Salinity in Calcarosols occurs through the presence of sodium, chloride, bicarbonate and sulfate ions, is caused by sodicity, and leads to decreased osmotic potential

Context Salinity occurs in sodic soils in Australia, but its effect in Western Australia is poorly understood. Aims We determined the cause of salinity, the ions responsible, and their potential significance as constraints to crop growth on sodic soils at Merredin and Moorine Rock. Methods Soil was collected from 76 profiles to depths of 1.0–1.4 m (388 samples). Samples were analysed for EC1:5, pH, texture, and exchangeable and soluble ions. Results Exchangeable cations were best calculated as the difference between total cations (determined from BaCl2/NH4Cl extracts) and soluble ions (determined from water-soluble extracts). Profiles showed increasing sodicity, alkalinity and salinity with depth. The major soluble cation responsible for salinity was Na+; the major soluble anions were Cl−, HCO3−, SO42−, and CO32−. High salinity in subsoils (depth > 0.2 m) was strongly correlated with dispersive charge (adj. R2 = 0.73). Osmotic potentials were calculated for two levels of gravimetric soil water, the water content of the soils at sampling, or assuming 30% (dry mass basis) soil water. At Moorine Rock, soils mostly had osmotic potentials less than −1.5 MPa. Increasing soil water content to 30% made osmotic potentials less negative. At Merredin, there was strong stratification of osmotic potentials; surface soils mostly had osmotic potentials between 0 and −0.5 MPa, but subsoils mostly had osmotic potentials between −1.0 and −1.5 MPa. Conclusions Crop growth in these landscapes is likely to be constrained by salinity, particularly in dry years.