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Aluminum and Iron Ion Products in Acid Extracts of Samples from Various Depths in a Menfro Soil¹

ABSTRACT

Samples from ten horizons of a Menfro soil were equilibrated for 16 and 65 days with dilute HCl. Aluminum, iron, and phosphorus contents and pH and E (platinum) were determined on the solution portion. The values of pAl + 3pOH, pAl + 2pOH + pH₂PO₄, pFe + 3pOH, and pFe + 2pOH + pH₂PO₄ were computed for each horizon.

The aluminum hydroxide ion product was found to be pH dependent. Gibbsite shows the same pH dependence under similar conditions. Comparisons between pAl + 3pOH values of gibbsite and Menfro soil made at the same pH indicated the aluminum oxides in the Menfro soil had solubility characteristics similar to gibbsite. Values of pAl + 3pOH corrected for pH dependence were the same at all depths in the Menfro profile. The pAl + 2pOH + pH₂PO₄ values in the Menfro soil, although pH dependent, were similar to variscite.

Iron hydroxide solubility was found to be similar to goethite. Values of pAl + 3pOH were found to be lower in the lower B and upper C horizon than in other parts of the profile. A mean value of pFe + 3pOH of 41.1 was found for iron oxides in the Menfro soils. Iron phosphates were more soluble in the lower portion of the Menfro soil. The pFe + 2pOH + pH₂PO₄ values in the upper horizons were greater than 35 and were near 34 in the lower horizons. These data indicate iron phosphates in the Menfro soil have solubilities similar to strengite.

¹ Contribution from the Missouri Agr. Exp. Sta., Journal Series number 6248.

² Associate Professor and Professor of Agronomy, respectively, Univ. of Missouri-Columbia 65201.

Received for publication December 9, 1971. Accepted for publication July 26, 1972.