Redefining the Fragipan to Improve Field Recognition and Land Use Relevance

The fragipan subsoil horizon can be a significant limitation to land use and management because of its ability to perch water, restrict root growth, and reduce available water storage. Recognition of the presence of a fragipan is used to describe some diagnostic horizons (mollic, umbric, cambic) and to identify the limits of the particle size control section when classifying a soil according to the U.S. soil taxonomic system. While soil scientists often disagree on whether fragipan diagnostic criteria are met, they will typically agree that the horizon has fragic soil properties. To potentially improve field identification agreement, we propose that the fragipan be redefined to meet criteria more like fragic soil properties. We also propose to include some current criteria for the fragipan denoting pedogenesis, slaking, and HCl reaction. In addition, we propose a new requirement be added for redoximorphic features above the fragipan as additional evidence of its restrictive nature. Changing the fragipan's definition will do little to alter soil land use interpretations given most interpretations are based indirectly off the effects of the fragipan. Analysis of existing series indicates that our proposal should not remove series now classified with a fragipan but would eliminate some ambiguities in field identification. Whether the fragipan's spatial extent would increase or decrease is dependent on regional mapping efforts.