In texture-contrast soils a significant proportion of stored soil water is held in the B-horizon, which often has seriously-limited physical and chemical properties. The extent to which physical properties limit soil water availability on the texture-contrast soils of the upper Eyre Peninsula, South Australia, is thought to be significant, but this has never been quantified. In this study, the concept of the Integral Water Capacity (IWC), which uses weighting functions to account for various physical constraints on soil water, was applied to calculate an ‘effective’ water capacity in line with plant-water uptake in the field. The IWC quantified the limitations and allowed an estimate to be made of the potential benefits from any strategies applied to ameliorate the physical constraints.

Calculation of the IWC for a typical soil profile (0-0.8m) was only 37 mm, which was 90 mm less than that expected from a simple calculation of the difference between water contents at field capacity and wilting point. The large difference was caused by soil resistance exceeding 2.5 MPa in some horizons at matric heads near field capacity. Nevertheless, roots appeared to explore macropores in some strong horizons, which facilitated exploration to deeper layers. In these cases, weighting functions were not applied and the magnitude of the IWC was larger. In layers with no macropores and no roots, penetrometer resistance and low aeration were the principal restrictions to root growth. Appropriate weighting functions were thus applied to estimate the magnitude of the restriction on available water, and this corresponded well with estimates of plant-water uptake.