Release of Nonexchangeable Potassium from a Highly Weathered, Forested Quartzipsamment
- N. B. Comerford ,
- W. G. Harris and
- D. Lucas
Long-term K supply is often considered a function of the amount of K-bearing minerals present. Hydroxy-interlayered vermiculite (HIV) is a ubiquitous secondary mineral of the southeastern lower Coastal Plain and is now known to be associated with nonexchangeable K in these soils. Oxalate has also been shown to exist in millimolar quantities in these soil solutions. The objective of this study was to test the effect oxalate has on the potential solubility of K in a forested Quartzipsamment containing appreciable HIV in the < 50-µm fraction, but no other detectable K-bearing minerals. Samples were prepared, chemically extracted, and extracting solutions analyzed for K. We found that: (i) K can be released from clay and silt fractions by oxalate concentrations ranging from 1 to 150 mM and pH 3 to 6; (ii) sequential extractions with oxalate removed the most K during the first extraction with the silt being depleted of readily released K after three to four extractions but with the clay releasing K continuously over seven extractions; (iii) low pH was as effective in the release of K as oxalate at higher pH; (iv) after a 210- to 240-d incubation of the extracted samples, re-extraction resulted in high levels of K being released, even from the silt fraction that had been previously depleted; (v) electron microprobe analysis and K x-ray images suggested that K contents were higher in the center of the phyllosilicate grains than near the edges.
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