Forms and Functions of Sulphur in Plants

September 14, 2023 Agric4Profits

Another group of essential nutrient elements are the secondary nutrients; that is sulphur (S), calcium (Ca), and magnesium (Mg). These elements must be supplied to crops for their growth and production.

However, the secondary nutrients are not easily deficient in soils, and their need by crops is not as high as the primary nutrients.

As soil science students, you should understand the specific functions and deficiency symptoms on plants of each of the nutrients.

Amount and Functions of Sulphur in Plants

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

There is a high requirement for S in such crops as soyabean, cotton, tobacco, legume (for the manufacture of protein), cabbage family mustard, onion, and pepper which have a hot taste due to S-containing compounds such as glucosides.

The major functions of sulphur in plants could be summarized as follows:

Sulphur forms structural components of plants through three amino acids containing sulphur – cysteine, methionine, and cystine-by the disulphide bonds. There is serious human malnutrition whenever these amino acids are deficient.
Sulphur is important in metabolic processes.
It is involved in fatty acid synthesis and forms constituents of vitamins biotin, thiamine, and glucosides whose characteristic taste is found in onion and pepper. The mustard produced from sulphur in plants contains allylisothiocyanate CH2 = CH CH2CNS which has a high flavor as in the onion and mustard oil.
Sulphur is required for no chelation of the leguminous plants by Rhizobium, the nitrogen-fixing bacteria.
Sulphur also increases the oil content of oil-producing plants such as groundnut, oil palm, and soyabean.
Sulphur functions in proteins: in coenzymes for carbohydrates and lipid metabolisms.

Deficiency Symptom 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. The absence of S may also delay the 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 levels of atmospheric sulphur-bearing air.

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

Read Also: Forms and Functions of Potassium in Plants

Soil Sulphur

Soil sulphur occurs in both organic and inorganic forms. About 50 – 70% of the total S in topsoil is in an 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 – SO₄^2- and as elemental sulphur, S, in some soils. Most S in soils comes from parent materials, the S- containing minerals such as FeS, CuS, and NiS which could be oxidized by microorganisms to form water-soluble sulphates.

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 Ca CO₃ 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.

Weathering of minerals as mentioned above.

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.

This is referred to as Acid Rainfall which 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 the atmosphere.