ABSORBENT METHOD FOR MEASURING SOLUTE ACTIVITIES IN UNSATURATED SOILS- THERMODYNAMICS AND PRELIMINARY EXPERIMENTS

Characterizing the physico-chemical environment in unsaturated soils h as been difficult due to lack of methods for measuring solute activiti es 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 infer red from the measured amounts of sorbed water and solutes and a genera lized pressure membrane technique, Preliminary evaluations of the meth od used Whatman no, 42 filter paper as the absorbent. Air-dry filter p apers 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 w ater contents and K2HPO4 molalities (mel sorbed K2HPO4 kg(-1) sorbed w ater) related to water potential in a way consistent with theory, To e xamine general trends and variability of measurements in actual soil s ystems, filter papers were equilibrated with soil receiving different chemical pretreatments, at soil water matric potentials between -7 and -540 J kg(-1). Coefficients of variation for Ca, Mg, and K molalities in the filter papers were usually <20% for a given treatment. Predict ably, cation molalities generally increased with decreasing soil water content, In CaSO4-treated soil, however, Ca molalities decreased sign ificantly, perhaps reflecting increased soil adsorption of CaSO4 at lo w water potentials, Adding Ca electrolytes to the soil generally incre ased K and Mg molalities, as expected from ion exchange.