Physical properties of some highveld Vertisols

Abstract

Surface (0–20 cm) soils were sampled at 100 m intervals along a 3 km transect through Vertisols derived from dolerite and Ecca group mudstone near Standerton, Transvaal. The two geologically correlated soil bodies, which historically appear to have formed the basis for use of either crusting or self-mulching surface habits as local classification criteria, could be clearly distinguished and compared from two main segments of the transect. Comparison of physical properties suggested that the soil bodies could be separated by the following values of each property, air-water permeability ratio of 63; coefficient of linear extensibility: 10%; modulus of rupture: 70 kPa; liquid limit: 45%; and plastic limit: 24%. The self-mulching, dolerite-derived soils had values generally in excess of these limits, The high MOR values for the latter soils indicate that strong crusting potential and self-mulching behaviour are not mutually exclusive and therefore cannot be used as classification differentiae. It is suggested that an alternative basis for separating the two soil bodies may be sought in a revised definition of the vertic A horizon.