RETENTION AND TRANSPORT OF CALCIUM NITRATE IN VARIABLE CHARGE SUBSOILS

Abstract

Salt retention has been observed in many variable charge soils. However, studies on the extent of the nitrate salt retention in tropical and subtropical soils are not documented in the literature. No attempts have been made to relate the extent of nitrate salt retention with soil mineralogy and the parameters of the soil chemistry, such as cation and anion exchange capacities (CEC and AEC) and soil solution pH and ionic strength (IS). Our objective was to provide evidence of the retention of Ca(NO 3 ) 2 in variable charge subsoils with extremely diverse mineralogical and chemical characteristics and to estimate the change in the extent of salt retention as a function of pH and IS. Laboratory leaching experiments were conducted with disturbed subsoil materials collected from the southeastern United States and other areas. Four lime treatments and four Ca(NO 3 ) 2 solutions of different concentrations were used to test the effect of pH and IS on nitrate salt retention. The results show that the transport of Ca(NO 3 ) 2 is affected by soil mineralogy and chemistry. The magnitude of salt retention was higher in subsoils with appreciable AEC and equivalent amounts of CEC, where both kaolinite and Al/Fe oxides dominate the clay fraction. In the subsoils with the highest magnitude, the electrical conductivity breakthrough curves (EC BTC) have an extended plateau of low EC values, and the first pore volumes of leachate contain virtually no salt. The cation and anion of the leaching electrolyte (Ca ++ and NO 3 - ) are adsorbed with no net release of other ions from exchange sites. Both pH and IS of the subsoil solution affect ion transport and alter the extent of Ca(NO 3 ) 2 2 retention. This study leads to the conclusion that mineralogy, surface charge, pH, and IS affect the magnitude of salt retention in variable charge subsoils, and their effects should be considered when predicting ion mobility and transport in these subsoils.