Ion interactions and constraints to plant nutrition in Australian sodic soils

Abstract

Many of the arable soils in Australia are affected by salinity and/or sodicity. Nutrient deficiency and ion toxicity may occur in both saline and sodic soils. Ho-ever, the mechanism for these constraints on plant growth in sodic soils differs from that of saline soils. Fertility of sodic soils with low nutrient reserves is compounded by the low supply of water and oxygen to roots in profiles with dispersive clays. Nutrient constraints in sodic soils are created by the electron and proton activities (pE and pH) in an environment of degraded soil structure. Australian sodic soils accumulate relatively low levels of organic matter. High sodium, high pH and low biological activity, commonly found in these soils, are not conducive for both the accumulation of organic matter and its mineralization. As a result, these soils are deficient 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 concentrations of sodium than of calcium in these soils are the major cause of both physical and nutritional problems. Therefore, amelioration of sodicity is the logical first step in improving the chemical fertility of sodic soils. However, fertilizer application and improvement of soil organic matter are essential to increase yields to match the potential yield predictable from climate.