Effectiveness of different precipitated phosphates as phosphorus sources for plants

Eleven precipitated phosphates were evaluated as sources of phosphorus (P) for plant growth by comparing their effectiveness with that of monocalcium phosphate, a source of water soluble P that is generally considered to be fully plant available. The precipitated phosphates comprised struvites recovered from waste water discharges (mainly magnesium ammonium phosphate), laboratory synthesised struvites, a synthetic iron phosphate and a recovered calcium phosphate. Precipitating phosphates in these forms could be a way for removing P from waste water before it is discharged to rivers, so reducing the risk of eutrophication. Application to agricultural land would be one potential use for such phosphates. Evaluation was by pot experiments with a sandy loam soil and with a sandy clay loam soil using perennial ryegrass (Lolium perenne) as the test crop. The soils differed in pH (6.6 and 7.1) and in Olsen P (28 and 11 mg L-1). Measured variables were grass dry matter (DM) yield and grass P concentration which were used to calculate offtake of P in the harvested grass. DM yields of ryegrass and P offtakes given by the synthetic and recovered struvites were not significantly different statistically either between themselves or to MCP applied at the same rate. On this basis these struvites could be used to recycle P to similar soils and the effect of the P on crop yield should be similar to that of MCP.