Anion exclusion effects on estimates of soil chloride and deep percolation

Abstract

Anion exclusion affects estimates of deep percolation determined using chloride mass balance models. This paper evaluates potential errors arising from ignoring anion exclusion in both soil analysis methods and model assumptions. Soil chloride analysis from saturated paste and 1:5 soil/water extracts are compared with that of field soil water extracted using a filter paper technique. The influence of anion exclusion on estimates of soil water chloride concentration (Clsw mmol/L) and chloride content per unit mass of soil (CIT mmol/kg soil), both which are required by chloride mass balance models, is examined for these methods. The relative importance of total electrolyte concentration and sodium absorption ratio (SAR) on the anion exclusion volume of a clay was also determined using a simple laboratory leaching technique. The chloride concentration of soil water at field moisture contents was more closely related to estimates calculated from saturated paste extracts than from 1:5 soil/water extracts. Although the amount of excluded water estimated in the saturated paste was larger than that estimated at the field water content, the proportion of non-excluded water in each was similar. The ClT was overestimated when analysis from saturation extracts was used. Electrolyte concentration and SAR were shown to have similar (but opposite) relative effects on the exclusion volume. However, because these variables were positively correlated, their effects were not apparent in the field samples. For the clay soil under study, deep percolation could be overestimated by up to 1.64 times if anion exclusion effects were ignored when using 1:5 extracts to estimate Clsw. The errors were considerably less if saturation extracts were used to estimate Clsw and ClT.