PHYSICAL AND CHEMICAL CHARACTERISTICS OF DUPLEX SOILS AND THEIR DISTRIBUTION IN THE SOUTH-WEST OF WESTERN AUSTRALIA

Duplex soils are widespread in Western Australia and occupy about 60% of the south-west agricultural area. The physical and chemical properties of these soils reflect their origin as erosional surfaces of old laterite profiles and the predominance of underlying, quartz-rich, granitic materials. The resulting combination of high contents of quartz sand, kaolin clay, and iron and aluminium oxides has important consequences on water-holding capacity, fertility, nutrient leaching, phosphate fixation, anion and cation exchange capacity, and buffering capacity. These soils are generally held to have a number of physical and chemical problems. Among these, low fertility and low permeability of the clay B horizon have greatest effect. While data on the chemical properties of these soils are limited, significant data sets on the physical properties of duplex soils are available from several studies of catchment hydrology. These studies reflect concern with runoff, waterlogging, and the development of secondary salinisation. Depth to clay varies from 5 to >100 cm. The shallower depths (10-40 cm) predominate and, in medium and high rainfall areas, combine with low permeability in the clay to result in widespread waterlogging. Commonly, the sand surface can be hardsetting and can have high strength from structural decline, from the presence of cementing agents, and from traffic effects. Sodicity is widespread and has effects on hardsetting in the A horizon and on permeability of the B horizon. Management strategies are available to treat some of these problems, but generally, much has yet to be done to develop sustainable agricultural systems on these soils.