A factor in soil aggregation and root development

The third shortcoming centers around the need for a good method for the determination of stable aggregates. The wet-sieve method, now commonly used, is generally recognized to be far from satisfactory. This is illustrated by the experiments of Harris et al. (1963), discussed below and shown in Fig.16.1. If a method had been used that measured true stable aggregates, as occur in nature, the values obtained would doubtless have been zero, or near zero, instead of values ranging up to 100%.. 

In aggregate stabilization we are by definition (Martin et al. 1955) concerned with both the forces and conditions that exist within the aggregate, and with those that exist on the outside. The former are so much more important that the latter are often ignored. 

Most studies of aggregate formation, as already stated, have been made with surface soils that contain considerable organic matter and other stabilizing substances. This fact should be kept in mind continually in considering the discussion of aggregate formation that follows. Undoubtedly the presence of these stabilizing substances does under some conditions affect aggregate formation. 

In the present discussion, dealing with the factors involved in aggregate formation, and later with aggregate stabilization, reference will be primarily to synthesis from single grains rather than to the disintegration of clods. This simplifies the discussion, and also covers the phase of the subject of most immediate interest and the least understood. 

When thawing occurs, soils are subjected to the effects of wetting, discussed above, and some of these effects cause aggregate deterioration. It is not always possible to draw a clear line of distinction between the freezing and wetting effects. 

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