Influence of Soil Moisture Content on Soil Solution Composition

Despite the importance of the soil solution to plant growth, few studies have considered the influence of moisture content on the composition of the soil solution. Indeed, soil solution has seldom been extracted from soils below field capacity, despite the relevance of such conditions to plants grown in the field. Soil solution was extracted from a variable‐charge soil (Oxisol) and from several permanent‐charge soils (Vertisols) at various moisture contents (potentials ranging from −5 to −230 kPa) using polyacrylonitrile hollow‐fiber filter elements and pressure chamber apparatus. For the Vertisols, a decrease in moisture content resulted in a proportionate increase in the soil solution ionic strength, a behavior similar to that expected from a solution without any solid‐phase interaction. In contrast, for the Oxisol, the ionic strength remained constant as the soil moisture content decreased due to “salt adsorption.” For soils containing excess gypsum (CaSO 4 ·2H 2 O) or lime (CaCO 3 ), the dissolution (with increasing moisture) and precipitation (with decreasing moisture) of these minerals controlled the solution concentrations of their constituent elements. The soil solution composition appeared to conform to that expected from the ratio law (valency effect). The results of this study provide evidence that the method used can extract representative soil solution from soils at moisture contents lower than field capacity. They also demonstrate that changes in soil solution ionic strength and composition, while obeying simple control mechanisms, could not be readily predicted without a detailed understanding of the soil's surface charge and exchange behavior.