Phosphate Rocks Compacted with Superphosphates vs. Partially Acidulated Rocks for Bean and Rice

Phosphate rocks (PR), which have low reactivity and contain large amounts of Fe and Al oxides, may be both ineffective as direct-application fertilizers and unsuitable for partial acidulation. Such PRs can be made more effective by mixing them with soluble phosphates and compressing the mixture into pellets. The agronomic effectiveness of such pelletized mixtures was compared with that of partially acidulated phosphate rocks (PAPR), in pot trials with upland rice (Oryza sativa L.) and kidney bean (Phaseolus vulgaris L.) as indicator crops. Capinota PR, (CPR) which is low in reactivity and contains 8.8% Fe2O3 + Al2O3, and moderately reactive Huila PR (HPR), with 2.3% Fe2O3 + Al2O3, were mixed with triple superphosphate (TSP) to provide 50% of the total P in water-soluble form in the mixture. The mixtures were then compacted into pellets. A Hartsells silt loam (fine-loamy, siliceous, thermic Typic Hapludult, pH 4.5) was used in the study. With TSP as the standard (assuming 100% effectiveness), pelletizing it with CPR increased the CPR's relative effectiveness from 43% before treatment to 80% after for rice grain production and from 51% before treatment to 92% after for bean biomass yield. Likewise, P uptake by rice increased from 47 to 79% and uptake by beans from 51% to 92% of that of TSP for CPR compacted with TSP. The moderately reactive HPR, low in Fe2O3 + Al2O3, the Huila PAPR (HPAPR), and the compacted HPR pus TSP were 83 to 86% as effective as TSP in increasing grain yield of rice and 75 to 81% as effective in promoting P uptake. For bean, the relative effectiveness of HPR, HPAPR, and compacted HPR plus TSP was 88 to 95% for biomass yield and 80 to 88% for P uptake. This study confirms that, for some PRs that have low reactivity and high Fe2O3 + Al2O3 contents, compaction with soluble P fertilizers is agronomically more effective than partial acidulation.