Comparison of methods to estimate occluded and resistant soil phosphorus

Weathering, pH changes, and biotic interactions through geologic times have created a gradation of easily released (labile) to strongly stabilized (resistant) P pools, While methodologies for fertility indices and labile P are well established, methods for quantifying less labile P, such as the occluded and resistant or residual P, are less well established. We reexamined existing methods for these pools, and suggested new procedures to improve their precision and timeliness. Specifically we compared two methods for reductant-soluble (occluded) P, and two existing and a nea procedure for the resistant P, Occluded P was different to reproduce from a sequential extraction procedure because of problems associated with molybdate blue reaction, which required extra molybdate or persulfate oxidation to minimize citrate interference. However, use of inductively coupled plasma (ICP) spectroscopy eliminated this problem. Measurement of P by ICP for surface and occluded P in the total free Fe oxide pool was more quantitative and reproducible leverage coefficient of variation (CV) = 5%] than in the sequential extraction (overage CV = 7%) procedure. Results for the proposed resistant P method (total soil P minus acid extractable P in an ignited sample) approximated the two sequential extraction procedures (total soil P - total acid-and base-extractable organic and inorganic P) and is easier, and more reproducible. Both procedures for all soils except the Molokai showed essentially the same amounts of occluded P, An average of about 26% of the total soil P (TP) was resistant, with the more weathered Cecil soil containing about 50% resistant P.