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Soil horizons identified as "buried" typically require a new material mantle above. The existing threshold for mantle thickness in soil classification is under discussion. A proposal to set a constant thickness of 50 cm for the mantle was tested on various soil series, showing little impact on classification. Removing the sliding scale for mantle thickness between 30-49 cm would have minimal effect on existing soil series and surveys, simplifying Soil Taxonomy.
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Removing the “Sliding Scale” Thickness of the Mantle in Soil Taxonomy – Discussion PointsJohn M. Galbraith1and Shawn McVey21 Virginia Tech, john.galbraith@vt.edu; 2 USDA Nat. Soil Survey Center,shawn.mcvey@lin.usda.gov Introduction Soil horizons (other than C or R) are identified as “buried” by the letter of “b” in the horizon name. However, a buried soil must have a mantle of new material of minimum thickness above the buried horizon sequence. Paraphrased from the 10th Ed. of the Keys to Soil Taxonomy: "A surface mantle of new material ..snip… may have a diagnostic surface horizon and/or a cambic horizon, but it has no other diagnostic subsurface horizons. However, there remains a layer 7.5 cm or more thick that fails the requirements for all diagnostic horizons, overlying a horizon sequence that can be clearly identified as the solum of a buried soil …snip." The minimum mantle thickness is presently set at 50 cm or more, or a sliding-scale from 30 to 49 cm thick that is at least half of the thickness of preserved diagnostic horizons below. There is a review of the buried soil criteria posted as Technical Note 10 at URL: http://soils.usda.gov/technical/technotes/note10.html In summary, the presence of a buried soil affects the reference depth of the soil surface and most differentiae that are considered for placement in taxa above the family level (see Keys to Soil Taxonomy, Chapter 1 “Buried Soils” and Chapter 4 (just before the “Key to Soil Orders” on your Kindle!). See example soil #1. Mantle (manuring) and a buried soil Results of a Review of Official Soil Series Descriptions A recent proposal was made to set the minimum mantle thickness to a constant 50 cm. The effect of such a proposal was tested on the typifying pedons of 1491 soil series of the Fluvents suborder, Fluvaquents great groups, Fluventic and Fluvaquentic subgroups. Cumulic subgroups were not tested. Only 283 (~19 %)of the series had a mantle of new material, mainly the Fluvents. Only 8 (~0.5 %) soil series had a mantle between 30 and 49 cm thick that met the minimum thickness criteria, and none would change in classification if the minimum mantle thickness were raised to 50 cm. It is very unlikely that any existing soil series would be split or combined by the proposed change. Conclusions Eliminating the rule fragment that allows a thin mantle 30-49 cm thick would have minimal effect (if any) on existing soil series, soil surveys, or interpretations. The proposed change would have a positive effect to simplify Soil Taxonomy.
