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The Chemistry of the Reclamation of Sodic Soils with Gypsum and Lime¹

ABSTRACT

Sodic soil reclamation was theoretically evaluated assuming equilibrium chemistry and piston movement of soil solution. The effective solubility of gypsum when mixed with a sodic soil is increased because the exchange phase acts as a sink for Ca²⁺ until both the gypsum dissolution and exchange reactions reach equilibrium. The electrical conductivity of a soil solution in equilibrium with both gypsum and an exchangeable sodium fraction (E_Na) of 0.0 and 0.43 is 2.3 and 14 dS m^–1. Thus, mixing gypsum into the soil hastens reclamation and provides higher solution concentrations for the maintenance or improvement of the soil hydraulic conductivity. The amount of gypsum dissolved, expressed in moles of charge (equivalents), is a linear function of the moles of exchangeable Na⁺ replaced; r² values typically exceeded 0.98. The slope of the regression line decreased with increasing final E_Na. Typical values were 1.40, 1.27, and 1.20 moles of charge gypsum dissolved per mole of exchangeable sodium replaced at final E_Na's of 0.05, 0.10, and 0.15. The inclusion of lime equilibrium reduces these slopes by 3, 6, and 9% for P_CO2's of 1, 4, and 10 kPa (1, 4, and 10% CO₂). Gypsum requirements for calcareous, sodic soils based on quantitative replacement of exchangeable sodium should be increased by factors of 1.3 to 1.1 depending on the desired final levels of exchangeable sodium.

¹ Contribution from U.S. Salinity Laboratory, AR-SEA, USDA, and Agricultural Research Organization, The Volcani Center, Bet-Dagan, Israel. Financial support of AR-SEA and the Department of Soil Science and Agricultural Engineering, University of California, Riverside, for the junior author is gratefully acknowledged.

² Soil Scientists, U. S. Salinity Laboratory, and The Volcani Center, respectively.

Received for publication June 25, 1979. Accepted for publication October 4, 1979.

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