Anion Adsorption by Allophanic Tropical Soils: III. Phosphate Adsorption

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 0.05 M mixed H 3 PO 4 ‐NaH 2 PO 4 solution. As much as 130 mmole P/100 g were taken up from H 3 PO 4 at pH 2.4. A B‐horizon sample, San Gregorio from Michoacan, Mexico, adsorbed P from H 3 PO 4 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 H 3 PO 4 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 H 3 PO 4 appeared limited by the supply of H‐ions to protonate sites or react with displaced OH. San Gregorio B horizon took up phosphate from NaH 2 PO 4 , but with low affinity. Phosphate uptake from NaH 2 PO 4 was accompanied by coadsorption of Na. At a given pH and level of tightly bound P established with H 3 PO 4 , addition of NaH 2 PO 4 resulted in low‐affinity adsorption of P and coadsorption of Na. Around half of the P bound from NaH 2 PO 4 was readily eluted with water. Phosphate bound from H 3 PO 4 was virtually insoluble in water. Some P was eluted by 0.5 M 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.