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The nitrogen in a legume crop residue becomes available to a following cereal chiefly after

  • A.The cereal roots directly absorb the intact residue
  • B.The residue is oxidised chemically by soil minerals
  • C.Rainfall dissolves the nitrogen straight out of the residue into the soil
  • D.Soil microorganisms decompose the residue and mineralise its nitrogen

Answer: D

Plants take up nitrogen only as ammonium or nitrate, so the organic nitrogen in the residue must first be released by decomposers. That is why the benefit to the following crop is spread over weeks rather than being available the moment the residue is incorporated.

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