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| Communicating Soil Processes |
| Author: Brenda J. Buck Created on: 8/31/2012 MessageID: 1561 |
| All of the discussions regarding soil taxonomy miss the point of both proposals, which has to do with changing genetic designations, not soil taxonomy. If, after this decision is reached, specific details inside soil taxonomy have to be changed, then that is a separate topic and discussion that should occur at that time. We should not be basing our decisions regarding genetic horizons on what impact they will have on soil taxonomy. That is putting the cart before the horse. We should not be concerned about the workload that this decision may eventually impact. What is most important is to get the science right, and do the best job we can of communicating soil processes through the use of genetic designations. As Katherine’s post noted, there are very many other disciples that use genetic horizons to communicate soil processes. Our decision here will impact them as well. The decision as to whether a new master horizon should be created versus a new suffix depends on whether or not there is something genetically different about the formation processes of vesicular horizons as compared to other master horizons. If the processes that form vesicular horizons are primarily surface organic matter additions plus eluviation, then a suffix added to an A horizon would be appropriate. If the formation processes of vesicular horizons are primarily eluvial then a suffix added to an E horizon would be appropriate. If vesicular horizons form primarily in the subsurface through illuviation, then a suffix added to a B horizon would be appropriate. If none of these processes adequately describes the formation of vesicular horizons, then a new master horizon is appropriate. Many of the references cited in the Turk et al. proposal show that several decades of research have found that vesicular horizons form from eolian additions at the surface that are sealed which result in trapping those eolian grains as well as trapping the air and forming the distinctive vesicular pores. The sealing has been shown to occur by several mechanisms including a layer of stones (desert pavement), biological crusts, or physical crusts. An important component of this is the lack of significant vegetation, which is one of the major reasons why vesicular horizons are common in arid and semi-arid climates, in hot and cold deserts including the arctic. Quade’s paper on the subject points out that several feedback loops exist between vesicular horizons and vegetation. As vesicular horizons form, water infiltration is decreased and vegetation decreases, but if disturbances occur such that water is able to establish vegetation then vesicular horizons are destroyed and that soil material is mixed into the resulting surface horizon. Once the vesicular horizon is destroyed, then organic matter input and eluviation now become the dominant soil processes and a designation of an A horizon is likely to be appropriate. So what are the major soil processes that occur in vesicular horizons? The formation of vesicular horizons can be argued to be primarily that of geologic surface processes but since biologic crusts can form vesicular horizons this is not completely accurate. One could argue that the dominant soil process in vesicular horizons is the restriction of soil processes. Organic matter input is exceedingly retarded unless it is a component of the original dust being trapped. Eluviation and illuviation are extremely limited because water movement is limited. Yet, these horizons form at or near the surface and their structure and characteristics are not the result of geological processes therefore the use of a C horizon also does not adequately communicate what is occurring here. Once the vesicular horizon has been formed, the majority of water does not enter into it because of the pore structure and thus eluviation is greatly restricted. Because of this, it is utterly contradictory to describe these horizons as E horizons with some suffix added. Eluviation is definitely not the main process occurring in these horizons even if they are light colored or even if some clay or salts are able to migrate out along the hexagonal crack system that commonly develops. Vesicular horizons should never be described as master E horizons. Similarly most vesicular horizons do not have significant illuviation occurring and as such do not fit into the definition of B horizons either. Because by their very nature, vesicular horizons limit water infiltration and are destroyed as vegetation growth occurs, the processes of organic matter input and eluviation that are a distinctive signature of A horizons is contradictory. This is alluded to in Chiaretti’s proposal in that in order for the master A horizon to be used with vesicular horizons, we need to expand the definition of what an A horizon can encompass. Do we really want to expand the A horizon definition to include surfaces that in their most developed state are devoid of vegetation? Is this really furthering communication of soil processes to lump such disparate processes together? In my opinion, acceptance of a suffix to describe vesicular horizons will greatly confuse all users of soil genetic descriptors. The processes that form vesicular horizons are not the same as those that form the other master horizons and that is why I believe it is acceptable to create a different master horizon to describe them. I don't understand the reluctance to embrace a new master horizon versus a new suffix, especially when use of the suffix implies significant contradictions in soil processes. |
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