THE EFFECT OF CLAY ADDITION AND GYPSUM APPLICATION ON THE PHYSICAL-PROPERTIES OF A HARDSETTING RED BROWN EARTH, AND THE RESPONSE OF IRRIGATED COTTON

The growth of furrow-irrigated cotton on red-brown earths (Alfisols) of the lower Macquarie Valley in New South Wales (Australia) is often restricted by slow water penetration and poor seedling emergence, associated with structural instability and hardsetting of the soil surface. However, local landholders have overcome the problem to some extent by increasing clay content at the soil surface by using deep mouldboard ploughing (to 0.45 m) and subsequent gypsum applications. A study was carried out to determine how mouldboarding had changed the physical condition of a red-brown earth near Narromine, why the water relations had been improved, and why the effect of this operation does not appear to be permanent. Cotton growth was measured at two sites, one ploughed and treated with gypsum (PG) and the other without these treatments (C). Both sites were also deep ripped just before planting. Field studies were complemented with laboratory experiments in an attempt to separate the effects of texture modification and gypsum application. The hydraulic conductivities of surface soil and a surface-subsoil mix, each with and without gypsum, were measured. The time for columns of these soils 0.05 and 0.2 m in height to wet to the surface, and their degree of expansion or slumping on wetting, were also determined. Cotton growth was over 100% greater where the PG treatment had been applied. This improvement was associated with greater, and more uniform, water penetration into the furrows and ridges. Despite a reduction in organic matter content and exchangeable Ca: Mg ratio, and an increase in exchangeable sodium percentage, soil from the PG site had a lower dispersion index and strength, and higher hydraulic conductivity and electrical conductivity near the surface. It also had more clay in the surface, and a higher content of smectite. The hydraulic conductivity experiment suggested that gypsum may have a similar beneficial effect on both sites. In contrast, the rate of wetting experiment, which gave a better simulation of soil physical processes in the field, showed that even with gypsum the C treatment could not maintain a favourable structure. The need for further experimentation is discussed.