THE EFFECT OF TILLAGE AND GYPSUM APPLICATION ON THE MACROPORE STRUCTURE OF AN AUSTRALIAN VERTISOL USED FOR IRRIGATED COTTON

Structural degradation of Vertisols (cracking grey clay soil) as a result of growing cotton under furrow irrigation has resulted in declining cotton yields. This degradation may be ameliorated with suitable management. A field experiment at Warren, N.S.W., Australia was set up to assess the long-term effects of repeated deep ripping plus gypsum, as compared with the more traditional shallow cultivation, on the physical condition and cotton lint yield of a sodic Vertisol. The soil was sampled 7 years after the experiment began for an assessment of soil macropore structure. Soil samples from successive depth intervals down the profile were impregnated with resin and the macropore structure described visually and quantitatively using image analysis. Other physical and chemical attributes such as shear strength, bulk density, pore space relations, pH, exchangeable sodium percentage, electrical conductivity, total cations, calcium to magnesium ratio and organic matter, were also measured from the same soil profiles. Results from both image analysis and conventional soil measurements indicated there was a structurally degraded zone from 25 to 60 cm in the shallow cultivated treatment. This was in contrast to a more porous and weaker soil in the deep ripped plus gypsum treatment that produced slightly larger cotton yields. The image-analysis data were a valuable supplement to the more conventional methods of quantifying soil structure.