CATION-EXCHANGE, HYDROLYSIS AND CLAY MOVEMENT DURING THE DISPLACEMENTOF SALINE SOLUTIONS FROM SOILS BY WATER

Deterioration of soil physical conditions occurs when rain or irrigati on water displaces soluble salts during reclamation and subsequent man agement of saline-sodic soils. Damage, which depends primarily on the presence of exchangeable Na+, appears to be ameliorated during leachin g by exchange of Ca2+ and Mg2+ for Na+ and loss of exchangeable Na+ by hydrolysis. The extent of these processes has been measured by leachi ng columns of repacked soil with water after preparation with Na+ and Ca2+ or Na+ and Mg2+ as the exchangeable cations and high or low (1 or 0.1 mol(c) l(-1)) initial salinities. Structural deterioration was mo nitored by changes in flow rate, and soil properties were measured bot h initially and after cutting the leached columns into layers. Prelimi nary studies established reliable methods for measuring exchangeable c ations and cation exchange capacity in the saline soils. In a sandy lo am (Na-Ca system), clay dispersion and movement occurred particularly in the upper layers as measured both by decreases in CEC and by the am ount of clay in the leachate. Cation exchange and hydrolysis of exchan geable Na+ during leaching reduced the exchangeable Na+ percentage, al though cation exchange was restricted to columns with high initial sal inity. In a clay textured soil (Na-Ca system) there was negligible cla y movement, and cation exchange and hydrolysis occurred in columns wit h both high and low initial salinities: cation exchange may have been encouraged by diffusion limited preferential release of Na+ from aggre gates during by-pass flow. In the sandy loam (Na-Mg system) Mg2+ incre ased the preference of the soil for exchangeable Na+ compared to the N a-Ca system. There was no cation exchange even in columns with high in itial salinity. The amounts of clay movement and hydrolysis were simil ar in the two systems. Conditions conductive to cation exchange are a high initial salinity, a Na-Ca rather than a Na-Mg system and, possibl y, restricted release of the divalent cation from within soil aggregat es. Attempts to model these changes are complicated by difficulties in predicting the effects of hydrolysis and by-pass flow.