Potassium and magnesium in irrigation water quality assessment

• Impacts of Na, K, Mg, and Ca on threshold electrolyte concentrations of a Sodosol were evaluated.
• We examine the relative impact of K on soil dispersion as compared to Na, and of Mg as compared to Ca in stabilizing soils.
• CROSS, a generalization of SAR, accounted quantitatively for impacts of Na, K, Mg, and Ca on soil dispersion.
• Optimization of CROSS suggests that the effect of K on soil dispersion is about one-third of that of Na.
• Mg concentrations need to be about 10 times larger than Ca to have the same beneficial effect on stabilizing soils.

There is a growing trend of K replacing Na in wastewaters from agricultural industries to reduce the adverse effects of irrigation with these waters on soil hydraulic properties. However, problems with soil physical properties caused by wastewaters with high concentrations of K have been reported in both Australia and California. A review of the literature dating back to the 1930s supports the general conclusion that the relative order of deleterious effect on soil hydraulic properties of the four common cations in soils is Na > K > Mg > Ca. This paper explores the capabilities of the Cation Ratio of Structural Stability (CROSS), a generalized Sodium Adsorption Ratio incorporating all four cations, as an improved irrigation water quality parameter. This new parameter includes a coefficient for K that accounts for its relative deleterious effect as compared with Na and another coefficient for Mg that accounts for its relative beneficial effect as compared with Ca. Based on optimizing CROSS using threshold electrolyte concentration data for a Sodosol from the Riverina Region of Australia, the deleterious effect of K is estimated to be about one-third of that of Na, while the concentration of Mg needs to be about an order of magnitude larger than Ca to have the same beneficial effect.