Redefining the Fragipan to Improve Field Recognition Accuracy.

The fragipan subsoil horizon is one of the most land use limiting soil horizons worldwide due to its ability to perch water and limit deep rooting.  Historically, multiple theories of fragipan genesis have been proposed for discrete parent materials with limited agreement in processes, but general agreement to common properties and geography.  How a soil horizon with such similar characteristics occurs over such a broad geographic range with contrasting major land resource areas and geological history still remains for debate. Recognition of the fragipan’s presence is often essential in correctly describing many epipedons and subsurface diagnostic horizons, and in classifying a soil according to US Soil Taxonomy. However, soil scientists commonly disagree on whether a fragipan horizon meets all of the diagnostic criteria, especially when working in unfamiliar regions. In counter to this trend is the extremely high proportion of soil scientists who typically will agree that the horizon has at least fragic soil properties; “the essential properties of a fragipan. They [fragic soil properties] have neither the layer thickness nor volume requirements for the fragipan.” We propose that the fragipan be redefined to simply meet the criteria of fragic soil properties. Generalizing the definition will do little to change soil land use interpretations given most interpretations are based indirectly off the fragipan’s effect, such as when a perched water table is present. No change to Soil Taxonomy would be needed either if the descriptive terms were switched; where fragipan is now one would simply use fragic soil properties instead. The usefulness of fragic soil properties currently at the subgroup level of Soil Taxonomy is debatable especially given the arbitrary choice of depths and percent of the horizon having the feature; its use does little to add to land use interpretation. Whether the spatial extent of the horizon would increase or decrease is dependent on regional mapping efforts and likely local convention.