Relationships between clay mineralogy and the hardsetting properties of soils in the Carnarvon horticultural district of Western Australia

We investigated the relationship between clay mineralogy and the hardsetting properties of soils used for intensive irrigated horticulture in the Carvarvon horticultural district of Western Australia. Hardsetting soils disintegrate when wetted due to a combination of slaking and dispersion processes, resulting in a structureless mass of soil when dry. Soil samples were taken from several horizons of six profiles with hardsetting problems. The profiles were located on six farms in the Carnarvon horticultural district. We measured the cation exchange capacity (CEC) on bulk samples and after saturating with sodium (Na), measured the tensile strength of remoulded cores. We also measured the clay mineralogy on clay separates using X-ray diffraction (XRD) analysis, and CEC by X-ray flourescence (XRF) analysis after barium (Ba) saturation. Samples were also investigated using transmission electron microscopy (TEM). The tensile strength of the Na saturated remoulded cores was positively and highly correlated(p ≤ 0.001) with both clay % and CEC of the soil. XRD indicated that the soil clays mainly consisted of illite with some kaolinite, but with very little smectite. However, the measured CECs (by Ba saturation) were higher than expected for this mineralogy. A detailed clay mineral interpretation using Pseudo-Voigt peak-shape algorithms confirmed the presence of appreciable concentrations of interstratified illite/smectite in all soil clays, which accounted for the CEC results. A detailed TEM study revealed different d spacings (corresponding to different clay minerals) in the same crystal, thus confirming the presence of interstratified materials. The degree of slaking and dispersion on wetting of these soils increases with the charge on sodium saturated soil clays; as a result, illite/smectite interstratified materials will disperse more than pure kaolinites or illites, thus enhancing hardsetting properties. Dispersion can be controlled by addition of calcium electrolytes, such as gypsum (CaSO₄-2H₂O), to the soil. Our results point the way to managing these difficult, hardsetting soils, particularly with respect to how much gypsum should be applied.