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Improving the Iron Oxide Sink Method for Extracting Soil Phosphorus
Author(s) -
Myers R. G.,
Pierzynski G. M.,
Thien S. J.
Publication year - 1995
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/sssaj1995.03615995005900030032x
Subject(s) - sink (geography) , filter paper , soil water , chemistry , calcareous , oxide , iron oxide , mineralogy , soil science , geology , chromatography , paleontology , cartography , organic chemistry , geography
Iron oxide coated filter paper can estimate labile P in soil by acting as a P sink. Variations in procedures and contamination from soil adhering to the sinks limit effective data interpretation, however. We evaluated two papers with pores <5.0 µm and one with pores 20 to 25 µm. Iron oxide coatings were formed by exposing FeCl 3 ‐treated papers to NH 3 vapor or 2.7 M NH 4 OH. Coated papers were either fixed in a screen assembly or allowed to move freely during shaking. The screen assembly eliminated abrasion loss of the FeO coating. Paper circles (5.5 cm) proved satisfactory and eliminated the need to cut 2 by 10 cm strips from coated papers. Fixed, small‐pore circles had less soil contamination (0.8–1.9 mg) than fixed, large‐pore circles (4.7–8.1 mg), a condition confirmed by x‐ray diffraction. Similarly, free, small‐pore circles retained less soil contamination (3.5–9.0 mg) than free, large‐pore circles (16.1–21.7 mg). Two sources of acid‐extractable P inflated the labile P pool. First, acid‐extractable P from soil adhering to papers increased sink‐extractable P by 7.5 to 24.1 mg P kg −1 , an error of 28 to 65%. Also, paper prepared in NH 3 vapor acidified the extracting solution with Kahola (fine‐silty, mixed, mesic Cumulic Hapludoll) and Haynie (coarse‐silty, mixed [calcareous], mesic Typic Udifluvent) soils and increased their extractable P an average of 13 mg P kg −1 , or 56%. To eliminate some analytical problems associated with the FeO‐sink extraction of soil P, we recommend preparing small‐pore circles (<5 µm) in NH 4 OH solution and securing them during shaking.