Properties of Iron—Manganese Nodules from a Sequence of Eroded Fragipan Soils

Iron‐manganese nodule concentrations increase in Ap horizons of Fragiudalfs in the lower Mississippi River Valley as profile thickness is reduced by erosion. This study was conducted to determine how nodule mineralogy and chemistry change with profile depth and after incorporation into the plow layer. Nodules larger than 1.0 mm were separated from each horizon of uneroded, slightly, moderately, and severely eroded pedons. Nodules were ground to <45 µm and analyzed for organic C, in addition to Fe, Mn, and P following extraction with dithionite‐citrate‐bicarbonate (DCB) and acid ammonium oxalate (AAO). Nodules from the E/Bx horizons of the least eroded pedons had the highest DCB‐extractable Fe (Fe d ) contents. Distributions of AAO‐extractable Fe (Fe o ) fluctuated with depth; however, Fe o /Fe d ratios reached a minimum in E/Bx horizons. In A horizon nodules, Fe d increased with degree of erosion, ranging from 70 (uneroded) to 99 g kg −1 (severely eroded). Conversely, Fe o and Fe o /Fe d decreased in Ap horizon nodules as erosion increased. Phosphorus contents in DCB extracts followed the Fe d depth distribution curve. Manganese extracted by DCB decreased with profile depth and severity of erosion. Although Fe o /Fe d ratios suggested the presence of ferrihydrite, goethite was the only Fe‐oxide mineral identified from differential x‐ray diffraction (DXRD) scans. Goethite XRD peak intensities tended to reflect Fe d contents, reaching a maximum in the E/Bx horizons and A horizons of the more severely eroded pedons. The color of the crushed nodules from the A horizons became progressively lighter as degree of erosion increased, due to increases in Fe d and decreases in Mn d and organic C. Manganese had the greatest effect on color, accounting for 93 and 85% of the variability in value and chroma, respectively. However, no crystalline Mn‐oxide minerals were identified. Incorporation of E/Bx nodules in Ap horizons by gradual erosion and removal of finer materials is probably responsible for the observed changes in nodule color, chemistry, and mineralogy.