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Abstract: The casing layer has received comparatively little attention by investigators over the last two decades. This has been due Iargely to the successful use of Sphagnum peat as a component of casing soils in many of the major mushroom producing countries. Suitable peats are however not available in many parts of the world and even when available, reserves are diminishing. The use of loam soil as a casing medium introduces a wide range of variables and in some regions suitable soils may also be unavailable. This situation prompted a wide range of investigations aimed to understand more adequately, (a) the function of the casing layer in inducing the formation of Agaricus bisporus fruitbodies, (b) the factors which govern productivity and (c) the relationship of these functions and; factors to the selection and management of alternative casing materials for commercial culture. Hayes and Shandilya (1977) described the use of decomposed cow manure as a satisfactory casing medium for use in the remote regions of the Indian Himalayas in Himachel Pradesh. Recently, Hayes, Cresswell, Yeo and Jakeman (1978), Yeo and Hayes (1979), Cresswell and Hayes (1979), described the use of a by-product of the pulp and paper making industry and termed PMB (paper mill by-product), as a satisfactory alternative to peat under the conditions of cultivation practised in Great Britain. Although the requirements for a productive casing layer were outlined by Atkins (1974) and Hayes (1978), the precise definition of factors which are relevant to productivity are difficult to measure and are restricted to broadband often vague statements on requiranents for an adequate water holding capacity, structure, texture and pH. Laboratory studies have shown that biological (especially bacteria) and chemical factors may be equally important in the transition which occurs in the casing layer from vegetative to reproductive growth (see especially Urayama, 1957; Eger, 1961; Hayes, Randle and Last, 1969; Park and Agnihotri 1969; Hume and Hayes, 1972; Smith and Hayes, 1972; Hayes, 1972; Hayes and Nair, 1976; Nair, Short and Hayes, 1976; Peerally, 1979; Cresswell and Hayes, 1979; Yeo and Hayes, 1979). There is little doubt that the difficulties encountered in defining, especially, the chemical and biological requirements, is associated with the inherent complexity of the interraction between a wide range of physical, chemical and biological variables operating in the casing soils of commercial practice. Furthermore, laboratory studies on artificial media may bear little or no relevance to commercial systems, since Yeo and Hayes (1979) and Cresswell and Hayes (1979) demonstrated that the underlying compost exerts a considerable influence on both the chemistry and bacteriology of the casing layer.
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