A high percentage of the P in grassland soils is organic. Where the NaHCO3- extractable P is below the adequate level, plants must rely on mineralization of organic P. This paper attempts to show the nature, transformations, and relative sizes of the organic P pools for five North American grassland soils. We evaluated four such pools: (1) a labile pool, extracted with 0.5 M NaHCO3 (pH 8.5); (2) a moderately libile pool, consisting of acid-soluble organic P and alkali-soluble inorganic P; (3) a moderately resistant fraction (fulvi-acid P); and (4) a highly resistant fraction (humic-acid P). Recovery of organic P from the extracts averaged 7, 34, 47, and 12 percent in the labile, moderately labile, moderately resistant, and highly resistant fractions, respectively. The P in phytic acid was not recoverable in either acid or alkali extractions, although ribonucleic acid was recovered in the labile fraction.
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Bowman et al. (1978) studied this question.