Partially acidulated phosphate rock as fertiliser and dissolution in soil of the residual rock phosphate

Abstract A field trial was conducted for two years to determine the effectiveness of partially acidulated phosphate rock (PAPR) — in this instance, a reactive North Carolina rock acidulated with 20, 30, and 50% of the phosphoric acid needed for complete acidulation. The soil was a yellow-brown loam (Typic Vitrandept), and triple superphosphate (TSP) was used as the standard fertiliser. The fertilisers were applied at four rates to a ryegrass-white clover permanent pasture. The soil was highly P retentive and the dominant clay was allophane. The trial design enabled measurement of the residual effectiveness of the fertilisers in the second year. The ability of soil tests (namely, Olsen, Bray-I 1 min, Bray-I 30 min, and anion exchange resin) to indicate soil P availability to plants was assessed. The rate of dissolution of unacidulated rock phosphate in PAPR was calculated from inorganic P fractionation in soil samples collected at intervals. Dry matter yields and P uptake indicated that, as a maintenance f...