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) CO 2 ‐saturated water (P‐CO 2 ), (2) ammonium acetate EDTA (P‐AAEDTA), and (3) sodium bicarbonate (P‐NaHCO 3 ) 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 33 P after 1 week of incubation, and the specific activity (SA = 33 P / 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‐CO 2 extracted between 20 and 100 times less P than P‐AAEDTA and P‐NaHCO 3 . The SA of P‐CO 2 was not different from the SA of water‐extractable P, showing that P‐CO 2 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‐NaHCO 3 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.