A simple incubation test for determining soil dispersion and gypsum rates

Abstract

Context High levels of exchangeable sodium or potassium in soils can lead to issues such as dispersion and deterioration of soil structure, reduced hydraulic conductivity and inadequate aeration. Gypsum is the most common soil amendment used to treat these issues but current methods to determine application rates (e.g. based on exchangeable sodium percentage or addition of dissolved calcium to diluted soil extracts) have limitations. Aim The aim of this study was to develop a simple soil incubation test to determine gypsum rates that are sufficient to eliminate soil dispersion. Methods Accurately weighed amounts of gypsum (to represent rates between 0 and 10 t ha−1) were mixed with soil (n = 8, three replicates) in small columns (50 mL plastic syringe-valve) and incubated under wet conditions for different periods. The turbidity, electrical conductivity, pH and cation concentrations in the column leachate were measured. Key results As gypsum rate increased, turbidity in the leachate decreased and this relationship was fitted with a natural logarithm function for each soil (average R2 = 0.71). A desired leachate turbidity target of ≤10 NTU was used in the equation to estimate the gypsum rate when dispersion was overcome for each soil. Testing suggested that a 1-day soil incubation time was sufficient. Conclusions Advantages of this new method include (a) non-reliance on exchangeable cation measurements, (b) direct relationship of the turbidity measure with soil dispersion and (c) the influence of soil salinity on dispersion being implicitly considered. Implications This method should provide improved decision support for management of dispersive soils.