Cation exchange, hydrolysis and clay movement during the displacement of saline solutions from soils by water

Abstract

Deterioration of soil physical conditions occurs when rain or irrigation water displaces soluble salts during reclamation and subsequent management of salinesodic soils. Damage, which depends primarily on the presence of exchangeable Na⁺, appears to be ameliorated during leaching by exchange of Ca²⁺ and Mg²⁺ for Na⁺ and loss of exchangeable Na⁺ by hydrolysis. The extent of these processes has been measured by leaching 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_cl⁻¹) initial salinities. Structural deterioration was monitored by changes in flow rate, and soil properties were measured both initially and after cutting the leached columns into layers. Preliminary studies established reliable methods for measuring exchangeable cations and cation exchange capacity in the saline soils. In a sandy loam (Na-Ca system), clay dispersion and movement occurred particularly in the upper layers as measured both by decreases in CEC and by the amount of clay in the leachate. Cation exchange and hydrolysis of exchangeable Na⁺ during leaching reduced the exchangeable Na⁺ percentage, although cation exchange was restricted to columns with high initial salinity. In a clay textured soil (Na-Ca system) there was negligible clay movement, and cation exchange and hydrolysis occurred in columns with both high and low initial salinities: cation exchange may have been encouraged by diffusion limited preferential release of Na⁺ from aggregates during by-pass flow. In the sandy loam (Na-Mg system) Mg²⁺ increased the preference of the soil for exchangeable Na⁺ compared to the Na-Ca system. There was no cation exchange even in columns with high initial salinity. The amounts of clay movement and hydrolysis were similar in the two systems. Conditions conductive to cation exchange are a high initial salinity, a Na-Ca rather than a Na-Mg system and, possibly, restricted release of the divalent cation from within soil aggregates. Attempts to model these changes are complicated by difficulties in predicting the effects of hydrolysis and by-pass flow.