Field evaluation of the liming value of two phosphate rocks and their partially acidulated products after 16 years of annual application to grazed pasture

Measuring pH of soil samples ( at four to five depths down to 300 mm) collected three times from a long-term (16 years) field trial involving annual application of six forms of phosphate fertilizers at the rate of 30 kg P ha(-1) yr(-1) showed that soil acidity in all treatments, including the untreated control, increased with time. The rates of acidification (pH unit yr(-1) during the first 10 years) in the topsoil ( 0 - 75 mm depth) were in the order, diammonium phosphate (0.038)> control, single superphosphate> Jordan partially acidulated phosphate rock (JPAPR)> North Carolina partially acidulated phosphate rock (NCPAPR), Jordan phosphate rock (JPR)> North Carolina phosphate rock (NCPR) (0.010). Of the 480 kg P ha(-1) applied over the 16 year period, 71 and 57% of P from NCPR and JPR dissolved. The theoretical liming values derived from the dissolution of NCPR and JPR were 1698 and 1303 kg CaCO3 ha(-1) respectively. Liming values of the two PRs calculated from the increase in soil pH over control treatment (Delta pH) down to 300 mm soil depth were 640 and 414 kg CaCO3 ha(-1) for NCPR and JPR, respectively. The lower liming values estimated from the DpH method is probably due to proton transfer resulting from the secondary reactions of dissolved fertilizer phosphate with soil constituents, the unaccounted liming effect of the PRs below 300 mm soil depth and the lower soil pH buffering capacities measured from a short-term pH titration method used in the estimation of the liming values. The results of this long-term field study showed that continuous use of certain phosphate rocks ( PRs) can significantly slow down the rate of acidification in pastoral soils.