Response of wheat (Triticum aestivum L.) and cowpea (Vigna unguiculata L.) to foliar wetting with low pH mine waters containing acid-generating metal cations

Acid mine drainage (AMD) contains metals that have detrimental effects on crop growth if present in excess in plant-available form. The use of untreated AMD from coal mines for crop irrigation on strategically limed soils is considered a potential option for mine water management, especially in remote areas without access to water treatment infrastructure. However, leaf scorching and trace element enrichment of plant tissue are of potential concern, as these waters often contain high concentrations of potentially toxic, acid-generating metals (Al3+, Fe2+ and Mn2+) which may cause foliar damage and hyper-accumulate in plant tissue. The objectives of this trial were to quantify any foliar injury and metal accumulation in wheat (Triticum aestivum L.) and cowpea (Vigna unguiculata L.) vegetative material after foliar wetting with simulated AMD enriched with metal cations, and to ascertain if biomass production will be affected. In this trial, crop water requirements were not met through mine water irrigation, as this was only a foliar wetting investigation. Two pot experiments were set up, with wheat grown in the winter and cowpea in the summer season of 2021. Sulphuric acid was used to lower pH to 2, after the addition of low, intermediate and high concentrations of individual acid-generating metal cations, as well as a combination of all three cations. Areal foliar injury was greatest for cowpea (18.7%) with a combination of Al, Fe and Mn at the highest concentration. The crop was not able to recover at this injury level. No injury was recorded in wheat. Both crops accumulated only limited quantities of the metal cations. Calculated hazard quotient for cattle ranged from 0.022 to 0.53, indicating that such fodder would be safe to consume. It is concluded that there are large differences in crop specie susceptibility to leaf scorching after foliar wetting with acidic metal cation-rich mine waters. It is recommended, therefore, that because large volumes of mine water need to be managed, and centre pivot overhead irrigation is likely to be utilized to this end, that studies to screen more species to select crops tolerant to scorching through foliage wetting with mine water irrigation be conducted. Thereafter, studies to better understand root zone effects of irrigation with such waters need to be conducted on species that can tolerate foliar wetting with these waters.