Plinthite--Quantification and Interpretive Importance in Soils of the Southeast U.S.

Plinthite is common in kaolinitic, weathered soils in the southeastern U.S.  Iron is segregated by redoximorphic processes. Iron-rich pedogenic concentrations can develop into a continuous, cemented subsoil layer as plinthite, which restricts the penetration of roots and movement of water.  Identification and quantification of plinthite can be subjective due to its weak cementation and leads to inconsistent classification and correlation of soils.  Eleven pedons representing 5 soil series (Kandiudults and Kanhapludults) from South Carolina were examined in this study with objectives to (1) quantify the amount of plinthite, (2) evaluate the impact of this plinthite on soil properties, and (3) discuss the importance of properly recognizing this material for soil interpretation.  This study has shown that pedon descriptions of similar soils often vary widely in both the quantity of plinthite materials (identified via volume estimates from standardized charts) and in the identification of the “degree of cementation” . These differences have resulted in inconsistent correlation and pedon classification from one region to another. An established procedure was modified to more consistently estimate the volume of plinthite.  Weakly-cemented plinthite was identified (beginning between 61 to 96 cm) and amounts varied from 0 to 30%.  Horizons rich in plinthite (Btv and Btvx) had bulk densities ranging from 1.58 to 1.78 g cm^-3, with citrate dithionite-extractable Fe up to 4.2%.  There was no general relationship of percent plinthite with clay, citrate dithionate extractable Fe (Fed), or bulk density.