DYNAMICS OF PHOSPHORUS DURING RECLAMATION OF SODIC SOILS

Abstract

This study describes the phosphorus status of sodic soils formed under the influence of sodium carbonate, aquic natrustalf, and its possible leaching during their reclamation. Uncultivated, barren sodic soils contained high amounts of Olsen's extractable phosphorus, which decreased with depth, and the amounts were highly correlated with the electrical conductivity of the soil. The incubation studies conducted at saturated moisture content with differential amounts of gypsum at 0, 25, 50, 100, and 200 percent of laboratory-determined gypsum requirement (GR) showed that Olsen's P decreased with increases in the rate of gypsum applied. The leaching studies conducted in the field showed that phosphorus in sodic soils moved down, along with other soluble salts, though the peak P concentration lagged behind the other salts. Leaching studies conducted in plexiglass columns in the laboratory brought out that substantial leaching of P, up to 60 ppm P, could occur when sodic soils are leached as such or after surface application of gypsum. It seems that under natural conditions, Na2CO3, which is the predominant salt in sodic soils, reacts with native calcium phosphate to form highly soluble sodium phosphates, which are leached during reclamation. Gypsum, mixed with soil at a rate of 100 percent GR and above, decreases the leaching of phosphorus. It seems due to (1) immobilization of soluble P by CaSO4 through the formation of less soluble Ca-P compounds; (2) common ion effect to decrease solubility of Ca-P compound; and (3) lowering of soil pH, which in turn decreases the soluble P in the pH range of 8.3 to 10.