Diffusion and cation exchange during the reclamation of saline-structured soils

Undisturbed blocks of a clay subsoil were equilibrated with solutions of 0.1, 0.3 and 1 molc l−1 of NaCl+CaCl2 and NaCl+CaCl2+MgCl2, with sodium adsorption ratio (SAR) values between 8 and 12. These were then immersed in water and the cumulative release of Na+ and Ca2+ by diffusion was measured over 90 days. The composition of the solution in the soil and the amounts of exchangeable Na+ and Ca2+ were measured before and after diffusion. The reduction in salinity in the blocks caused exchange of Ca2+ for Na+, hydrolysis of Na+ and a reduction in exchangeable sodium percentage (ESP) due both to dilution and to the more rapid diffusion of Na+ than Ca2+ out of the soil. The exchange caused a reverse concentration gradient to be established for Ca2+ so that between 15 and 100 h after immersion, Ca2+ was diffusing back into the soil. During the leaching of heavy textured, structured, saline, sodic soil containing both mobile and immobile water, the differential rates of diffusion of Na+ and Ca2+ may contribute to the reclamation of sodicity as salt is removed.