Ameliorating dense clay subsoils to increase the yield of rain-fed crops

The yield of rain-fed crops needs to increase to meet the requirements of a growing human population. One way to achieve this is to store more rain in the root zone of soils and increase its use by crops. This will be difficult where poorly structured, dense clay subsoils occur. These subsoils limit crop yields by reducing root growth, and the uptake of soil water and nutrients. Reduced root growth results from physical factors, such as low porosity restricting the movement of air and water, high soil strength and temporal water logging which impedes the roots. Chemical factors such as alkalinity, salinity, and sodicity may further restrict root growth in neutral pH and alkaline clay subsoils. This review will focus on the challenging task of improving structure in these dense clay subsoils and to overcome physicochemical constraints in the subsoil, in order to increase crop yields. Early attempts to ameliorate these subsoils using tillage practices, and amendments in some cases, were rarely successful. A more recent and successful approach has been to deep-band nutrient-rich organic amendments in the subsoil, and to follow with a field crop. This increased root growth in the subsoil, and was often associated with improved subsoil structure. It was able to produce substantial and prolonged increases in crop production. A key benefit was an increase in plant available water in the subsoil and the utilization of this water by the crop. The impact of the practice depends on the amount and distribution of rainfall, and this results in increased financial risk for the farm business. Specialized machinery will need to be developed to handle the amendments, while the form, rate of incorporation, and placement depth of amendments will need to be optimized to reduce high upfront costs of the practice.