Impact of sodium adsorption ratio of irrigation water on the structural form of two Vertosols used for cotton production

Abstract

In recent years, the production of cotton in Australia has been limited by the availability of irrigation water. To overcome this problem, poorer quality (Na+-rich) irrigation sources have been used in some situations, despite the effects elevated levels of Na+ may have on soil physical and chemical properties. This paper reports on changes in the surface-connected structural form attributes of two Vertosols from eastern Australia (one Red Vertosol, one Black Vertosol) after treatment with a range of different water-quality solutions. Intact soil columns from each of the Vertosols were irrigated through six wet–dry cycles using one of six treatment solutions with varying Na+ concentrations. Replicate columns for each treatment of each soil were analysed post-irrigation for selected chemical attributes. A second set of replicate columns was impregnated with a fluorescent resin post-irrigation, horizontally sectioned, and photographed under ultraviolet light. Image analysis was carried out on the section photographs to yield quantitative estimates of porosity (P), surface area (Sv), solid and pore star lengths (ls* and lp*), and solid and pore genus (gs and gp). Generally, the soil treated with the low-Na+ solution had the most desirable structural form attributes (larger P, Sv, and gp and smaller ls* and gs), while the soil treated with the high-Na+ solution had the least desirable structural attributes. The structural attributes and chemical properties of the Red Vertosol changed more markedly with water quality than did those of the Black Vertosol. The difference in response to water quality between these two soils is presumed to be related to the clay mineral suites and the exchange capacity of these soils; the Black Vertosol contains appreciably more smectite and has a much larger effective cation exchange capacity than the Red Vertosol.