Iron Oxide Sink Method for Extracting Soil Phosphorus: Paper Preparation and Use
- R. G. Myers ,
- G. M. Pierzynski and
- S. J. Thien
Since the FeO-sink method to estimate plant-available P in soils was first introduced in 1983, modifications have continued to improve the procedure. Recently FeO-coated paper circles (5.5-cm diam.) were used in place of the traditional 2 by 10 cm strips. The objective of this study was to more clearly describe the preparation and use of the FeO-coated circles. We used Whatman no. 50 hardened filter paper (2.7-µm pore openings). Standard methods for FeCl3 treatment of the papers were used. After the papers had been air dried, one paper was secured between two 10 by 10 cm fiberglass screens. By simultaneously immersing stacks of 25 of these paper-screen units in 2.7 M NH4OH for a 30-s immersion time, 100 papers could be uniformly coated with FeO in about 10 min. Immersion times of 30, 60, and 180 s in NH4OH solution showed that papers immersed for 30 s in NH4OH solution extracted P as effectively as those immersed for longer times. One 5.5-cm FeO circle was as effective as three circles in extracting P from most soils. A desorption model for soil P was used that can relate soil-test P to desorption kinetics. In soils with maximum reversible P ranging from 25.1 to 106.5 mg kg−1, desorption constants ranged from 0.141 to 0.232 h−1 with a mean of 0.191 h−1. Low-P soils released P slower than did high-P soils. Such information may be of interest in assessing P availability in soils.
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