S

The earth’s crust contains about 0.06 % sulphur. It is mostly present as sulphides, sulfates and in organic combinations with C and N. More than 95% of total sulphur in soil is present in organic matter under temperate conditions. Under tropical and sub tropical conditions, the organic matter content in the soil is generally less than the temperate soils. The initial source of all sulphur is the sulphide contained in plutonic rocks. Through weathering the sulfides are released.

In humid region organic sulphur, while in arid zone, the sulfates of calcium, Mg, Na and K predominate. Another source of sulphur is the atmosphere. Owing to industrial activity or through combustion of fuel, sulphur dioxide is produced which is brought down by rain.

The continuous use of high analysis fertilizers, particularly those which are devoid of sulphur such as urea, triple super phosphate, ammonium nitrate and ammonium chloride may lead to sulphur deficiency in soils.

Forms of sulphur in soils: Sulphur is present in the soil in many different forms, both organic and inorganic.

The inorganic forms are readily soluble sulphate, adsorbed sulphate, insoluble sulphate co-precipitated with calcium carbonate and reduced inorganic sulphur compounds.

1. Easily soluble sulphate: Sulphur is normally taken up by plants as the SO4= ion. In sulphur deficient soils the amount of readily soluble SO4= is frequently between 5 and 10 ppm. Sandy S deficient soils often contain less than 5 ppm. SO4= when associated with monovalents like Na+ are more prone for leaching. Medium losses when bound to divalent like Ca2+ or Mg2+ and minimum losses when bound to Al3+ or Fe3+.

2. Adsorbed sulphate: Adsorbed sulphate is an important fraction in soils containing large amounts of hydrous oxides of iron and aluminium like in Oxisols and Ultisols. Adsorbed sulphate can account for upto 1/3rd of total sulphur.

3. Sulfate coprecipitated with calcium carbonate: It is an important fraction in calcareous soils.

4. Sulfides: Under anaerobic conditions in water logged soils, there may be accumulation of H2S formed by the decay of organic matter. Also, SO4= present in the soil serves as an electron acceptor for sulphate reducing bacteria and it is reduced to H2S. This reduction takes place at a redox potential of less than -150 mV and at a pH range of 6.5 to 8.5.

5. Elemental sulphur: It is an intermediate product formed during oxidation of sulfides by chemical processes.

6. Organic form: Most of the sulphur in surface horizons of well drained agricultural soils of humid, temperate, semi arid and sub tropical regions is present in the organic form. It accounts for more than 90 %.

Average C : N:S ratio for calcarious soil 113 : 10 : 1.3 and for non calcareous soils it is 147 : 10 : 1.4. The N:S ratio of most soils is in the range of 6 to 8 in the form of amino acid like cystein, cystine and methionine.

Sulphur Transformations: The transformations of sulphur in soil are many and varied with changes often being cyclic. Also, the element converts back and forth from inorganic and organic forms due to the presence of living organisms.

When plant and animal residues are returned to the soil they are attacked by microorganisms thus releasing some of the sulphur as sulphate. Most of the sulphur, however, remains in organic form and eventually becomes part of the soil humus.

1. Mineralization: Mineralization of sulphur is the conversion of organic sulphur to inorganic SO4= by microbes. It depends on the sulphur content of the decomposing material. Mineralization of sulphur takes place at or below C/Sweight ratio of approximately 200: 1. Heterotrophic microorganisms bring about mineralization. Sulfatase enzyme plays an important role in mineralization.

R-O-SO3- + H2O ROH + HSO4-

Mineralisation is more in cultivated soils than fallow due to rhizospheres effect.

2. Immobilization: Sulfur is immobilized in soils in which the ratio of either C : N:

S is too wide. At a C : S ratio of above 200 : 1, immobilization or tie-up of SO4= with various organic forms is favored. Particularly, if the ratio is greater than 400: 1

i.e., when low sulfur containing organic materials are added to soils not plentifully supplied with inorganic sulphur immobilization of inorganic sulphur takes place.

The energy rich material stimulates microbial growth and the inorganic sulphur

(SO4=) in the soil is locked in the microbial tissues. Only when the microbial activity subsides inorganic SO4= again appear in the soil solution.

Sulphur oxidation – factors affecting oxidation in soils – sulphide injury- causes – symptoms – and remedial measures.

