Loess Toposequences in the Lower Mississippi River Valley: I. Fragipan Morphology and Identification

Despite large acreages of agronomically important fragipan soils in the Lower Mississippi River Valley, relatively little information is available regarding their morphology, particularly in regard to fragipan definition and formation. This study was designed to evaluate morphological differences between nonfragipan (memphis [fine‐silty, mixed, thermic Typic Hapludalf]) and fragipan (Loring [fine‐silty, mixed, thermic Typic Fragiudalf] and Grenada [fine silty, mixed, thermic Glossic Fragiudalf]) soils occurring on the same landscape. A secondary objective was to determine if morphological characteristics of the Loring and Grenada pedons are consistent with taxonomic criteria. Five representative sites were selected based on published soil survey maps and landscape relations. Morphological characteristics associated with fragipans are present in all the Loring and Grenada pedons. The most obvious and consistent indicators of a fragipan are vertical gray seams and associated redoximorphic features. Because brittleness is subjective and a function of moisture content, it is not always a reliable indicator of fragipan occurrence. Likewise, root distribution is not particularly useful since deep‐rooted vegetation did not occur on these soils. Several Memphis pedons exhibit protofragipan development, but based primarily on the organization of gray seams, these should not be considered fragipan soils. Overall, the published criteria are adequate, with the notable exception of slaking, and the characteristics of the Loring and Grenada pedons are consistent with them. Greater relative root penetration and less brittleness also assist in differentiating between the nonfragipan horizons within the Memphis pedons and fragipan horizons of the other members of the toposequence, but these evaluations have to be made in detail from deep pits, particularly in borderline situations.