Long-term effects of gypsum on the chemistry of sodic soils

Abstract

Soils with high exchangeable sodium contents have structural and hydrological problems that limit agricultural productivity. Gypsum is commonly applied as an ameliorant but there are few field studies into its long-term (> 10 years) effects on soil chemistry, especially at depths greater than 0.4 m, and no long-term validation of the gypsum requirement model. The aim of this study was to address these two knowledge gaps. This was achieved by a) carrying out a field study of the effects of repeated gypsum applications, irrigation, and sugarcane cultivation to 1.25 m depth over a 26-year period in Australia, and b) assessing the ability of the gypsum requirement model to predict changes in exchangeable sodium percentage (ESP) more broadly, using 53 unique cases from 17 field studies (> 1 year). In our field study, the greatest reductions in exchangeable sodium (up to 5.49 %; p < 0.05) occurred between 0.25 and 1.00 m depth, where initial concentrations were highest. The amount of sodium (16.47 cmolc/m2), magnesium (2.49 cmolc/m2), and potassium (0.65 cmolc/m2) displaced was less than the amount of calcium gained (36.76 cmolc/m2) over the profile. Most calcium was retained in the application layer (0.00–0.25 m) but some moved down to at least 1.25 m. Reductions in exchangeable sodium were smaller than in all previous long-term studies, possibly due to lower deep drainage rates (39–45 mm/year) and increased pH. Assessment of the gypsum requirement model showed that predicted final ESP deviated widely from measured ESP in many cases (95 % of predictions deviated by >10 %), presumably due to interactions between pH, clay type, carbonate, climate, management, and cation exchange equilibria. By adjusting two model parameters, the effect of gypsum on ESP was more accurately predicted (20 % of predictions were within 10 % of measured ESP) making it a useful tool for farmers.