Plinthite Formation in the Coastal Plain Region of Mississippi

Plinthite is a redoximorphic feature of many highly weathered soils and is considered to be an iron-rich and humus-poor pedogenic product that hardens irreversibly on exposure to sunlight. Soil conditions and the pedogenic processes responsible for plinthite formation are understood only within a general framework, lacking a minimum set of the conditions necessary and sufficient to predict which soils contain plinthite. Four plinthite-bearing pedons are described, classified, and analyzed to determine their physical, chemical, and mineralogical properties. In addition, detailed elemental analysis of the whole soil, clay fraction, plinthite nodules, and excised zones of Fe-accumulation and Fe-depletion was performed to estimate if Fe and Sc may be depleted or accumulated preferentially in these fractions. Iron, preferentially associated with the clay fraction, accumulated in the illuvial horizons as a result of the clay eluviation-illuviation processes. Iron is also preferentially incorporated into zones of Fe-accumulation, including plinthite nodules; however, Sc does not segregate between zones of Fe-accumulation and Fe-depletion. The concentration divergence of Fe and Sc suggests that plinthite is primarily an oxidation-reduction process involving diffusion processes. Plinthite formation occurs in conjunction with Btx horizon degradation and in a zone of Fe-accumulation having a firm to very firm consistency.