Dynamics of calcium‐sodium exchange under unsaturated plow conditions

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Abstract
An experiment was conducted in soil columns of 84 cm length and 5.7 cm inner diameter to determine the effect of degree of water saturation on the extent of replacement of sodium by calcium from the exchange sites. The experiment was performed by running 0.20 N calcium chloride solution through a saline‐sodic soil into a water table at the lower end at different rates less than the infiltration rate of the soil, thus obtaining unsaturated flow conditions. The effective or the apparent calcium exchange capacity of the soil was reduced significantly and nearly proportionally with a decrease in the degree of water saturation during the flow process. It is suggested that part of exchange sites become inaccessible during the unsaturated flow and that there must be some isolated locations away from the main water stream in which mixing or molecular exchange by diffusion is negligible.