Soil Moisture and the Ecology of Soil Fungi

Summary Some basic concepts concerning soil physical factors have been briefly discussed. The importance of evaluation of the total soil moisture stress, as indicative of the energy which must be used by an organism to gain water from the soil, is shown. This total stress can be divided into tensiometric and osmotic components, of which the latter is important only in saline or heavily fertilized soils and is but briefly mentioned. Tensiometric suction is discussed and shown to be best described along with a measure of the texture and structure of the soil by means of the moisture characteristic. From such a characteristic, additional information concerning porosity and aeration can be obtained. Although there are few published experiments in which suction has been measured, none suggests a limitation of fungal growth directly due to suction factors in the soils between saturation and the permanent wilting point of mesophytic higher plants. Evidence derived from controlled humidity experiments reveals that most fungi can exert sufficient force to absorb water from atmospheres at a lower relative humidity than those in equilibrium with soil at the permanent wilting point. It is then suggested that absorption of soil water per unit area of absorptive surface will need to occur much less rapidly in the case of soil fungi than of higher plants, so that the growth of the former will not be limited to the same extent by reduction in the hydraulic conductivity of the soil. The conclusion is drawn that the majority of fungi will probably be able to grow in soils somewhat, or even considerably, drier than the permanent wilting point of higher plants. From an examination of the evidence it is concluded that in a compact soil spores are dispersed by the soil water only when that water is moving through a nearly saturated soil. In natural soils, however, where cracks, root channels, etc., are present, rapid movement of water, and thus of spores, may take place to a considerable depth even when the general soil mass is far from saturated. The literature reveals that such factors as moisture content, texture and structure, which affect soil aeration, have a profound influence on fungal activity and it is postulated that aeration is, in fact, the effective agent. Although the evidence is far from adequate, it appears that both excess of carbon dioxide and an insufficiency of oxygen are implicated as inhibitors. I wish to pay particular tribute to Professor N. Collis‐George who provided the initial stimulus for me to begin investigations in the field which forms the topic of this article and who has been of much assistance to me in discussion. I also wish to thank Dr E. C. Childs and Dr S. D. Garrett for discussion and for reading and criticizing various sections of the manuscript and to thank Dr D. J. Greenwood for advice concerning soil aeration. I am grateful to Professor H. Godwin for providing accommodation for me at the Botany School, Cambridge, for a period during which this article was written.