DEPTH-OF-EROSION ASSESSMENT USING IRON-MANGANESE NODULE CONCENTRATIONS IN SURFACE HORIZONS

Abstract

Iron-manganese nodules are commonly observed on the surface of loess-derived fragipan soils of the lower Mississippi River Valley in quantities that appear to be related to degree of erosion. Thus, this study was conducted to determine if accumulations of these nodules could be used to estimate relative amounts of prior erosion among sites and to identify the process by which these materials become concentrated in Ap horizons. Generally, depth to fragipan provides a good estimate of relative degree of erosion. Therefore, soil samples were collected from four sites that differed with respect to depth to fragipan and relative amounts of past erosion. These included a virgin site (uneroded) in addition to slightly, moderately, and severely eroded sites. Average soil depths above the fragipan were 92, 59, 43, and 19 cm, respectively. Soil samples collected from the surface horizons of these sites were analyzed for nodule concentrations in the >2.0-, 2.0–0.50-, and 0.50–0.125-mm fractions. Average concentrations in the >2.0–0.125-mm range for the four soil depths totaled 13.6, 19.4, 36.9, and 76.0 g kg-1, respectively. Regression analysis indicated that depth to fragipan explained 88% of the variability in nodule concentrations. Nodule distributions in soil profile samples from these same areas indicated that, initially, maximum concentrations occurred in lower B-horizons but gradually shifted to Ap horizons as erosion progressed. A rainfall simulator study conducted on these sites showed that nodules accumulate because of their greater stability than soil aggregates of similar size, and that nodules tend to accumulate in proportion to their size.