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Anion Adsorption by Allophanic Tropical Soils: III. Phosphate Adsorption¹

ABSTRACT

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.

¹ Contribution from the Dep. of Soil Science and Agr. Eng., Univ. of California, Riverside 92502. Support of the National Science Foundation through Grant no. NSF-GB-11711-(Pratt) is gratefully acknowledged.

² Postgraduate Research Soil Scientist and Professor, respectively, Dep. of Soil Science and Agr. Eng., Univ. of California, Riverside 92502.

Received for publication September 10, 1973. Accepted for publication September 17, 1973.