STUDIES ON EXCHANGE OF SODIUM FROM SOILS BY LEACHING WITH CALCIUM SULFATE

Abstract

The exchange of Na from soils and clay minerals was studied by leaching columns of the materials with calcium sulfate. The exchangeable Na percentage was rapidly reduced to 5 to 10%; further reductions were slow and required large amounts of leaching solution. After Na no longer appeared in the leachate, from 3 to 6% exchangeable Na remained on the clay. Complete replacement of Na by leaching was not possible. Replacement of Mg did not begin until the rapid exchange of Na was completed. Differences in solute mixing among the different clays were more important than differences in Na-Ca exchange constants, resulting in an efficiency of replacement in the order: attapulgite > kaolinite > halloysite illite > mont-morillonite. The initial rapid exchange of Na lead to a uniform concentration of Na remaining where less than symmetry amounts of Ca were used. When solid Ca sulfate is mixed with soil at the top of the column, it reaches a maximum concentration in leaching water of about 1/3 its saturated concentration. A small fraction appears to remain undissolved, even after prolonged leaching.