Improving grain yields on a sodic clay soil in a temperate, medium-rainfall cropping environment

Soil constraints are a major limitation to grain production on waterlogging-prone sodic soils in the medium-rainfall zone of southern Australia, and several options have been proposed to overcome these constraints. A field experiment commenced in 1999 to compare the effectiveness of different management strategies, including improved crop nutrition, soil amelioration by using gypsum with or without deep ripping, applying organic matter, using raised beds or delayed sowing on improving the growth and grain yields of four consecutive crops including wheat (Triticum aestivum) in 1999 and 2002, barley (Hordeum vulgare) in 2000, and faba beans (Vicia faba) in 2001. Improving crop nutrition alone generally did not significantly improve grain yields, whereas adding ameliorants such as composted pig bedding–litter or deep ripping+gypsum produced grain yield increases in all crops by up to 48% compared with the control. Similar increases in grain yields were produced when crops were grown on raised beds, even in seasons when growing-season rainfall was well below average. Greatest yield increases were recorded when both raised beds and ameliorants were used (up to 2t/ha, or 63%). Spring-sown crops consistently produced lower grain yields than the (autumn-sown) control. For the three cereal crops (two wheat and one barley), increases in grain yields resulting from soil amelioration generally were not associated with increased harvest index or kernel size but were associated with greater tiller number and number of grains per m2. For the pulse crop, faba beans, yield increases were associated with greater dry matter production and increased number of grains per m2. All management strategies significantly increased crop nitrogen (N) uptake, although this did not necessarily translate to increased grain protein because of a dilution effect in the highest yielding treatments. Increases in grain yield coincided with improved root growth throughout the profile (up to 140cm depth). All physical amelioration treatments either reduced the degree of temporary waterlogging, as indicated by shallow piezometers, or improved soil structure, as indicated by reduced cone penetrometer resistance, compared with the control. Reduction in soil exchangeable sodium percentage on this highly sodic clay soil, measured within the first season after implementation, was less clear-cut. Increases in grain yield, however, appeared related to improved N supply rather than greater water use. Large increases in grain yields across a range of seasonal conditions appear possible on these soil types in medium-rainfall environments provided both soil structure and nutrition are improved.