Agronomic effectiveness of partially acidulated phosphate rock fertilizers in an allophanic soil at near-neutral pH

The agronomic effectiveness of five partially acidulated phosphate rocks (PAPRs) and an unground phosphate rock (PR) were compared against single superphosphate (SSP) in a glasshouse experiment using a high phosphorus (P) retention soil at a near-neutral pH (pH 6.5), and corn (Zea mays L.) as the test crop. The PAPRs were prepared by acidulating unground North Carolina PR with either phosphoric or sulphuric acid (expressed as Phos-PAPR and SA-PAPR, respectively) and at three levels of acidulation (20, 33, and 50%). The relative agronomic effectiveness (RAE) and substitution value (SV) of the test fertilizers, calculated with respect to SSP using the standard "vertical" and "horizontal" comparisons, showed that 50% phosphoric acidulated PAPR performed as effectively as SSP whereas the other fertilizers were less effective. The PR treatment showed a small yield response. The dry matter yield and P uptake were linearly related to water-soluble P of the fertilizers up to 66% of total P and there was no advantage in acidulating fertilizers above this level of water-soluble P using reactive PR. Whereas very little of the directly-applied PR dissolved (3.4% of PR applied), PR applied as a component of PAPRs dissolved up to 22%. The dissolved proportion of added PR component increased with increasing water-soluble P content of the fertilizer. The results suggest a greater efficiency of PAPR than SSP as a P supplier to plants.