Exchangeability of phosphate extracted by four chemical methods

Isotopically exchangeable phosphate (P) is a major source of P for plants. In practice, however, plant-available P is assessed by chemical extractions solubilizing a mixture of P forms the availability of which is ill defined. We undertook an isotopic approach to assess the exchangeability of P extracted by (1) CO2-saturated water (P-CO2). (2) ammonium acetate EDTA (P-AAEDTA), and (3) sodium bicarbonate (P-NaHCO3) compared to the exchangeability of P extracted by water. Five topsoils with similar P-fertilization histories but different soil properties were studied. Phosphorus was extracted from soils labeled with carrier-free P-33 after 1 week of incubation, and the specific activity (SA = P-33 / (31) P) of the extracts was compared with the SA of P extracted by water to calculate the amount of P isotopically exchangeable that had been solubilized during the extraction. P-CO2 extracted between 20 and 100 times less P than P-AAEDTA and P-NaHCO3. The SA of P-CO2 was not different from the SA of water-extractable P, showing that P-CO2 solubilized similar forms of P as water and that these forms can be considered as available. The SA of P extracted by the two other methods ranged between 25% and 63% for P-AAEDTA and 66% and 92% for P-NaHCO3 of the SA of water-extractable P. The fraction of exchangeable P extracted by AAEDTA decreased linearly with increasing soil pH, suggesting that this method dissolves slowly or non-exchangeable P from calcium phosphates.