There is a need for a conceptual and analytical model to describe and interpret the common shrinkage curves of structured soil samples. In this work, we propose a new device for direct shrinkage measurement of unconfined structured soil samples and discuss the relevance of parametric models of the shrinkage curve to fit the experimental data.The experimental procedure consists of simultaneous and continuous measurements of the diameter, height, and weight of an initially saturated soil sample as it dries. The shrinkage measurement can be completed for the full moisture range in a short time (2-3 d), and all shrinkage phases can be established easily and accurately identified.

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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.