|Response from authors of master V proposal|
|Author: Judith Turk Created on: 8/30/2012 MessageID: 1552|
|We appreciate the comments by Rindfleisch. We offer these responses:|
“There is no question whatsoever that vesicular horizons have an important place in soil taxonomy …”: We have proposed a new morphologic horizon designation, not an addition to soil taxonomy. If others feel that adding vesicular horizons to the taxonomic system is critical, that is a task that we are happy for them to undertake!
1. Thickness of vesicular horizons: All of us have views shaped by our own experiences, but desert lands are so vast (1/3 of Earth’s land surface) that is not possible for one observer to see it all. To overcome these inherent biases, our proposal used the NRCS database to obtain widespread, objective data. These were supplemented by published, detailed investigations from around the globe.
From the soil databases, the median thickness of the vesicular horizons is 8 cm in the Great Basin, and 5 cm in the Mojave and Sonoran deserts (NOT predominantly 1-3 cm thick). The infiltration data do not come from sites with exceptionally thick vesicular horizons. For example, Young et al. measured infiltration rates of 0.3-0.6 cm/hr for vesicular horizons 3 to 6 cm thick -- very typical of what we have seen in the soil databases.
Texture of vesicular horizons: They are silt-rich relative to the underlying horizons, but are not all silt loams. Based on the soils descriptions from the databases, the most common textures in the vesicular horizon are loams, sandy loams, silt loams, and fine sandy loams (for more data see Turk and Graham, 2011. Distribution and properties of vesicular horizons in the western United States. Soil Sci. Soc. Am. J. 75:1449-1461). The vesicular horizons studied by Young et al. significantly reduced infiltration rates compared to underlying horizons, including soils with varied vesicular horizon textures.
2. Definition of the A horizon: Definitions of the master horizons are presented in italics in Keys to Soil Taxonomy. The current definition of the A horizon does not include vesicular horizons. The statement below the definition makes the exception, but it is not part of the definition. An odd situation that, until our proposal came forward, no one seemed inclined to address.
The definition of the A horizon presented by the Merriam-Webster dictionary (“the uppermost dark-colored layer of a soil”) closely follows definitions and pictures found on NRCS websites that target the public, as do the definitions in hundreds of other sources (dictionaries, encyclopedias, textbooks from elementary school to university levels, etc.). They aren’t wrong, they reflect the well-accepted definition of the A horizon as promoted by the NRCS itself.
3. Thresholds and extent: Even vesicular horizons 3 cm thick significantly reduce infiltration rates. The median thicknesses of vesicular horizons in the Great Basin, Mojave, and Sonoran Deserts all exceed this. Vesicular horizons that significantly decrease infiltration are very widespread in desert landscapes.
Role of desert pavement: This is described in the proposal and in numerous publications cited therein. For a review, see Turk and Graham, 2011 (cited in #1 above).
4. Use of transitional horizons: As with any master horizon, the transitional designation would be used in exceptional cases where the properties of another horizon are present in addition to the primary master horizon. We don’t know where the idea came from that it would be routine to use them with V horizons. The use of transitional horizon designations would NOT be routine for the vesicular horizons any more than it is for other horizons.
“vv” suffix is confusing: Anyone who has taught pedology in the western U.S. has run into this confusion with plinthite (v) and the doubling up of letters to sometimes indicate more of a substance (kk) and sometimes not (jj). The system should make sense even to those who do not yet have “familiarity and experience with arid soils”.
The basis for master horizons: All master horizons are based on soil features. That is not what distinguishes the master from the suffix. See the comment by Hirmas (7/25/2012). The basis is to communicate genetic relationships. The V horizon does this effectively. Expanding the A horizon definition obscures genetic interpretation.
5. Workload: Changing the designations on OSDs and other records will happen with either of the proposals, the change will either involve V or Avv. So, once it is agreed a change is needed, the workload is there in either case. The current NRCS Soil Data Join and Recorrelation effort seems to provide an ideal time to embrace this project to improve data on arid zone soils.
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Support for Recognition of Vesicular Horizons
Response from authors of master V proposal (current)