EVALUATION OF THE AGRONOMIC EFFECTIVENESS OF NATURAL AND PARTIALLY ACIDULATED PHOSPHATE ROCKS IN SEVERAL SOILS USING P-32 ISOTOPIC DILUTIONTECHNIQUES

The agronomic effectiveness of two natural phosphate rocks (PRs) from North Carolina (USA) and Togo and their 50% partially acidulated produ cts (PAPRs) was evaluated in two greenhouse experiments using P-32 iso topic dilution techniques, namely L and A(L) values. In the first expe riment rye grass was grown in a soil from Ghana. While the proportion of P in the plant derived from the P fertilizer (Pdff) ranged on. the average from about 10% for the PRs up to 80% for the PAPRs, the P fert ilizer recovery was less than 1% for a 60-day growth period. In the se cond experiment, average values of P in the maize plants derived from the PAPRs ranged from 35% to 75% in 3 different soils. Both PRs were i neffective with the exception of North Carolina PR in the Seibersdorf soil. The P fertilizer recovery was 0.25% for the North Carolina PR in this soil whereas the recovery values ranged from 1.2% to 1.6% for th e PAPRs. Mean values of the relative fertilizer efficiency estimated f rom the L values of each soil were less than 1% for the PRs whereas th e values for the PAPRs which were dependent on soil type ranged from 2 0% up to 45%. The coefficient of relative effect of partial acidulatio n, that was calculated from the ratio of A(L) values for PR and PAPR i n each soil indicated that partial acidulation increased the effective ness of the natural PRs in all soils under study. This study showed th at the use of P-32 isotope dilution techniques allows an accurate meas urement of the P availability from natural and modified PR products to crops. Another advantage is that quantitative comparison of the P sou rces under study, PRs and PAPRs in this case, can be made even in soil s where there is no response to the applied P sources.