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The earthy smell of freshly turned soil is produced by

  • A.Methane from methanogens
  • B.Geosmin released by actinomycetes
  • C.Ammonia released by nitrifiers
  • D.Hydrogen sulphide from sulphate reducers

Answer: B

Actinomycetes release geosmin as they die, and it is this compound the human nose detects at very low concentrations after rain or ploughing. The other gases listed do occur in soil, but hydrogen sulphide smells of rotten eggs and neither ammonia nor methane gives the characteristic earthy note.

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