Prediction of nitrate and chloride ion concentrations in soil solution using water extracts

Soil solution composition is one of the most important factors governing the uptake of nutrients by plants; however, sampling and analyzing soil solutions is usually time-consuming. We tested a simple method to predict anion concentrations in soil solutions from their concentration in water extract on 126 soil samples including 41 allophanic Andisols. In this method, the concentration of an anion in soil solution was predicted by multiplying its concentration in water extract with a dilution ratio (i.e. the ratio of water content at water extraction to that at soil solution separation). The method worked successfully for NO3 and Cl with average relative errors of 1.8% and 17%, respectively. The method, however, greatly overestimated the SO4 concentration in soil solution because of the dissolution of gypsum and desorption of SO4 during water extraction.