ION INTERACTIONS AND CONSTRAINTS TO PLANT NUTRITION IN AUSTRALIAN SODIC SOILS

Many of the arable soils in Australia are affected by salinity and/or sodicity. Nutrient deficiency and ion toxicity may occur in both salin e and sodic soils. Ho-ever, the mechanism for these constraints on pla nt growth in sodic soils differs from that of saline soils. Fertility of sodic soils with low nutrient reserves is compounded by the low sup ply of water and oxygen to roots in profiles with dispersive clays. Nu trient constraints in sodic soils are created by the electron and prot on activities (pE and pH) in an environment of degraded soil structure . Australian sodic soils accumulate relatively low levels of organic m atter. High sodium, high pH and low biological activity, commonly foun d in these soils, are not conducive for both the accumulation of organ ic matter and its mineralization. As a result, these soils are deficie nt in N and S. Australian soils are highly weathered and have moderate to low reserves of many plant nutrients such as Cu, Mn, Mo, Zn and P. Solubility of phosphorus is generally increased in sodic soils. Poor leaching conditions accumulate boron in soil layers. Higher concentrat ions of sodium than of calcium in these soils are the major cause of b oth physical and nutritional problems. Therefore, amelioration of sodi city is the logical first step in improving the chemical fertility of sodic soils. However, fertilizer application and improvement of soil o rganic matter are essential to increase yields to match the potential yield predictable from climate.