A) Sulphur oxidation : Elemental sulfur, sulfides and several other inorganic sulfur compounds can be oxidized in the soil by microbial oxidation,though chemical non biological oxidation also takes place to a lesser extent. Therate of biological oxidation depends on three factors like

i) the microbial population ii) Characteristics of sulfur source, iii) Environmental

conditions in the soil

CO2 + S + H2O2 + 2 H2O (C H2O)n + SO4= + 2 H+

Two classes of bacteria are specially adapted for sulfur oxidation. These are chemo lithotrophic sulfur bacteria which utilize the energy released from the oxidation.

eg : Thiobacillus thiooxidans.

Thiobacillus thioparus

Factors affecting sulphur oxidation in soils

1. Soil micro flora: The more the number of sulphur oxidizing bacteria like Thiobacillus thiooxidans the more is sulphur oxidation.

2. Temperature: Temperature between 25o and 40oC will be close to ideal, theprocess is slow below 10oC.

3. Soil pH: Optimum pH range is 6.5 to 7.2.

4. Soil type and properties: S oxidation is not very dependent on soil texture, organic matter content etc. Rapid sulphur oxidation is observed in well fertilized soil than one low in P and K. Thiobacillirequire NH4+ and NO3- is injurious. Organic matter is not essential as Thiobacilliare autotrophic but not heterotrophic.

5. Fertilizer interactions: Sulphur when applied in combination with N and P increases the rate of sulphur oxidation.

B) Sulphur reduction or Sulphide Injury or H2S injury

Under anaerobic conditions in water logged soils there may be accumulation of H2S formed by the decay of organic matter. Also, SO4= present in the soil serves as an e- acceptor for sulphate reducing bacteria viz., Desulfovibrio, Desulfato maculum and SO4= gets reduced to H2S. This reduction of SO4= is both redox potential and pH dependent. Sulphide injury occurs at a redox potential of less than -150 mV and in the pH range of 6.5 to 8.5

2 R CH2-OH + SO4= 2 R COOH + 2 H2O + S=

In some rice paddies, high in organic matter and low in active metallic elements such as iron, the free H2S may be released which is harmful to rice roots.

This H2S injury to rice in paddies is referred to as “Akiochi”.

Symptoms of Akiochi

Ø The upper leaves become progressively shorter.

Ø Lower leaves drying to a dirty yellow color with brown spots on the surface.

Ø Roots are pale white, mingled with black or rotten ones giving rotten egg smell and when roots are exposed to sun after washing they turn to reddish colour.

Ø Soil pH is around 6.0.

Ø Bubbles will be coming out from the affected areas.

In normal submerged soils well supplied with Fe, the H2S liberated is almost removed from solution by reaction with Fe2+ forming FeS2 which forms protective coating on roots and prevent rotting.

Fe2+ + S-2 FeS2 (Iron sulphide)

Remedial measures

Ø Use of Ammonium sulphate or any S containing materials should be avoided.

Ø Application of material containing iron such as laterites or lake mud.

Ø Providing aeration to soil by draining the field.

Sulphur is absorbed by plant roots as So42- ions. Concentration of S in plants range between 0.1 and 0.4%.

Functions of sulphur

1. Essential for synthesis of sulphur containing aminoacids cystine, cysteine and methionine.

2. Essential for synthesis of other metabolites including Co-enzyme A., Biotin, Thiamin of vitamin B and Glutothione.

3. Synthesis of chlorophyll.

4. It is a vital part of ferredoxins i.e Fe – S – protein occurring in the chloroplasts.

5. Responsible for the characteristic smell or odor and taste of mustard, onion and Garlies. (Puncy smell)

6. Enhances the oil formation in crops (Soyabean)

7. Increaseeasing root growth.

8. Stimulate seed formation.

9. Promote nodule formation – Leguminous species.

Deficiency of sulphur

a. Stunted growth pale green to yellow colour.

b. Immobile in plants and plants symptoms start first at younger leaves.

c. Poor seed set in rapeseed.

d. Tea –Tea yellows.

Micronutrients Zn, Fe, Mn, Cu

Micronutrients are equally important in plant nutrition as macronutrients; they simply occur in plants and soils in much lower concentrations. Plants grown in micronutrient-deficient soils it similar reductions in productivity as those grown in macronutrient-deficient soils.

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