Empirical equations for prediction of major cation concentrations in soil solution using concentrations in water extracts

Soil solution composition is one of the most important factors governing the uptake of nutrients by plants. However, sampling soil solution is usually time and labor consuming. We analyzed soil solutions and water extracts from 126 soil samples including 41 allophanic Andisols for major cations and anions. The multiplication of the concentration of a cation in the water extract by the dilution ratio (i.e. the ratio of the water content at water extraction to that at soil solution separation) always overestimated its concentration in soil solution. For soils in which the soil solutions were unsaturated with respect to gypsum, there were highly significant linear correlations between the concentration in soil solution and the concentration in water extract multiplied by the dilution ratio for Ca, Mg, K and Na. The linear regression analyses gave the following equations for predicting cation concentration in soil solution: [Ca]_ss = 0.8553 r_d [Ca]_wex - 0.9282, [Mg] _ss = 0.9346 r_d [Mg]_wex - 0.6645, [K] _ss = 0.4978 r_d [K]_wex - 0.8984, [Na] _ss = 0.7092 r_d [Na]_wex - 0.5283, where [ ] refers to the concentration in mmol L^-1, the subscripts ss and wex are soil solution and water extract, respectively, and r_d is the dilution ratio. We also proposed a practical criterion to evaluate the saturation status of soil solution with respect to gypsum.