Assessing the agronomic effectiveness of wastewater‐treated Allophanic soil as a phosphorus source for plant growth

Domestic sewage treatment plants ( STP s) that discharge partially treated wastewaters to surface waters contribute to eutrophication. A number of Allophanic soils have shown promise as a filter substrate to remove dissolved reactive phosphorus ( DRP ) from wastewaters. Once the phosphorus (P) sorption capacity of the soil is exhausted, it can potentially be applied to land as a soil amendment to recycle P for productive use. In this study, Allophanic soil that was P‐enriched by wastewater treatment ( WTS ) was evaluated for its agronomic effectiveness in a glasshouse pot experiment in which ryegrass ( Lolium multiflorum ) was grown on Ohakune silt loam. The ability of WTS to supply P for ryegrass growth was compared with a soluble phosphorus source (mono‐calcium phosphate, MCP ). The WTS had a total P ( TP ) concentration of 8128 mg TP/kg oven‐dried soil. As the initial P status of the Ohakune soil was very low, the cumulative dry matter yields increased significantly with increased additions of WTS and MCP . The WTS was highly effective at increasing available P in the soil, as measured by Olsen P and ryegrass yield and P uptake. The soluble fertiliser P value of WTS was estimated to be equivalent to 61% of MCP applied at the same rate. Therefore, the results show that WTS is an effective P source for plant growth and its application to soil has the potential to recycle both the soil and the P it contains.