Field Monitoring of Soil Salinity and Sodicity Under Commercial Furrow Irrigation: A Case Study from the Lower Burdekin Delta, Australia

Field-scale monitoring of soil and water processes under commercial irrigation remains uncommon, despite its value for understanding salinity and sodicity development outside laboratory settings. This study monitored two furrows on a commercial sugarcane farm in the Lower Burdekin Delta, Australia, spanning a clay-content gradient of 44.9–51.9%, with soil sampling following irrigation with surface water, groundwater, and a farmer-managed blend. Continuous monitoring showed wetting and solute redistribution concentrated in the upper 35 cm, while deeper layers remained comparatively stable. Deep percolation represented 8–49% of applied irrigation. Electrical conductivity (EC), Na+, and sodium adsorption ratio (SAR) increased along the furrow (Spearman ρ = 0.89–0.95; Friedman p ≤ 0.006), with the finer-textured end showing the greatest accumulation. Irrigation-water risk increased from surface water (EC 0.24 dS m−1; SAR 1.05) to blended water (0.90; 4.45) and groundwater (1.48; 6.48), with groundwater exceeding guideline limits for EC and SAR. At the furrow end, exchangeable sodium percentage (ESP) exceeded the 15% root-growth threshold (20.3% baseline; 23.5–32.2% after irrigation), while indicative saturated-extract EC (4.8–9.0 dS m−1) exceeded the 1.7 dS m−1 sugarcane threshold. These findings demonstrate the value of continuous field monitoring and identify spatial variability and water source as candidate factors warranting confirmation under replicated designs.