A review of the fate of potassium in the soil-plant system after land application of wastewaters.

Irrigation with wastewaters from agri-industry processes such as milk factories, piggeries, wineries and abattoirs is commonplace. These wastewaters all have high levels of potassium (K). Potassium concentration in effluents from domestic wastewater sources are relatively low, reported to vary between 10 and 30 mg L(-1). Higher levels of potassium are reported for effluents from olive oil mills, 10,000-20,000 mg KL(-1), wool scouring, 4200-13,000 mg KL(-1), cheese and lactic whey and potato processing, approximately 1800 mg KL(-1), piggery effluent, 500-1000 mg KL(-1) and winery wastewaters, up to 1000 mg KL(-1). Application of wastewaters with these high potassium levels has been found to increase the overall level of soil fertility, with the exception of alkaline effluents which can dissolve soil organic carbon. Long-term application of such wastewater may cause the build-up of soil potassium and decrease the hydraulic conductivity of the receiving soils. These potential impacts are uncertain and have been inadequately researched. Regulatory limits for potassium in drinking water have been set only by the European Union with no toxicological or physiological justification. The literature shows that grasses and legume herbages accumulate high levels of potassium, up to 5% dry weight, and some grasses, such as turfgrass are particularly tolerant to high levels of potassium, even under saline conditions. This adaptation is considered useful for increasing potassium immobilization and sustainable practices of land wastewater disposal. Potassium availability is significantly affected by the cation ratios of the wastewater, the existing soil water solution and of soil exchange sites.