INFLUENCE OF SALT CONCENTRATION AND SODICITY ON THE SOLUBILITY OF PHOSPHATE IN SOILS

The effects of electrolyte concentration and exchangeable sodium (sodicity) on the solubility of native phosphate (P) in some Brown Chernozemic soils (Haploborolls) from Saskatchewan, Canada were evaluated using a flow (miscible displacement) technique. Phosphate concentration was measured in leachate from soil columns treated with a series of NaCl-CaCl2 solutions having fixed sodium adsorption ratio (SAR) with electrical conductivity (EC) decreasing from about 10 to 0.1 dS m-1. The results show that the P content of the leachate was strongly influenced by both EC and SAR and that these two variables were interactive in their effect. Leachate P was usually low (< 0.1 mg L-1) when EC was high, but it increased as EC decreased, particularly when SAR was high. Values as high as 3 to 5 mg P L-1 were attained under conditions of low EC and high sodicity (SAR 20-40). Samples of leached soil were subjected to a chemical fractionation procedure to determine how the various forms of inorganic P responded to the imposed salinity and sodicity treatments. In the absence of Na (SAR 0), there was relatively little change in the distribution of P during the leaching sequence but, as sodicity increased, the water-extractability of P increased substantially. The main effect of sodicity was that it made the weakly adsorbed P fraction (NaHCO3-P) more labile. Increasing sodicity had little effect on the more strongly adsorbed P fraction (NaOH-P) or on Ca-bound P (HCl-P). The results of this study suggest that electrolyte concentration and sodicity may have an important bearing on the P nutrition of crops grown on Canadian prairie soils. However, the widely-used Olsen test appears to be insensitive to the influence of these variables. It is possible that the poor performance of this test noted in recent reports is partly related to salt and/or sodicity effects.