Biosolids and the need for Potash

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Biosolids (sewage sludge) provide valuable plant-available nutrients and have useful soil conditioning properties. They are subject to regulations which require that the application rates of specific heavy metals and their concentrations in soils are not exceeded, that disease risks to stock and humans are minimised and that applications should match the requirements of crops. Rates of applications are therefore important and so is the balance of nutrients. Water companies go beyond the regulations by agreement with stakeholders. Potential contaminants have been greatly reduced over the years by cooperation with industry, with their removal at source, and by legislation that has prevented the manufacture and sale of hazardous substances. Biosolids provide useful quantities of nitrogen and phosphate, but only modest amounts of potash and magnesium because these elements are quite soluble and are washed out in the treated water. Lime stabilised biosolids are a useful liming material.

Because most plant species require a greater total supply of potash than any other nutrient it is vital to complement biosolids with appropriate applications of potash fertiliser.