Va. Snyder et al., ABSORBENT METHOD FOR MEASURING SOLUTE ACTIVITIES IN UNSATURATED SOILS- THERMODYNAMICS AND PRELIMINARY EXPERIMENTS, Soil Science Society of America journal, 59(6), 1995, pp. 1524-1531
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.