Abstract
Abstract Addition of a separate acid hydrolysis step in the determination of watersoluble phosphorus was found necessary in the official volumetric method but not in the quinoline molybdate method. By carefully controlling the phosphorus and ammonium citrate concentrations in the direct determination of available phosphorus, phosphorus can be completely precipitated as quinoline phosphomolybdate. The nitration of the ammonium citrate digestate in preparing the combined water-soluble and citrate-soluble extract for direct determination of available phosphorus can be eliminated. The latter two observations were supported by the results of a collaborative study by 8 analysts on 8 fertilizer materials.
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