This method is used to estimate noncrystalline and poorly crystalline Fe and Al in soils by extracting with an acid ammonium oxalate solution adjusted to pH 3. The extraction solution removes noncrystalline inorganic forms of Fe and Al and organic complexed Fe and Al. Fe and Al are quantitated by ICP-OES. The method has detection limits of approximately 10 ppm for Fe and Al.
Ross, G. J. and Wang C. 1993. Extractable Al, Fe, Mn, and Si. pp. 239-246. In: Carter, M. R. (ed), Soil Sampling and Methods of Analysis, Canadian Society of Soil Sciences, Lewis Publishers, Ann Arbor, MI.