A comparison of fractionation methods for forms of phosphorus in soils

We used 16 soils to compare the Hedley method for soil phosphorus fractiona tion to an alternative method recently developed by Ruttenberg to different iate among P fractions in marine sediments. For forms of labile and Fe-boun d P in soils, these methods were poorly correlated, with the Hedley fractio nation showing a greater ability to discriminate among variations in plant- available P. For Ca-bound P, total organic P, and total P, the methods were well correlated (r(2) = 0.93, 0.48, 0.74, respectively), although the sum of P measured in the Ruttenberg extractions is only 45% of the total P reco vered by the Hedley fractionation. The Hedley fractionation seems superior when an index of plant-available phosphorus and a separation of organic and inorganic forms is needed, whereas the Ruttenberg method allows a separati on of CaCO3-bound P from apatite-P, which is potentially useful in calcareo us soils.