Understanding the Inhibition of Color Change in Problematic Red Parent Material Hydric Soils

Abstract

Core Ideas Problematic red parent material (PRPM) soils lack typical hydromorphological characteristics. Hematite mineralogy was suspected to limit redox‐induced color change in PRPM soils, but mechanisms were previously unknown. Physical occlusion of Fe oxides in fine (silt and sand sized) lithic fragments did not induce color change resistance. Significant Al for Fe substitution in hematite of PRPM soils was not observed, excluding that mechanism. The large size of hematite crystals limits Fe reduction resulting in color change resistance in PRPM soils. Problematic red parent material (PRPM) soils resist redox‐induced color changes and development of redoximorphic features, posing a challenge to hydric soil and wetland identification. Previous studies suggested that color change resistance was a function of the mineralogical properties of hematite inherited from soil parent materials, but the underlying cause of the phenomenon has remained uncertain. In this study, several hypotheses (i.e., physical occlusion, Al substitution, and crystal size) were investigated to explore the mechanism of PRPM soil color change resistance. The physical occlusion hypothesis was assessed by comparing Color Change Propensity Index (CCPI) values between size fractions for both PRPM and non‐PRPM soils. Persistence of color change resistance (low CCPI) in the finest (clay) fractions of all PRPM soils resulted in removing the physical occlusion hypothesis from consideration. Substitution of Al for Fe in hematite was compared between PRPM and non‐PRPM clay fractions by observing X‐ray diffraction (XRD) peak shifts. Substitution of Al in hematite remained low for the PRPM samples (mean 0.8 mol%), compared with the non‐PRPM (mean 6.8 mol%) suggesting that Al substitution is not the cause of observed color change resistance. Finally, mean hematite crystallite size was evaluated using Scherrer equation calculations. Mean hematite crystallite size in PRPM soils was significantly larger than non‐PRPM soils (P = 0.0039), suggesting that color change resistance and the PRPM soil phenomenon are derived from the large size of hematite crystals.