Ringkasan Penggunaan langsung fosfat alam (FA) ke dalam tanah sebagai sumber pupuk P telah dilakukan selama bertahun-tahun melalui beberapa macam cara penggunaan. Kualitas FA di Indonesia umumnya rendah dan ketersediaan bahan baku yang berkualitas untuk produksi pupuk fosfat terlarut relatif terbatas. Beberapa mikroba asal tanah yang SummaryThe direct application of rock phosphate (RP) to soils as a source of phosphorus (P) fertilizer has been employed with varying popular methods over the years. The RP in Indonesia has low quality for plant fertilization and the availability of the raw material with good quality for production of soluble phosphate fertilizers is limited. Many common soil microbes that can dissolve insoluble inorganic phosphate have been extensively studied. However, there is little information on mechanism of P-solubilization from local RP of Indonesia and inorganic P by Aspergillus niger BCC F194 isolated from tropical acid soils. A laboratory study was conducted to determine the ability of phosphate solubilization A. niger BCC F194. Agronomic evaluation of bioactivated local RP, i.e. Cileungsi and Madura phosphate rocks (CRP and MRP) for direct application in greenhouse experiment was also conducted. A. niger BCC F194 was able in solubilizing different types of hardly-soluble phosphates, i.e. CRP and MRP, Ca 3 (PO 4 ) 2 , and AlPO 4 compounds. Inorganic P solubilization was directly correlated to the organic acid production and increasing proton (H+) activities causing pH to decrease and production of organic acid. The major acidic metabolites produced by A. niger BCC F 194 in modified liquid culture Pikovskaya medium were oxalic acid (3.75 mM), citric acid (2.0 mM), and gluconic acid (0.9 mM). The solubilization of Cileungsi RP was higher than that of Madura RP, and the solubilization of Ca 3 (PO 4 ) 2 was better than that of AlPO 4 . No correlation between solubilization of inorganic P and enzyme activities, although high activities of phosphatase enzyme were detectable in the medium. A biosuperphosphate formula had been constructed by reacting local RP with liquid culture supernatant (LCS) replacing sulfuric acid. Results of cocoa, rubber, and oil palm seedlings experiments in greenhouse indicate that both granular and powder biosuperphosphate prototypes commonly had a comparable relative agronomic effectiveness value to that of the conventional SP.

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