Differential availability of partially sulphuric and phosphoric acidulated phosphate rocks I. Plant response

A glasshouse study was conducted to determine the influence of soil pH on the agronomic effectiveness of partially phosphoric (Phos-PAPR) and partially sulphuric (SA-PAPR) acidulated phosphate rocks (PR). For Phos-PAPR ground North Carolina PR (NCPR) was acidulated with 10, 30 and 50% of acid needed for complete acidulation. For SA-PAPR a blend of NCPR, Arad and Khouribga PRs were acidulated with 60% of the acid needed. The relative agronomic effectiveness of these PAPRs were compared with superphosphate (SSP) and ground NCPR. A highly phosphate (P) retentive and P deficient pasture soil was used. Prior to addition of fertilizers to soil, the pH of soil was adjusted to 5.1 (initial soil pH) 5.4, 5.7 and 6.1 by applying varying amounts of Ca(OH)₂. Ryegrass (Lolium perenne) was grown as the test plant over a period of eight months. Fertilizers were applied at three rates plus control. Soil pH was monitored and continuously adjusted to the desired levels throughout the experimental period.The dry matter yields and P uptake in SSP treated pots were not influenced by soil pH. With increasing soil pH, agronomic performance of Phos-PAPRs and NCPR significantly (P<0.01) decreased but that of SA-PAPR was not affected. On the basis of per unit water-soluble P applied, uptake of P by plants was greater from PAPRs than SSP. Using the P uptake values of SSP and NCPR (which was used to prepare the PAPRs), the dissolution of P from the residual PR component of the PAPRs were calculated. The residual PR component of the Phos-PAPRs apparently dissolved in greater quantities than unacidulated NCPR. Dissolution of the residual PR was enhanced with increasing degree of acidulation. However, in the case of SA-PAPR, the agronomic performance of the PAPR was mostly dependent on the water-soluble P component of the PAPR. The uptake of P from the residual PR component of the SA-PAPR was insignificant.