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SumnmyThe micropore (< IOO A) size distributions of compacted clay mineral systems have been examined by means of low-temperature Nt sorptionisotherms to saturation. The calculation of pore sizes was based on the application of the Kelvin equation, corrected for the adsorbed f i l m thickness, to the parallel plate model.Clay-mineral systems have been shown to exist with a high degree of parallel alignment of the plate-shaped crystals. This results in a high proportion of microporosity and in relatively discrete pore sizes.In montmorillonite clap much intercrystalline overlap area in the dry matrix is inaccessible to N, sorption. The structure of these systems may be governed partly by the effect, in suspension, of electrostatic interaction between the charged particles on crystal size, but largely by mechanical interaction on sedimentation and the size of the exchangeable catiom present.

A model has been presented to illustrate the way in which the influence of exchangeable Na on the fundamental processes of dispersion and flocculation on Na-Ca systems affects the various soil physical properties in the field. Most cultivated soils slake (breakdown into microaggregates) when subjected to rapid wetting, giving rise to a surface seal and a reduction in infiltration rate. However, slaking alone may not neccessarily reduce the soil's productivity, e.g. surface aggregates of the highly productive self-mulching black earths slake even when in the virgin state. If dispersion follows slaking, in most cases it will lead to poor physical properties which may manifest as poor drainage, surface crusting. hardsetting and poor trafficability or workability of the soil and eventually lead to reduced crop yields.It is the dispersion phase that is affected by the presence of excessive sodium on the exchange complex of the soil, and this may have a profound effect on the soil's physical properties and behaviour.This paper reviews the possible mechanisms by which excessive sodicity may manifest in undesirable soil physical behaviour. It also attempts to relate observations made in the laboratory on pure Na-Ca-clay systems to the behaviour of the soil in the field.The effect of sodium on the dispersive behaviour of a soil is discussed in relation to its hydraulic conductivity and the processes of infiltration, redistribution and evaporation of water which in turn affects the subsoil water storage in a soil profile. The presence of sodium is also discussed in relation to changes in soil strength characteristics, the soils workability and ease of tillage and ultimately the soil's productivity. Data are presented which show that the validity of a threshold ESP and the exclusive use of ESP as a measure of sodicity are open to question.

To date no experimental technique has been capable of directly and repetitively measuring spatial distributions of soil water content in a non-destructive manner. The potential of computer assisted tomography (CAT) to overcome this problem has been examined in this paper. The results obtained from a commercially-produced X-ray CAT scanner and a conventional gamma scanner suggest that CAT scanning can be used to determine spatial changes in soil water content with adequate resolution for soil-plant studies. The technique can clearly be used to resolve spatial changes in soil water content with time. Application of the technique to water uptake by a single plant root shows that CAT scanning presents an extremely exciting possibility for studies of soil-plant water relations.

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