Composts on which the mushroom mycelium fails to make normal growth are a common cause of poor yields and crop failures. No investigation into the matter appears to have been made, though other aspects of mushroom nutrition have received attention. A good deal of research has been directed towards the discovery of the principal mineral (3, 5, 6, 7, 11) and organic (1, 2, 4, 6, 7, 8, 9) nutrients required by the mushroom. Environmental factors such as temperature (2), partial pressures of gases (12), concentration of soluble materials (6, 7), hydrogen-ion concentration (10, 11) and moisture content (7) of the compost have been studied. But failures occur, of the kind described, when these factors appear to be favourable and when the compost contains ample food material. It has been suggested (l, 2) that poor composts may be unsuitable in physical condition or contain injurious compounds. The influence of the physical or physicochemical condition of the compost on mycelial growth deserves attention, since a compost is a colloidal system composed of solid, liquid and gas phases. In such a system it is possible, without changing the chemical identity of the materials, to bring about great changes in free surface energy, kinetic energy, interfacial tensions, dispersion and hydration of colloids, electrokinetic phenomena, diffusion and osmotic pressures—all of which are probably of vital importance in the nutrition of the mushroom. The properties of stability, swelling and degree of dispersion of most colloidal systems are determined by the nature of the adsorbed ions, especially the adsorbed cations.