Phosphorus availability of recycled calcium phosphorus produced from sewage sludge

'Heatphos method' is a technology to recycle phosphorus by adding calcium to heated sewage sludge. In this study, two calcium-rich phosphorus materials air dried or roasted at 700 deg C were used as phosphorus fertilizers. These two phosphorus materials were named dried recycled calcium phosphate (dRCP) and roasted recycled calcium phosphate (rRCP), respectively. The monomial efficiencies of dRCP and rRCP were compared with that of single superphosphate (SSP) by the following. 1) The effect of each added phosphorus fertilizer (SSP, dRCP, rRCP) on phosphorus availability on five soil types. 2) The response of three crops [maize (Zea mays L.), soybean (Glycine max L. Merr.), and groundnut (Arachis hypogaea L.)] grown in pots to the three phosphorus fertilizers. 3) Phosphorus adsorptions on the five soil types incubated for six months as evaluated from the three phosphorus fertilizers. Available P measurements show no significant difference among the three fertilizers on Red soil, Gray Lowland soil, Andosol or Non-allophanic Andosol, but the available P of Terrestrial Regosol (pH 8.2) showed that both dRCP and rRCP were inferior to SSP. The effect of dRCP on P uptake by maize was found to be equal to or better than that of SSP in Andosol and Red soil (pH 5.9 and 4.7, respectively); however, those of dRCP and especially rRCP were much lower than SSP in the Terrestrial Regosol (pH 8.2). These results indicated that the effect of RCPs may be affected by soil pH. dRCP showed high efficiency in relatively low pH soils; this result suggests that it is possible to use dRCP as P fertilizer as well as SSP at low pH soil. However, rRCP was less soluble than the other two P sources (SSP and dRCP) because the roasting procedure may develop the crystallinity of calcium phosphate. In Non-allophanic Andosol and Red soil, the influence of incubation on P uptake by crops was different from expectation from P absorption coefficient of soil. It may be necessary to develop a new evaluation method for P adsorption in soil.