Soil properties, root responses and production of irrigated pasture on a red-brown earth after subsoil modification

Soils used for growing irrigated pasture in northern Victoria have shallow (0.10-0.15 m) topsoils and dense, clay subsoils. Reducing soil limitations to root growth and function has the potential to increase pasture production. Therefore, subsoil modification treatments, comprising loosening and fragmentation of the B horizon to depths of 0.2, 0.4 and 0.6 m, the incorporation of calcium ameliorants, superphosphate and application of organic matter to the soil surface, were compared with unmodified soils and a 0.4 m deep topsoil, in a red-brown earth over a 2-year period (1994-96). Subsoil modification improved the physical condition of the soil. Air-filled porosity of the deep-modified soils (0.4 and 0.6 m deep), 24 h after irrigation, exceeded 15% at 0.1 and 0.3 m depths in both years, and was always higher than in the unmodified soils. Penetrometer resistance was markedly lower in the deep-modified subsoils in both years. Greater earthworm activity was associated with these improved soil physical conditions. However, the soil treatments did not result in greater root length or root mass beneath the pasture. In the first year, pasture yield was greater on the deep-modified soils (25-31 t DM/ha) than on the unmodified soils (19 t DM/ha). In the second year, the yield advantage of the deep-modified treatments decreased to 11%, despite the maintenance of improved soil physical conditions. In both years, pasture yields were highest where there was an initial surface application of 20 t DM/ha organic matter to a modified soil. We conclude that structural modification of the B horizon can increase pasture yields on this soil. However, further research is necessary to maintain these yield increases beyond the initial year and to develop more practical systems of soil modification.