Hydrostructural characteristics of two African tropical soils

This paper describes the characteristic shrinkage curve of a soil as it dries. It also introduces a new method to measure specific soil moisture states such as permanent wilting point and field capacity using the shrinkage curve. The model considers the soil fabric as a non-rigid, aggregated and unsaturated soil water medium. Its functional parameters (defined as pedohydral parameters) are the independent parameters of the shrinkage curve, which defines two major aspects of the structure of the fabric: (i) the volumetric functional elements of the soil fabric, and (ii) the arrangement of solid, water and air as functions of water content. Two African tropical soils were characterized and analysed according to a structural model with parameters of their shrinkage curve. The parameters of four soil horizons for the two soil types were determined continuously along their shrinkage curves. They were then used as descriptive variables representing the soil's hydrostructural behaviour in a canonical variate analysis, the results of which showed the A horizons to be distinct from the lower horizons, which appear as continua down the profiles. Analysis of the results reveals the importance of the clay and of the iron/clay ratio in the hydrostructural properties of the two soils. Transitional points of the shrinkage curve matched well some particular moisture states of the water potential curve, such as wilting points and field capacities. A more precise method of calculating water-holding characteristics and other structural properties such as air capacity and swelling index can be obtained from the characteristic shrinkage curve.