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Comparison of Methods to Estimate Occluded and Resistant Soil Phosphorus
Author(s) -
Bowman R. A.,
Rodriguez J. B.,
Self J. R.
Publication year - 1998
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1998.03615995006200020007x
Subject(s) - molybdate , chemistry , extraction (chemistry) , soil water , phosphorus , coefficient of variation , soil test , environmental chemistry , chromatography , soil science , environmental science , inorganic chemistry , organic chemistry
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 new procedure for the resistant P. Occluded P was difficult 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 [average coefficient of variation (CV) = 5%] than in the sequential extraction (average 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.