about 0.05% forming the –SH and the –S-S groups in plant cells.

There is high requirement for S by such crops as soya bean, cotton, tobacco, legume (for manufacture of protein), cabbage family mustard, onion and pepper which has hot taste due to S-containing compounds such as glucosides.

Amount and Functions of Sulphur in Plants

The major functions of sulphur in plant could be summarised as follows: Sulphur forms structural components of plants through three amino acids containing sulphur – crysteine, methionine and crystine-by the disulphide bonds.

There is serious human malnutrition whenever these amino acids are deficient. Sulphur is important in the metabollic processes. It is involved in fatty acid synthesis, forms constituents of vitamins biotin, thianire and glucosides whose characteristic taste is found in onion and pepper.

The mustard produced from sulphur in plants contains allylisothio-cyanate CH2 = CH CH2CNS which has high flavour as in onion and mustard oil. Sulphur is required for nochelation of leguminous plant by Rhizobium, the nitrogen fixing bacteria.

Sulphur also increases oil content of oil producing plants such as groundnut, oil palm and soyabean. Sulphur functions in proteins: in coenzymes for carbohydrates and lipid metabolisms.

Sulphur Content of Soils

Deficiency Symptom

Deficiency symptoms of S occurs as a general yellowing of entire plants; and this characteristic yellowing may be difficult to distinguish from severe nitrogen deficiency. Plants are usually small and spindly. Nodulation of the roots of legumes is reduced.

Absence of S may also delay maturity of fruits and seeds and reduce the quality of protein in plants. Sulphur-deficiency is prevalent in the tropics compared to the temperate regions due to parent materials, high leaching rates and low level of atmospheric sulphur-bearing air.

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Excess S has been found to lead to accumulation of NO-3 in plants and also the reduction in the absorption and utilization of molybdenum, Mo, by plants.

Some small amount of sulphate is held on positively charged sites of soil colloids. FeS and CuS are referred to as primary minerals which have not been weathered. Sulphur is also found in secondary minerals.

Often found along with CaCO3 and Ca SO4 in arid or semi-arid soils. There are few S-deficient soils in the world. In normal agricultural soils, S is added in many ways;

Acid Rain

Rainfall dissolves the sulphur oxides SO2 evolved during the burning of wood, coal, fuel oil or from range and forest fires. Rainwater combines with sulphur oxide to form sulphuric acid making the soil to be acidic. Natural gas has high level of SO2 which is usually dissolved in rain water.

This is referred to as Acid Rainfallwhich is produced in large amounts especially in oil-prospecting areas causing acidification of lakes and rivers. During the dry seasons, dry deposition of SO2 may occur on leaves and soil surfaces.

Soils generally are becoming S-deficient due to reduced use of S-containing agricultural inputs but are also still receiving their S from atmosphere.

In summary, soil sulphur occurs in both organic and inorganic forms. Total S content in soils is about 0.005 to 0.1% depending on the soil type. Organic S occurs as organic sulphates and carbon sulphides.

Most S in soils comes from parent materials, the S containing minerals such as FeS, CuS, NiSwhich could be oxidised by microorganisms to form water soluble sulphates.

Like phosphate, sulphur is taken up by plants as the sulphate form,

SO4 Sulphur concentration in plant tissue is about 0.05% forming the –SH and the –S-S groups in plant cells.

The major functions of sulphur in plant could be summarised as follows:

Sulphur forms structural components of plants through three amino acids containing sulphur – crysteine, methionine and crystine-by the disulphide bonds.

Deficiency symptoms of S occurs as a general yellowing of entire plants; and this characteristic yellowing may be difficult to distinguish from severe nitrogen deficiency.

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