Author : Castillo, Edgardo H.

Summary : Phosphate deposits are in the Philippines are located in vast areas where soils are acidic and phosphorus-deficient. Thus, this study aimed to seek some ways of using indigenous phosphate rocks (PRs) for direct application as phosphorus (P) fertilizers for ultisols. PR samples were obtained from Palawan caves and were ground to pass a 100-mesh sieve before they were analyzed and used as P fertilizers. Mineral contents and chemical reactivities of the PRs were determined using X-ray diffractometer and chemical extractants, respectively. Calcining a variscite at 100-200 degree centigrade was also tried. Greenhouse trials assessed the effect of application rate, Chromolaena odorata ammendment, and ordinary superphosphate (OSP)on the effectiveness of medium and low reactive PRs applied to Aborlan sandly loam with corn as test crop. A field trial tested some treatments based on the result of the greenhouse experiments. The major P minerals of the indigenous PRs were carbonate apatite, hydroxyapatite, flourapatite, strengite, and variscite. The chemical reactivities of these PRs were very low to medium based on their available P as extracted by neutral ammonium citrate, or 2 percent formic and citric acid extractants. The neutral ammonium citrate extracted P from PRs containing carbonate apatite but it did not extract P from the PRs containing variscite or strengite. The formic and citric acids extracted P from all the samples analyzed. Citrate-soluble P of the PRs was available to plant as that of OSP. OSP supplement did not increase the effectiveness of PRs. Among the organic matter-supplemented treatments, the medium reactive PR at 45 kg P (subscript 2) O (subscript 5) plus 45 tons of fresh herbage of C. odorata per hectare was the most effective in terms of corn dry matter yield. Soil available P after the greenhouse trials increased as the PR application rate increased with or without OSP or C. orodata supplements. In the lowland field test, low and medium reactive PRs were as effective as OSP.