Cation concentrations in the saturation extract and soil solution extract of soil salinized with various sulfate salts

Abstract

Abstract A laboratory study was conducted with 2 objectives: i. to determine the effects of salinization on the cation composition of saturation paste and soil solution extracts and ii. to determine the accuracy with which the saturation extract, the traditionally used method, reflects the composition of the soil solution, the ionic medium in which plant roots reside. A factorial experiment including 5 sulfate salt species each applied at 5 rates was established in a loamy sand soil and the composition of the 2 extracts was determined after equilibration. Na and Mg concentrations in both the soil solution and saturation extract were directly related to amounts of these cations applied. K concentrations tended to increase with increasing salt concentration because of release of sorbed K. Ca concentration in both extracts was increased by addition of low levels of salt (< 200 mmol/L), apparently by release of exchangeable Ca, but decreased with higher salt addition because of precipitation reactions. The decline in Ca concentration, which is exacerbated by a simultaneous decrease in activity coefficient, may have been sufficient to induce Ca deficiency and restrict crop yields. The estimated Ca activity declined from approximately 10 mmol/L to 2 mmol/L with increasing amounts of salts applied. The saturation extract provided an accurate measure of the Na, Mg, and total salt concentration of the soil solution, regardless of the type of salt applied. In contrast, Ca concentration in the soil solution was not accurately reflected by analysis of the saturation extract. The ratio of Ca to total cations in the soil solution, however, which is closely related to crop yield in sulfate‐dominated saline soils, was highly correlated to the same ratio in the saturation extract.