Absorbent Method for Measuring Solute Activities in Unsaturated Soils: Thermodynamics and Preliminary Experiments

Abstract

AbstractCharacterizing the physico‐chemical environment in unsaturated soils has been difficult due to lack of methods for measuring solute activities at low soil water content. We describe a method where a thin sheet of insoluble and chemically nonreactive absorbent material is allowed to absorb water and solutes from the soil. Solute activities are inferred from the measured amounts of sorbed water and solutes and a generalized pressure membrane technique. Preliminary evaluations of the method used Whatman no. 42 filter paper as the absorbent. Air‐dry filter papers that had been impregnated with different known amounts of K2HPO4 were allowed to absorb water by equilibrating through the vapor phase with salt solutions of known water potential. Measured filter paper water contents and K2HPO4 molalities (mol sorbed K2HPO4 kg−1 sorbed water) related to water potential in a way consistent with theory. To examine general trends and variability of measurements in actual soil systems, filter papers were equilibrated with soil receiving different chemical pretreatments, at soil water matric potentials between −7 and −5400 J kg−1. Coefficients of variation for Ca, Mg, and K molalities in the filter papers were usually <20% for a given treatment. Predictably, cation molalities generally increased with decreasing soil water content. In CaSO4‐treated soil, however, Ca molalities decreased significantly, perhaps reflecting increased soil adsorption of CaSO4 at low water potentials. Adding Ca electrolytes to the soil generally increased K and Mg molalities, as expected from ion exchange.