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Herbicides are often applied to crop residues, but their fate has not been well studied. We measured herbicide washoff from sugar cane trash during simulated rainfall, at 1, 8, and 40 days after spraying (DAS), to provide insight into herbicide fate and for use in modeling. Herbicides included are commonly used in the sugar industry, either in Australia or in Brazil. Concentrations of all herbicides and applied Br tracer in washoff declined exponentially over time. The rate of washoff during rainfall declined with increasing DAS. Cumulative washoff as a function of rainfall was similar for most herbicides, although the most soluble herbicides did have more rapid washoff. Some but not all herbicides became more resistant to washoff with increasing DAS. Of the total mass washed off, 80% washed off in the first 30 mm (∼40 min) of rainfall for most herbicides. Little herbicide remained on the trash after rainfall, implying nearly complete washoff.

Use of non-traditional and marginal quality saline sodic water will increase in water limited environments and methods to assess use suitability are required. The threshold electrolyte concentration (CTH) defines the soil solution concentration, for a given soil solution sodicity, at which an acceptable reduction in the soil hydraulic conductivity (10–25%) is maintained without further soil structural degradation. The traditional method of determining CTH is via leaching columns, which are laborious and often expensive. Dispersive potential (PDIS) is potentially a more rapid method with which to determine the CTH in a practical sense and make management recommendations for water quality use on a given soil. This work evaluated the PDIS method against known CTH data to determine the efficacy of use for non-dispersive soils irrigated with marginal quality saline sodic water. Results suggest that the PDIS approach to CTH did not reliably, or efficiently, determine the CTH in non-dispersive soils equilibrated with an irrigation solution. Using it to determine the aggregation and dispersion boundary for initially non-dispersive soil appeared to have merit, but only where the aggregates equilibrated with the irrigation solution were subject to rapid dilution with deionised water.

Use of non-traditional irrigation sources will increase with industry water demand, with many industry wastewaters (e.g. agri-industry processes such as milk factories, piggeries, wineries and abattoirs) containing appreciable potassium (K), which is known to result in soil structural decline if the concentration is sufficient. The threshold electrolyte concentration (CTH) is generally understood to represent the electrolyte concentration (directly proportional to electrical conductivity) at which a soil will remain stable when subjected to a solution of given sodium adsorption ratio, without limiting dispersion. However, current approaches to determine CTH do not incorporate K. Hence, this work seeks to investigate incorporation of K into the disaggregation model for CTH and validate this against equivalent sodium (Na) systems using an ionicity approach. A single generalised coefficient of equivalence for K relative to Na did not appropriately describe the system changes; this coefficient was specific to a soil and appeared to vary with the percolating electrolyte concentration. Incorporation of K into the disaggregation model, although not accurate with a universal coefficient of equivalence for K, was considered reasonable where no other approach could be used. This conclusion was drawn on the basis that the model would produce a conservative CTH under such circumstances, which would not cause undue degradation to the soil environment.