The Physical Properties of Mixtures of A and B Horizons and Evaluation of Profile Mixing for Management of Trangie Red-Brown Earth

Abstract

Conventional methods of seedbed preparation have caused crop yields to decline on red–brown earths. This yield decline is attributed to reduced physical fertility brought about by the collapse of soil structure. Regeneration of favourable structure may be achieved by several means, one of which is mixing of subsoil with topsoil. Deep mouldboard tillage, which mixes A and B horizon soil, has increased cotton yield on some irrigated red–brown earths in the Lower Macquarie Valley of New South Wales. The beneficial effect of deep mouldboard ploughing may be enhanced by gypsum application. Deep mouldboard tillage loosens the topsoil to approximately 400 mm and changes the texture of the plough layer by mixing clay subsoil with loam topsoil. However, the physical basis for the yield response to deep mouldboard tillage has not been thoroughly investigated. The morphology, particle size distribution and clay mineralogy of a red–brown earth on the Agricultural Research Centre, Trangie, N.S.W., indicates that it is prone to structural degradation when used for irrigated cultivation. The aim of this study is to quantify the compaction constraints to soil physical fertility of the Trangie red–brown earth and to determine the likely beneficial effects of deep mouldboard tillage. The results of an investigation of the penetration resistance, soil water and oxygen transmission properties affecting the physical fertility of Trangie red–brown earth soil as a function of compaction and soil horizon mixing is reported in this thesis. The effect of gypsum application on soil penetration resistance is also reported.