Extraction of organic phosphorus in Andosols by various methods

The amounts of inorganic (P_i) and organic (P_o) phosphorus extracted by the Truog, Bray II, Olsen, and Citrate methods, in upland soils consisting of three Andosols and a Yellow soil, with different levels of fertilization were determined. In the soils subjected to the application of both chemical fertilizers and organic matter, the amount of Po extracted by the respective extraction methods was much smaller than that of P_i. Especially, in the Andosol with yearly addition of manure for 28 y, the amount of extractable Po did not increase with the increase of the amount of manure applied even though the contents of total carbon, total phosphorus, and extractable P_i linearly increased. In the uncultivated high humic Andosols, on the other hand, P_o was the dominant form extracted by the Olsen and Citrate methods. In order to determine the P_o status, Pain each extract was further hydrolyzed by phosphatases (acid phosphatase, phytase, and alkaline phosphatase), and the amount of P_i released was measured. Citrate-P_o was hydrolyzed to a certain extent (30–40% by phytase, and 12–13% by acid phosphatase), whereas Olsen-P_o was not hydrolyzed by these phosphatases. These results suggest that the chemical structure or molecular size of the P_o extracted by these methods is different. Citrate-P_o may enable to evaluate plant available P to a certain extent, which is particularly important in Andosols with a low fertility.