Discussion
Immobilization of sulphur in soil occurs when the C : S ratio is :
Answer: D
Sulphur is immobilised when the C : S ratio is above 400 : 1.
Soil microbes need carbon and sulphur in a balance. When residues added to the soil are rich in carbon and poor in sulphur, the microbes take the sulphate they lack from the soil solution and build it into their own cells, so it is temporarily unavailable to plants: immobilisation. The usual rule is that net immobilisation occurs above a C : S ratio of about 400 : 1, while net mineralisation occurs below about 200 : 1, and between the two neither process dominates.
The options 'below 50', 'below 100' and 'below 200' all describe residues rich in sulphur, which release sulphate (mineralisation) rather than lock it up.
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