| Abstrakt: |
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 FeCl3‐treated papers to NH3vapor or 2.7 MNH4OH. 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 NH3vapor 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 NH4OH solution and securing them during shaking. |
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