Improving soil physical fertility and crop yield on a clay soil in Western Australia

In the low rainfall area of Western Australia, clay soils with massive soil structure form a major part of the area sown to wheat. Yield increases on such soils have been poor in the last decade compared with those on other soil types. An experiment was conducted over 4 years (1997-2000) using a factorial combination of soil ripping to 0.4 m, application of commercial grade gypsum at 2.5 t/ha, and addition of complete nutrients based on soil test each year. All crop residues were retained after harvest and returned to the soil. The experiment was conducted in a wheat-field pea rotation at Merredin, WA. Soil water infiltration rate, soil strength, bulk density, water-stable aggregates, cation exchange capacity, and wheat yields were measured. Grain yields of wheat and field peas were increased by deep ripping, the addition of gypsum, or the addition of complete nutrients in some years. The main treatment effects on yield were additive, as significant interactions between the treatments on yield were seldom found. However, all the main treatments also significantly improved many of the soil physical properties related to crop growth. In 2000, 4 years after the treatments were applied, soil water infiltration rate was increased by more than 200%, strength of the topsoil decreased by around 1600 kPa, and soil bulk density decreased by 20%. Gypsum application increased water-stable aggregates, but soil mixing caused by deep ripping reduced them. The combination of soil ripping and gypsum application in the presence of complete nutrients and annual return of crop residues to the soil is suggested to improve crop grain yield and soil physical fertility on a range of Western Australian soils.