Sulphur in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 51

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Sulphur in the soil
Background
Sulphur is a chemical element and is written in the chemical form as S. Plants and
animals require sulphur to build proteins that are important in the quality and
quantity of plant material produced, and in the health of animals. Sulphur can
give plants increased cold tolerance.
With the trend to high-analysis fertilisers that tend to be low in sulphur,
deficiencies can occur in soil over time. It will then be necessary to add a sulphur
rich fertiliser or gypsum to the soil at a rate indicated by a soil test to enable
quality crops to be grown.
Sulphur in the soil
Most sulphur is concentrated in the organic matter in the surface layer of the soil;
however, some is present in the subsoil where it has been deposited by leaching.
Sulphur combined with organic matter is not available directly to plants, but
must be converted to the sulphate (SO 4 ) form by bacteria to be plant available.
This happens best in warm, moist, well-aerated soil. However, when some bacteria
are decomposing crop residues high in carbon, or when cold, waterlogged
conditions are present, hydrogen sulphide (H 2 S) (a gaseous product) is formed,
which is lost from the soil. Sulphur availability in the soil can therefore be cyclic,
changing from available to unavailable forms as soil conditions change.