Anion Adsorption by Allophanic Tropical Soils: III. Phosphate Adsorption1
- H. Gebhardt and
- N. T. Coleman2
Andepts from Mexico and Hawaii bound 30–70 mmole P/100 g air-dry soil at pH 4.3–4.7 during a 1-hour reaction with 0.05M mixed H3PO4-NaH2PO4 solution. As much as 130 mmole P/100 g were taken up from H3PO4 at pH 2.4. A B-horizon sample, San Gregorio from Michoacan, Mexico, adsorbed P from H3PO4 and consumed H in nearly equimolar amount, with 38 mmole/100 g of each removed from solution in bringing soil from an initial pH of 5.4 to 4.3. Larger additions of H3PO4 dissolved appreciable Al, gave pH < 4, and yielded solutions that were supersaturated with respect to variscite. An apparent adsorption maximum of 38 mmole/100 g compares with maxima of 27 and 30 mmole/100 g for Cl and sulfate, respectively. High-affinity adsorption of P from H3PO4 appeared limited by the supply of H-ions to protonate sites or react with displaced OH. San Gregorio B horizon took up phosphate from NaH2PO4, but with low affinity. Phosphate uptake from NaH2PO4 was accompanied by coadsorption of Na. At a given pH and level of tightly bound P established with H3PO4, addition of NaH2PO4 resulted in low-affinity adsorption of P and coadsorption of Na. Around half of the P bound from NaH2PO4 was readily eluted with water. Phosphate bound from H3PO4 was virtually insoluble in water. Some P was eluted by 0.5M arsenate or selenite. The Andepts bound P through at least three mechanisms: high-affinity adsorption on protonated sites; low-affinity adsorption with co-adsorption of Na; and formation of insoluble variscite-like substances.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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