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

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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 fertiliser, even PAPR 20% was as effective as TSP from the first pasture cut. The residual effectiveness of fertilisers in the second year ranged from 97 to 108% of the effectiveness of freshly added TSP. All soil tests significantly correlated with dry matter yield and P uptake. In the Bray-I test, extractable P decreased with increasing shaking time and the values stabilised after 15 min shaking. Bray-I 30 min was found to be desirable for convenience and accuracy. No measurable amount of unacidulated rock phosphate in PAPRs dissolved in the first 3 months after fertiliser addition. The rate of dissolution in the next 15 months was 60% per year of that initially added.