Free Iron and Coloration in Certain Well‐Drained Coastal Plain Soils in Relation to Their Other Properties and Classification

Physical, chemical, and mineralogical studies were conducted on several well‐drained, moderately fine‐textured North Carolina Coastal Plain soils (primarily Orangeburg and Norfolk series) belonging to the Typic Hapludult subgroup and ranging in B horizon coloration from yellow to yellowish red. Results indicated that geomorphic landscape position and associated relative soil age influence some properties more than does coloration of the B horizon. It appears that to much emphasis has been placed on the hue of the B horizon as a series differentiating criterion in these reddish‐ and yellowish‐hued coastal plain soils. Except for horizons containing plinthite or intense iron mottling, nearly all of the free iron oxide in the soils investigated occurred in the clay fraction, predominantly in the < 0.2 µ clay. Goethite was the only X‐ray identifiable crystalline iron oxide compound found in clays and other iron‐rich soil fractions. There was no consistent relationship between amounts of free iron oxide and yellow to red Munsell hue × value indices of soil and clay fractions. However, a general trend of increasing free iron oxide content with increasing Munsel chroma was noted in various size fractions of these soils. Free iron and specific surface data indicated that coloration in these soils is not due to the relative thickness of adsorbed iron coatings on the clay surfaces. Heating experiments and infrared studies suggested that the form of the iron oxide (its degree of hydration or neutralization) is the dominant factor in influencing hue of the subsoil in these Coastal Plain soils.