Sorption and mineral equilibria controls on moisture chemistry in a C-horizon soil

Abstract

A study was made of the chemistry of soil moisture collected over a period of 12 months from sandy loam soils in central Pennsylvania. Samples were taken at depths of from 1 to 9 m by means of suction lysimeters. The prevalent chemical character of soil moisture was generally Ca 2+ > Na + > Mg 2+ ≈ K +, with HCO − 3 > Cl − > SO 2− 4 ≈ NO − 3 on both a mole and weight basis. Soil-moisture pH ranged from 5.20 to 6.74. Dissolved silica averaged 54 mg/l (as SiO 2) and was as high as 117 mg/l. The specific conductance ranged from 20 to 400 μS/cm. Total dissolved solids (TDS) in the water increased with decreasing atmospheric precipitation and increasing temperature. This was due to evapotranspiration, which increased TDS directly, and also indirectly by reducing soil relative permeabilities and increasing the contact time between soil minerals and water. A secondary cause was greater precipitation acidity during the warmer months and during periods of lower precipitation. Soil colloids are predominantly Fe- and Mn-oxyhydroxides, with usually smaller amounts of illite and kaolinite. Total cation exchange capacity is 2–3 meq./100 g. Regression analysis of dissolved-ion activities showed that the aquo-cations are strongly buffered by Donnan equilibrium and selective adsorption with selectivities in the order K + > H + > Mg 2+ > Ca 2+. Calculation of the degree of saturation of the soil moisture with respect to soil minerals showed that the soil moisture is undersaturated with respect to calcite, dolomite, amorphous silica and illite, and super-saturated with respect to quartz and probably also kaolinite. Sorption equilibria controlled major-cation concentrations at all depths in the soil, whereas solution-mineral equilibria did not limit concentrations of these cations.