Extraction of organic phosphorus in andosols by various methods

The amounts of inorganic (P1) and organic (Po) 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 P1. Especially, in the Andosol with yearly addition of manure for 28y, 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 P1 linearly increased. In the uncultivated high humic Andosols, on the other hand, P. was the dominant form extracted by the Olsen and Citrate methods. In order to determine the Po status, Po in each extract was further hydrolyzed by phosphatases (acid phosphatase, phytase, and alkaline phosphatase), and the amount of P1 released was measured. Citrate-Po was hydrolyzed to a certain extent (30-40% by phytase, and 12-13% by acid phosphatase), whereas Olsen-Po was not hydrolyzed by these phosphatases. These results suggest that the chemical structure or molecular size of the Po extracted by these methods is different. Citrate-Po hay enable to evaluate plant available P to a certain extent, which is particularly important in Andosols with a low fertility