Mineral Weathering and Iron Segregation in Plinthite-Rich Soils of South Carolina.

Plinthite is common in soils formed in Pliocene-aged loamy-textured fluviomarine deposits in the Upper Coastal Plain physiographic province of South Carolina. Seasonal water tables have created redox conditions, a process that resulted in segregation of Fe in plinithic horizons.  Four pedons (three Plinthic Kandiudults and one Arenic Plinthic Kanhapludult) were examined in this study with an objective to evaluate the degree of mineral weathering and pedogenesis in these soils, focusing on the distribution and mineralogy of Fe oxides within horizons and pedons.  Clay ranged from 3-4% in A and E horizons to > 40% in the subsoil (plinthite rich) horizons.  Plinthite ranged from 8 to 30% in Btvx horizons.  Sand mineralogy consists of > 90% quartz.  Lath-shaped mica is the most common weatherable mineral and is likely pseudomorphically altered to kaolinite.  Micromorphology of plinthite rich horizons shows that the micromass (clayey material) is variable in color, ranging from yellowish red to brownish yellow, with redox depletions of light gray. Areas of Fe depletion and enrichment (plinthite accumulations) are often intermixed, with gradual, diffuse boundaries between the two zones.   The Fe appears to have moved from reduced ped interiors and precipitated as oxidized hypocoatings along root channels and other voids, or superimposed on clay coatings.  The Fe depleted (gray) zones within horizons are slightly higher in clay than the red zones where Fe accumulates. Citrate dithionite-extractable Fe ranges from 0 to 3.2% higher in the redder zones, while acid oxalate-extractable Fe is similar in both materials ranging from non-detectable to 0.09%.  The degree of segregation of mineral constituents, as well as mineral type, between the oxidized and reduced zones likely influences the degree of plinthite cementation.  Instances of diffuse boundary conditions between the reduced and oxidized materials may play a significant role the ability of a soil scientist to recognize plinthite.