A Thirty Year Review of the Kandic Horizon in Soil Taxonomy.

The kandic subsurface diagnostic horizon was introduced in the Third ed. of the Keys to Soil Taxonomy (1987) based on the recommendation of the International Committee on Low Activity Clays (ICOMLAC).  The SSSA Soil Taxonomy Task Force has several guiding principles with regard to developing fundamental changes to the system, including “fundamental changes in Soil Taxonomy should lead to a more user-friendly product that can and will be used by more than experienced pedologists.”  The thirty year anniversary of the kandic horizon provides an opportune time to reflect on its impact and relevance with regard to this principle. Kandic horizons possess low activity clay dominated by kaolinite and sesquioxides, and share properties with the argillic (relatively finer textured subsoils) and oxic (low activity clay) diagnostic horizons.  Within the U.S., 83 series with kandic horizons exist in the southeastern U.S., Hawaii and Puerto Rico, with most of these being Ultisols in kaolinitic families within thermic temperature regimes.  These soils have been surveyed on over 16 million ha mostly within the Piedmont and Atlantic Coastal Plain physiographic provinces.  Over the years, issues with the kandic horizon’s cation exchange capacity (CEC) methodology and criteria, texture requirements, precedence within Soil Taxonomy, and overlap with other low activity soils both within the U.S. and internationally, have arisen. A review of pertinent data with regard to these issues with recommendations consistent with Task Force goals will be presented.