Current views that aggregation of soils results from by-products of microbial growth and that filamentous microorganisms contribute very little to physical binding are reviewed. The microscopical examination of soils and aggregates showed fungal mycelium to be persistent in well-aggregated soils, but relatively deficient in soils lacking structure. Cases are cited where physical properties of soils depend upon the presence of filamentous microorganisms: (1) Surface crusts on sands in open plant communities where algae dominate the coenoses with filamentous fungi; (2) the aggregation of deeper horizons of sands by adhesive fungal mycelium; (3) the sand capping to clay subsoils in solonetzic soils bound by abundant fungal mycelium and microbial gums; (4) the water repellence of sandy soils associated with the growth of microorganisms, particularly basidiomycete fungi; (5) the progressive improvement of crumb structure in fine-textured soils associated with increased development of persistent mycelium after several years under pasture.

The water repellence of sandy soils has been investigated at sites in the upper south-east of South Australia by observation of infiltration patterns and measurement of contact angle of wetting. Water penetrated into repellent sands through narrow channels, the intervening soil remaining dry. Such dry areas persisted through the winter months and were responsible for a mosaic pattern of bare and grassed areas in severely affected paddocks. The intensity of water repellence has been found to vary with species of plant cover, age of pasture, and also management practices.

For water entry into a vertical column of dry sand, the height of capillary rise, h, is defined for the present purposes as the maximum height of capillary rise in the sand, below which the moisture content is uniform. Previous experimental results on water entry into dry sand have been reviewed to show the validity and usefulness of this definition. The rise of the wetting front into a vertical column of sand was measured, the rate of rise of the wetting front was plotted against the reciprocal of the height of rise, and 1/h was found by extrapolating the line to zero rate of rise. For water-repellent sand a positive hydrostatic head was applied to the base of the sand to obtain an adequate number of points for the extrapolation. This pressure was adjusted so that the initial rate of advance of water into the sand was about equal to that of water into the ignited sand with no positive applied pressure. The advancing contact angle averaged over the wetted surface area of the sand was then calculated from the ratio of the values of h obtained with sand before and after ignition. The contact angle of a water-repellent sand has been shown to be higher than 90�. This explains the difficulty experienced in the field of wetting these sands. Two remedial measures are suggested: one is to cultivate and mix the soil to give a uniform average contact angle, the other is to cut slots so that a positive hydrostatic pressure can be applied to the deeper patches of high contact angle sand.

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