Soil-specific strategic irrigation: saline-sodic water as an irrigation resource

Soil-specific strategic irrigation: saline-sodic water as an irrigation resource

Declining water quality and quantity is a threat to the production of food and fibre worldwide. While irrigation using marginal quality saline-sodic (MQSS) water is emerging as a more common practice, it is still an under-utilised resource because of its potential detrimental impact on soil structure and crop production. The aim of the research was to enhance the current understanding of, and capability to, strategically utilise saline-sodic water as an irrigation resource through further investigation of the theory of threshold electrolyte concentration (CTH).

Soil structural response to irrigation water quality is known to be a function of sodium (Na) contained in the irrigation water and the electrolyte concentration of that water. The CTH is classically used to determine the suitability of water to be applied to a soil, and is usually conducted as a laboratory analysis utilising saturated hydraulic conductivity. This work aimed to validate the laboratory based semi-empirical disaggregation model approach to CTH against field soils where MQSS water had been applied for an extended period of time. Unirrigated locations proximal to long-term irrigation sites were paired to provide control conditions. Results showed that the disaggregation model is useful for proactive planning of irrigation systems with regard to water quality and a good measure for identification of MQSS water as a strategic resource. The applicability of these results to irrigation guidelines was discussed with demonstrated a required focus on removal of generalised guidelines and identification of soil-specific tolerable hydraulic conductivity reduction.

The traditional method of determining CTH is via leaching columns, which is a laborious and often expensive process Dispersive potential (PDIS) was potentially a more rapid method which allowed determination of the CTH in a practical sense, potentially providing a rapid means by which to 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 MQSS. 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.

The threshold used to define the tolerable reduction in hydraulic conductivity is generally the CTH — defined as between a 10% and 20% reduction in saturated hydraulic conductivity from stable condition — others have suggested that the aggregate-dispersion boundary may be used as this threshold instead. This boundary is also known as the threshold turbidity concentration (CTU). Using a saturated hydraulic conductivity approach, this work sought to quantify the extent of reduction at the CTU and compare this to traditional CTH approaches. It was found that saturated hydraulic conductivity reduced between 44 and 78% for the five Vertisol soils investigated. This indicated that the CTU varied between soils and was substantially more than the 10–20% reduction in hydraulic conductivity at the CTH. Quantification of this boundary condition allows more sensible selection of tolerable Ksat reduction that does not result in undue irreversible structural decline.

Use of non-traditional irrigation sources will increase, with many industry wastewaters containing potassium. Potassium is known to result in soil structural decline if the concentration of K is sufficient. Current approaches to determining CTH do not incorporate K. This work sought to investigate incorporation of K into the disaggregation model for CTH and validate this against an equivalent Na systems using an ionicity approach. It was found that a single generalised coefficient of equivalence for K relative to Na does not appropriately describe the system changes, rather that this coefficient specific to a soil and appears to vary with the percolating electrolyte concentration. Incorporation of K into the disaggregation model, while 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 serve to produce a conservative CTH under such circumstances, which would not cause undue degradation to the soil environment.

Relating the reduction in net negative charge to the rKsat was hypothesised to provide vital information concerning soil-specific reduction rates. The net negative charge, measured as zeta potential (ζ), was determined for three soils of distinct difference. The disaggregation model approach to CTH was used to determine rKsat with ζ measured at each treatment solution in the CTH methodology. Zeta potential was found to be a function of SAR and EC for a given pH with a general equation provided. Net negative charge and rKsat were very highly related (R2>0.8 for all three soils), although the slope of the relationship was distinctly different for the three soils, in keeping with literature describing the influence of clay content and oxide content on the reduction in hydraulic conductivity. Additional research into the effect of clay content, sesquioxide occurrence and ζ on rKsat is required to use ζ for prediction, but this work showed promise in moving towards a predictive model.

This research clearly established the feasibility of strategic MQSS water usage, but also identified several impediments in its use with reference to the soil-specific response, the methodology used to determine the suitability, the presence of magnesium (Mg) and potassium (K), the mechanisms controlling the soil response, and finally the guidelines used to determine the suitability.