A number of papers are being read to-day which will show how varied are the problems of horse manure composts and how much progress has been made in studying them.One may study the nature and composition of the fresh manure, the changes which occur during composting and during fermentation in the beds, the changes brought about by the mushroom, and the influence of physical and nutritional factors on growth and cropping. I shall give an account of experimental work, which I carried out some years ago on the physico-chemical properties of horse manure composts, the more important parts of which have already been published (1, 2). Of greatest importance to the mushroom grower was the experimental work which led to the recommendation to add ground gypsum (CaS04.2H2O) to horse manure as a routine practice before composting. Of lesser importance to the mushroom grower, but I think of considerable interest to him, was a study of the measurement of the pH of composts and the effect of pH of compost on mycelial growth. An account of this work has hitherto not been published (3), largely .because I had hopes of adding to it, but it deserves mention if only to show growers that pH is not a property of composts to which they need give very much attention. With regard to the work leading to the use of gypsum, I am not going to make this an occasion for repetition of published work only, but to stress certain aspects of the work to meet comments and criticisms which have since been made.
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