Monitoring changes in soil salinity and sodicity to depth, at a decadal scale, in a semiarid irrigated region of Australia

Soil salinity and sodicity are two of the most limiting constraints for agriculture in arid and semiarid landscapes, but long-term studies are scarce, and most solely focus on the topsoil. This study monitors the change in soil electrical conductivity (EC) and exchangeable sodium percentage (ESP) to 1.2 m depth with bivariate linear mixed models between 2002 and 2015 in a semiarid, irrigated cotton-growing region of south-west New South Wales, Australia. In this work, the impacts of shifts in rainfall, variability of irrigation water quantity and quality, and agricultural land uses, on soil salinity and sodicity are analysed. The study area possessed generally low levels of soil salinity, and shifts in EC were detected over time, but only isolated areas of the various sampling depths experienced statistically significant changes in EC. Some areas under irrigated cotton production experienced a desalination trend, whereas soil EC under irrigated perennial horticulture increased over time. This increase was attributed to the use of fertilisers that contain salts, and the varying quantity and quality of applied irrigation water. Sodicity was low to moderate in the upper 0.5 m of the soil profile but high in deeper layers, with a trend of increasing soil sodicity through time. Most of the statistically significant increases in ESP occurred in areas under irrigated cotton and horticulture, with this likely due to the continued addition of sodium to the soil system. This study also demonstrates that visible near infrared spectroscopy can be used in to predict soil ESP values to reasonable accuracy.