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 The review focuses on information from Australia (published and unpublished), using overseas data only where no suitable Australian data is available.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.

High ear number is key to achieving high wheat yields in the high-rainfall zone of south-western Australia

The growth and yield of spring wheat (Triticum aestivum L.) were examined to determine the actual and potential yields of wheat at a site in the high rainfall zone (HRZ) of south-western Australia. Spring wheat achieved yields of 5.5−5.9 t/ha in 2001 and 2003 when subsurface waterlogging was absent or minimal. These yields were close to the estimated potential, indicating that a high yield potential is achievable. In 2002 when subsurface waterlogging occurred early in the growing season, the yield of spring wheat was 40% lower than the estimated potential. The yield of wheat was significantly correlated with the number of ears per m2 (r2 = 0.81) and dry matter at anthesis (r2 = 0.73). To achieve 5–6 t/ha of yield of wheat in the HRZ, 450–550 ears per m2 and 10–11 t/ha dry matter at anthesis should be targetted. Attaining such a level of dry matter at anthesis did not have a negative effect on dry-matter accumulation during the post-anthesis period. The harvest index (0.36−0.38) of spring wheat was comparable with that in drier parts of south-western Australia, but relatively low given the high rainfall and the long growing season. This relatively low harvest index indicates that the selected cultivar bred for the low- and medium-rainfall zone in this study, when grown in the HRZ, may have genetic limitations in sink capacity arising from the low grain number per ear. We suggest that the yield of wheat in the HRZ may be increased further by increasing the sink capacity by increasing the number of grains per ear.

Grain yield production in relation to plant growth of wheat and canola following clover pastures in southern Victoria

The plant growth and grain yield of crops following a pasture phase in 1:1 pasture–crop rotations were studied in southern Victoria in 2001 (wheat and canola at Hamilton, and wheat at Streatham and Gnarwarre). Both the wheat and canola crops produced high grain yields with no application of nitrogen fertiliser. In experiment 1 (at Hamilton) where the crops were dependent on nitrogen input from subterranean clover pasture, canola produced 4.1 t/ha of grain and wheat averaged 6.0 t/ha. The 3 canola cultivars (Charlton, Mystic and Surpass 400) had similar grain yields. However, for wheat, the late-maturing spring wheat cv. Kellalac and the early-maturing spring wheat cv. Silverstar produced significantly higher grain yields (6.6 and 6.3 t/ha, respectively) than the late-maturing winter cv. Brennan (5.0 t/ha). The 3 cultivars of each crop differed markedly in their major yield components. The most striking differences were those shown by Silverstar, which had the highest yield, together with Kellalac, but had lower biomass and lower leaf area index than the 2 late-maturing wheats. Silverstar compensated by having 50% more grains per head than the late-maturing Brennan. While Silverstar flowered on average 34 days earlier than the 2 other wheats, it took some 3 weeks longer to mature after anthesis. In experiment 2, the wheat crop (cv. Silverstar) produced grain yields of 5.4 t/ha over 6 different treatments, with higher grain yields at Streatham (6.1 t/ha) than at Gnarwarre (4.7 t/ha). Across the 2 sites, the grain yields following clovers reached over 5.7 t/ha, in contrast with low grain yields from the continuous crop (3.7 t/ha) and fallow/crop treatments (3.7 t/ha). Grain yields were closely related to the herbage dry matter production of previous pasture legumes, indicating a positive crop response. This may, in turn, reflect the nutrient status of the treatments, particularly the nitrogen status.