PROPERTIES OF IRON-MANGANESE NODULES FROM A SEQUENCE OF ERODED FRAGIPAN SOILS

Iron-manganese nodule concentrations increase in Ap horizons of Fragiu dalfs in the lower Mississippi River Valley as profile thickness is re duced by erosion. This study was conducted to determine how nodule min eralogy and chemistry change with profile depth and after incorporatio n into the plow layer. Nodules larger than 1.0 mm were separated from each horizon of uneroded, slightly, moderately, and severely eroded pe dons. Nodules were ground to <45 mu m and analyzed for organic C, in a ddition 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) fluctu ated with depth; however, Fe-o/Fe-d ratios reached a minimum in E/Bx h orizons. In A horizon nodules, Fe-d Increased with degree of erosion, ranging from 70 (uneroded) to 99 g kg(-1) (severely eroded). Conversel y, Fe-o aad Fe-o/Fe-d decreased in Ap horizon nodules as erosion incre ased. Phosphorus contents in DCB extracts followed the Fe-d depth dist ribution curve. Manganese extracted by DCB decreased with profile dept h and severity of erosion. Although Fe-o/Fe-d ratios suggested the pre sence of ferrihydrite, goethite was the only Fe-oxide mineral identifi ed from differential x-ray diffraction (DXRD) scans. Goethite XRD peak intensities tended to reflect Fe-d contents, reaching a maximum in th e 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 chrom a, respectively. However, no crystalline Mn-oxide minerals were identi fied. Incorporation of E/Bx nodules in Ap horizons by gradual erosion and removal of finer materials is probably responsible for the observe d changes in nodule color, chemistry, and mineralogy.