IMPROVING THE IRON-OXIDE SINK METHOD FOR EXTRACTING SOIL-PHOSPHORUS

Iron oxide coated filter paper can estimate labile P in soil by acting as a P sink. Variations in procedures and contamination from soil adh ering to the sinks limit effective data interpretation, however. We ev aluated two papers with pores <5.0 mu m and one with pores 20 to 25 mu m. Iron oxide coatings were formed by exposing FeCl3-treated papers t o NH3 vapor or 2.7 M NH4OH. Coated papers were either fixed in a scree n assembly or allowed to move freely during shaking. The screen assemb ly 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 contaminat ion (0.8-1.9 mg) than fixed, large-pore circles (4.7-8.1 mg), a condit ion confirmed by x-ray diffraction. Similarly, free, small-pore circle s 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 NH3 vapor acidified the extracting solution with Kahola (fine-silty, mixed, mesic Cumulic Hapludoll) and Haynie (coarse-silty, mixed [calcareous], mesic Typic Udifluvent) soil s and increased their extractable Pan 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 im ) in NH4OH solution and securing them during shaking.