Phosphorus fertiliser value of sewage sludge ash applied to soils differing in phosphate buffering and phosphate sorption capacity

The global phosphate rock reserves that underpin phosphorus (P) fertiliser production are finite. Recovery of P from waste streams (e.g., by producing fertiliser products from sewage sludge) is an important way to conserve scarce P resources. This study evaluated the P fertiliser and soil pH amendment values of a municipal sewage sludge ash (SA) for pasture growth on three acidic soils with contrasting P buffering and P sorption ("fixation") characteristics. Growth of subterranean clover (Trifolium subterraneum L.) and incubation of the fertilisers with soil were used to compare the initial (following application) and residual (following incubation with soil) fertiliser values of SA with those of single superphosphate (SSP). Soil pH(Ca) was increased linearly by application of SA. The lime-equivalent value of SA was similar to 33% (product weight basis). The initial P-fertiliser value of SSP was substantially greater than that of SA in all three soils. For both P sources, initial fertiliser value was negatively related with the P buffering capacity of the soil. The residual P fertiliser value of SSP ranged from low (42% of initial fertiliser value) to relatively high (82%) indicating differences among the soils in their P sorption capacities. The residual value of SA was better than that of SSP in soils that exhibited strong P-sorption capacities. Standard soil-P tests (i.e., Colwell P, CaCl2-P, Bray-1 P) that are often used in combination with SSP, underpredicted the fertiliser value of SA. This indicated a need to recalibrate these soil tests for use with SA.