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Greenhouse and Field Evaluation of Indigenous Phosphate Rocks in an Ultisol: Effect of Application Rate and Chromolaena Odorata (L.) Ammendment on Phosphorus Availability and Yield of Rice and Corn
Dissertation Abstract:
Phosphate deposits are found in the Philippines where vast areas have acidic and phosphorus (P)-deficient soils. Thus, this study aimed to seek some ways of using indigenous phosphate rocks (PRs) for direct application as P fertilizers for ultisols.
PR samples obtained from Palawan caves were ground to pass a 100-mesh sieve then 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°C was also tried.
Greenhouse trials assessed the effect of the application rate, Chromolaena odorala ammendment, and ordinary superphosphate (OSP) on the effectiveness of medium and low reactive PRs applied to Aborlan sandy 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 Pas extracted by neutral ammonium citrate or two percent formic and citric acid extractants. The neutral ammonium citrate was able to extract P from PRs containing carbonate apatite but it did not extract P from the PRs containing variscite or strengite. Formic and citric acids extracted P from all the samples analyzed.
Citrate-soluble P of the PRs was available to plants as that of OSP. OSP supplementation did not increase the effectiveness of PRs. Among the organic matter-supplemented treatments, the medium PR at 45 kg Pp5 plus 45 t/ha of fresh herbage of C. odorata 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. odorata supplements.
In the lowland field test, low and medium reactive PRs were as effective as OSP, as indicated by P availability and rice yield. Available P, exchangeable Ca and pH of the soil in the PR-treated and control plots were statistically the same after harvest. Estimated net gain was greater with the use of OSP than with the use of PRs as P fertilizers.
