Subsoil constraints to crop production on neutral and alkaline soils in south-eastern Australia: a review of current knowledge and management strategies

Crop yield variability and productivity below potential yield on neutral and alkaline soils in the semiarid Mediterranean-type environments of south-eastern Australia have been attributed, in part, to variable rooting depth and incomplete soil water extraction caused by physical and chemical characteristics of soil horizons below the surface. In this review these characteristics are referred to as subsoil constraints. This document reviews current information concerning subsoil constraints typical of neutral and alkaline soils in south-eastern Australia, principally salinity, sodicity, dense soils with high penetration resistance, waterlogging, nutrient deficiencies and ion toxicities. The review focuses on information from Australia (published and unpublished), using overseas data only where no suitable Australian data is available. An assessment of the effectiveness of current management options to address subsoil constraints is provided. These options are broadly grouped into three categories: (i) amelioration strategies, such as deep ripping, gypsum application or the use of polyacrylamides to reduce sodicity and/or bulk density, deep placement of nutrients or organic matter to overcome subsoil nutrient deficiencies or the growing of ‘primer’ crops to naturally ameliorate the soil; (ii) breeding initiatives for increased crop tolerance to toxicities such as salt and boron; and (iii) avoidance through appropriate agronomic or agro-engineering solutions. The review highlights difficulties associated with identifying the impact of any single subsoil constraint to crop production on neutral and alkaline soils in south-eastern Australia, given that multiple constraints may be present. Difficulty in clearly ranking the relative effect of particular subsoil constraints on crop production (either between constraints or in relation to other edaphic and biological factors) limits current ability to develop targeted solutions designed to overcome these constraints. Furthermore, it is recognised that the task is complicated by spatial and temporal variability of soil physicochemical properties and nutrient availability, as well as other factors such as disease and drought stress. Nevertheless, knowledge of the relative importance of particular subsoil constraints to crop production, and an assessment of impact on crop productivity, are deemed critical to the development of potential management solutions for these neutral to alkaline soils.