Question

Discuss sulfur and its pathways in soil (i) plant uptake, (ii) sulfur mineralization, (iii) sulfate adsorption in acidic soils on iron oxides, (iv) the adsorption of sulfate on carbonates in alkaline soils, (v) the role of plant residues, (vi) reduction of sulfate to sulfide.

Answer 1

Sulfur is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent, and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula S₈

1. Major sulfur (S) inputs to soils originate from organic litter deposition and animal droppings. Most of this deposited S is organically bound (organo-S).this organic sulfur mineralised by fungi and bacteria and made available for plant uptake in form of sulfate.
2. Sulfur mineralization is a microbial driven decomposition which changes organic to inorganic forms.
3. In acid soils sulfate adsorption essentially involves the chemistry of Fe and Al. The hydrous Fe and Al oxides tend to form coordination complexes due to the donor properties of oxygen. The amount of sulfates adsorbed depends on the number of one-coordinated FeOH and AlOH sites,the surface area of adsorbent,the pH of the system.
4. sulphate occurs as a co-precipitated or co-crystallized impurity in the calcium carbonate.Sulphate adsorption was found to be negligible above pH 6.5 and adsorbed sulphate may be determined by aqueous extraction after increasing the pH above this value by addition of solid calcium carbonate.

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