Evaluation of methods for field diagnosis of sodicity of soil and irrigation water in the sugarcane growing districts of Queensland, Australia

Sodicity and related properties of soils and irrigation water restrict sugarcane yields and cause environmental problems such as turbid runoff. The aim of this work was to assess the usefulness of several field diagnostic tests for soils and waters of the Australian sugar industry. Tests were evaluated using over 500 soil samples collected from 0-0.75 m depth from the six main sugarcane growing districts. Exchangeable sodium percentage (ESP) in the topsoil (0-0.25 m) could be predicted by electromagnetic induction (Geonics EM38) in two districts (r2=0.64-0.82), and in the subsoil (0.25-0.5 m) in three districts (r2=0.63-0.86). Dispersion index (DI) was a function of ESP in all districts, with 54-67% of variation accounted for in four districts. pH was a function of ESP in all districts (59-73% of variation accounted for) except Bundaberg. ESP could be predicted from pH, electrical conductivity (EC) and Na concentration (by portable Na-selective electrode) of 1:5 soil:water suspensions (r2=0.57 for Bundaberg and r2=0.66-0.84 in all other districts). In irrigation waters (141 samples), measurements of Na concentration, by portable Na-selective electrode, and EC could be used to estimate Na (r2=0.85) and Ca + Mg (r2=0.87) concentrations, allowing sodium adsorption ratio to be estimated. Depending on the circumstances, these tests can provide useful estimates of sodicity and related properties in the field