Mineralogical Investigation of Color Change Resistance in Hydric Soils Derived from Problematic Red Parent Materials.

It has long been recognized that soils derived from certain red parent materials (RPM) resist color changes often associated with typical hydric soils. As a result, test indicator TF2, and more recently Field Indicator F21 (Red Parent Material), were developed and approved for testing in delineating hydric soils in the United States. Previous work suggests that this “problematic” resistance to color change comes from mineralogical characteristics inherited from their parent materials, but the true cause remains uncertain. During a collaborative, nationwide project to evaluate the extent and location of problematic RPM, this question of the fundamental cause of the “problematic” nature of RPM soils was explored. Using red soils derived from both problematic and non-problematic parent materials, several mineralogical hypotheses were examined including: 1) the physical occlusion of iron oxides within specific particle size fractions of RPM soils; 2) substitution of Al for Fe within the hematite structure of RPM soils; and 3) the crystal size of hematite in RPM soils versus non-RPM soils. This presentation will present the general results from the mineralogical investigations to test these three hypotheses, and to provide context for possible (updated) color requirements of the current F21 RPM Field Indicator.