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Abstract: Experienced mushroom growers are aware of the occurrence of highly reflective, tiny, whitish crystals that appear in mushroom compost prepared according to the "short composting method" of Sinden and Hauser (1950), These crystals can be found in the centre of compost piles by day 5 to 6 during Phase I composting, A search of the literature revealed no previously published information on this very interesting phenomenon. By making use of X-ray diffraction these crystals were identified as pure elemental sulphur by the Chemical Institute of the ' South African C.S.I.R. The question arose as to where these crystals come from. As a working hypothesis I proposed that they might originate from gypsum (CaSO4) which is used in the preparation of mushroom compost. Gypsum, which is added to composting substrates to improve their structure by dispersing colloids and thus keeping the compost from becoming "greasy", is usually added at a rate of 25 kg/tonne (Vedder, 1978). I reasoned that the CaSOLj might be reduced to hydrogen sulphide (H2S) under anaerobic conditions by the action of sulphate reducing bacteria presumably present in the anaerobic,core of the compost pile. The hydrogen sulphide gas could then be oxidised to elemental sulphur and water.
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