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Stability changes during ageing (sterile) or incubation (non-sterile) of both natural field aggregates and remoulded aggregates from five soils were studied for periods up to 30 days. Growth of fungal hyphae, estimated by ergosterol measurement, corresponded to temporary stability increases in both types of aggregates during the first 15 days. Thereafter, fungal hyphae disappeared and were replaced by actinomycetes and bacteria. Increased stability due to entanglement by hyphae was comparable to that due to thixotropy in remoulded aggregates. Bacterial growth accompanied the fall in stability associated with fungal decline, but had little direct effect in stabilizing soil aggregates. Destruction of polysaccharides by periodate oxidation greatly diminished aggregate stability. The role of bacterial polysaccharides in soil aggregate stability is discussed.

Mathematical models to describe the density of plant roots in the soil have been developed by analogy from the equations describing diffusion or heat flow. Results from an experiment with isolated plants (lettuces) shows that the equation for diffusion from a point source applies, while results from a row crop (onions) indicate that when allowance is made for the assymetry resulting from a preferred direction of growth, the equation describing diffusion from a line into a semi-infinite medium can be applied. Results reported by other workers for ryegrass are used to demonstrate that for a sward the appropriate equation is that describing diffusion from a plane into a semiinfinite medium.

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