Fragipan degradation and nodule formation in Glossic Fragiudalfs of the lower Mississippi River Valley

Glossic Fragiudalfs comprise several million hectares of agronomically important soils within the silty, uplands of the lower Mississippi River Valley (Major Land Resource Area [MLRA] 134). These soils are typified by a fragipan within 100 cm of the surface overlain bg a 5-cm-thick, or more, glossic horizon containing bleached coatings (albic material) along primary ped bees, indicating fragipan degradation. Concentrations of Fe-Mn nodules also occur in horizons above the fragipan. There is limited documentation regarding the in situ morphology of the nodules or their relationship to the underlying fragipan. The objectives of this study were to document (i) the profile distribution of nodules, (ii) variations in nodule morphology with depth, and (iii) the role of fragipans in nodule formation. Nodules were most common and largest directly above the fragipan horizons. Field and micromorphological observations suggested the nodules formed in remnants of the brittle fragipan matrix as a result of the conversion of Btx material to E' material (albic material) that isolated and surrounded brittle fragipan peds. The brittle peds that are the precursors to nodules appeared to decrease in size and became more highly impregnated with Fe and Mn oxides with distance above the fragipan. Spatial relationships of Fe concentrations and argillans within the nodules illustrated that the nodules were being formed in place and were not transported despite having distinct borders. With time the glossic horizon expands at the expense of the fragipan, resulting in a horizon consisting of Fe-Mn nodules and a few isolated brittle peds in a matrix of albic material.