Field evaluation of partially acidulated phosphate rocks in a Ferralsol from Cuba

Phosphorus (P) is needed in large areas of developing countries to improve soil fertility for crop production. The use of phosphate rock (PR) is an alternative to costly soluble P fertilizers, but it is ineffective usually in non-acid soils unless it is modified i.e. partially acidulated (PAPR). A laboratory incubation study using the isotopic exchange kinetic method of 32P and field experiments were undertaken on a neutral Ferralsol of Cuba to evaluate the effectiveness of PAPRs as fertilizers for common bean (Phaseolus vulgaris, L.). Sulfuric-acid based PAPR using 40%, 50% and 60% of the acid required to produce single superphosphate were studied. In the laboratory experiment Trinidad de Guedes PAPR was effective in providing P to the soil, through increases in isotopically exchangeable P and the percentage of P derived from fertilizer (% Pdff). In the three field experiments carried out to compare the P sources, yields of common bean were increased by PAPR, though the response was less than with triple superphosphate (TSP). The relative agronomic effectiveness (RAE) of PAPR was greater than that of unacidulated PR. Taking into account the RAE values and the current cost of the P sources, the choice of Trinidad de Guedes PAPR instead of TSP could be economic, although the RAE value for PAPR was lower than that of TSP. This result indicates that PAPR could be used in the soil understudy to obtain the best economic return. DM yield, P uptake and grain yield of common bean were significantly increased by applying P as 50% PAPR. Low cost improvement of the agronomic value of PR can be achieved by partial acidulation, so this modification of the phosphate rock show promise for utilization of PR reserves indigenous to developing countries. (author)