Does the high potassium content in recycled winery wastewater used for irrigation pose risks to soil structural stability?

Abstract

Sodicity, where a high proportion of cation exchange sites are occupied by monovalent Na+ ions relative to divalent cations Ca2+ and Mg2+, is a widespread and serious problem globally as this can result in soil dispersion, low hydraulic conductivity and poor aeration. It has been increasingly recognized that the presence of potassium (K+) can also be a contributor to soil structural stability problems. The aim of this research was to investigate the long term (>15 years) effects of irrigating treated winery wastewater on soil structural stability, for which K+ comprises a high proportion of total cation concentrations, at a vineyard trial site in the Barossa Valley, Australia. Analysis of the long-term soil dataset was undertaken along with laboratory experiments to determine the current or future impacts of high exchangeable K+ on soil hydraulic conductivity and dispersion. Field results showed build-up of K+ in subsoils and a corresponding decline in Mg2+. This appears due to the higher selectivity and exchange of K+ relative to Na+, and higher K/Na ratio in the wastewater. The potential dispersibility of the subsoils was demonstrated via a high turbidity of soil suspensions and high values of the Emerson Dispersion Index. Declining relative hydraulic conductivity occurred for subsoils leached with winery wastewater followed by low ionic strength water. Flocculation measurements were performed in which aliquots of CaCl2 were added to determine the amount of Ca2+ needed to overcome dispersion. We conclude that the long-term use of winery wastewater irrigation has potential to negatively impact soil structural stability via increasing exchangeable K+. Further field research is required but Ca or Mg based amendments could be used to reduce dispersion risks.