The management of soil physical properties limiting crop production in Australian sodic soils - a review

Sodic soils occur extensively on the agricultural lands of Australia. The poor crop productivity of sodic soils is often associated with their low infiltration rates and restricted drainage. This is caused by low macroporosity and macropore instability, due to the presence of sodium on the clay surfaces. To achieve long-term improvements, tillage techniques to increase macroporosity have to be combined with chemical and biological techniques to improve macropore stability. Macropore stability is improved by addition of chemical ameliorants and organic matter. Maintenance of macroporosity also requires protection of the tilled soil from recompaction during flood irrigation, raindrop impact and trafficking. Adverse effects of sodicity of surface soil layers can be corrected by incorporating gypsum and by using conservation farming practices to add organic matter and to protect the surface from mechanical disturbance and raindrop impact. Subsoil sodicity can be corrected by combining deep ripping with chemical ameliorant additions, but the beneficial effects are often quickly lost under flood irrigation and trafficking. Longer term increases in crop production can be achieved by providing surface and subsurface drainage, bed farming and gypsum-slotting. Advantages and disadvantages of these techniques, their application in dryland and irrigated cropping and the areas for future research are discussed. In soils such as Vertisols with high shrink-swell potential, strong-rooted crops such as safflower could be used for biological soil loosening, through deep soil profile drying. The effectiveness of soil ameliorative techniques can be evaluated by assessing the soil factors limiting crop growth during a growing season in a non-ameliorated soil, and the subsequent changes in these soil factors due to the ameliorative practices. A technique which can be used in field studies to monitor these changes through the cropping season, based on the concept of the 'non-limiting soil water range' for crop growth, is described. Irrigation management of sodic soils and re-use of saline drainage waters require an understanding of the changes in soil hydraulic properties with changes in water quality parameters. The 'equivalent salt solution' concept can be used to predict such changes in soil hydraulic properties. These predicted values could then be used in existing water flow models for assessing water and salt flow through irrigated sodic clay soils.