ELEMENTAL COMPOSITION OF SOIL NODULES FROM TWO ALFISOLS ON AN ALLUVIAL TERRACE IN MISSOURI.

Soils having fluctuating water tables and exhibiting redoximorphic color patterns commonly have Fe- and Mn-bearing nodules. Two soils were examined to determine the dominant soil forming processes influencing soil genesis, with particular attention directed toward understanding soil processes responsible for the chemical composition of these Fe- and Mn-bearing nodules. Somewhat poorly and poorly drained Alfisols developed in silty-textured Pleistocene terraces show a range in pH, exchangeable Na, and the distribution of Fe- and Mn-bearing nodules. Nodule formation is more evident in the near-surface soil horizons and roughly declines with increasing soil profile depth. Elemental analysis of the particle size separates shows that the sand fractions have substantially greater Fe and Mn concentrations and greater Mn/Fe ratios. Natric horizons exhibit a more alkaline reaction and show greater Mn and Fe concentrations in fine nodules when compared with fine nodules from more acidic horizons. Trace element analysis reveals that trace elements having multiple valence states are preferentially incorporated in larger nodules, whereas trace elements having solitary valence states are more concentrated in smaller nodules. If it is reasonable to suppose that coarse nodules experience a greater number of oxidationreduction episodes during their formation, then trace elements having multiple valence states may participate in the oxidation-reduction processes and have a greater large nodule abundance.