Wendy Vance

Physical subsoil constraints, such as high soil strength, low porosity or unfavourable pore characteristics, impair crop water use, either through effects on water availability or the ability of the crop to access the water. By reducing the capacity of the soil to store water or by impeding infiltration or drainage, physical subsoil constraints can alter the availability of water to the crop. By delaying root exploration, reducing ultimate rooting depth or reducing the efficiency with which water is extracted from a soil zone, they can reduce the crop’s ability to access water present. The resultant impact on crop water use is modulated by factors including the amount and distribution of rainfall, the soil’s water holding capacity and the depth and severity of the constraint. While the processes by which subsoil constraints influence crop water uptake are generally well-understood, important aspects still need clarification or quantification. There are still many questions regarding processes of water transfer from the bulk soil to the roots’ vascular elements. New knowledge will need to be effectively linked with our understanding of water uptake at the scale of the crop or soil profile. There is also a need to improve knowledge of the influence of agronomic management on pore size distribution, continuity and stability in terms of their influence on root system development. Finally, simulation studies that evaluate the interaction of access to water with differing soil types and climatic zones will provide important extrapolation to allow the agronomic importance of subsoil constraints to be quantified in the context of inter-annual variation in rainfall distribution.