Understanding a landscape’s susceptibility to erode by wind begins with determining the relationships between wind speed, surface cover and vegetation. Web Soil Survey32 is the USDA’s platform to find seamless soil survey data for the entire nation. The site is free to the public and is capable of printing professional reports including scientific soil descriptions, soil properties and qualities, suitabilities and limitations for use, and ecological site assessment- all with accompanying high quality color interpretative maps. This section describes the site’s capabilities in understanding the relationships of soils and vegetation in mitigating and preventing soil erosion.
Fundamentally, there is a close correlation between wind erosion and the texture of the surface layer, the size and durability of surface clods, rock fragments, organic matter, and a calcareous reaction. Soil moisture and frozen soil layers also influence wind erosion. Wind Erodibility Group (WEG) and Wind Erosion Index (WEI) -discussed in a later chapter- can be found on this tab under the Soil Erosion section. WEG and WEI are developed from an algorithm of the above properties.
Materials published through Web Soil Survey include some excellent descriptions and definitions of many of the site and soil properties discussed here, written by experts in the field. In an effort to take advantage of the work that has already been done by others and to avoid re-inventing the wheel, so to speak, relevant portions of these descriptions are provided as part of this section, unmodified from their original source and indicated with quotation marks.
The Ecological Site Assessment
The Ecological Site Assessment tab can be used to map out and determine ecological sites for a selected area of interest. From there, the ecological site name can be cross-referenced to ecological site database called Ecosystem Dynamics Interpretive Tool (EDIT), or alternatively the state FOTG, to obtain a reference site description for that ecological site. This description will provide a listing of structural groups of vegetation (grasses, forbs, shrubs/vines, and trees) and their canopy percentage that can be expected if the site is in its reference state. Other states, man-made or natural alterations to the site, are also described in narrative fashion. The description includes other characteristics of the site that directly affect its potential to erode by wind, including biological crusts, surface fragments, litter, bedrock, and bare ground. With both the vegetative composition and a detailed description of the soil surface, reasonable inferences can be made of the site’s potential to erode by wind.
Other interpretations available in Web Soil Survey include:
Organic Matter Depletion
Organic matter content in soils is an indicator of soil health. Organic matter is a soil binder and contributes to keeping soil in place when subjected to wind and rain. This interpretation rates the soil’s susceptibility to deplete organic matter on a scale of 0 to 1, where 1 represents a soil feature has the greatest ability to enable organic carbon depletion. Several soil features are evaluated, for example- high oxidation rate, low clay surface percentage, well aerated, low antecedent organic matter content, and others. These ratings are then compiled into a rating class.
“Rating class terms indicate the extent to which the soils enable the depletion of organic matter. 'Organic matter depletion high' indicates that the soil and site have features that are very conducive to the depletion of organic matter. Very careful management will be needed to prevent serious organic matter loss when these soils are farmed. 'Organic matter depletion moderately high', 'Organic matter depletion moderate', and 'Organic matter depletion moderately low' are a gradient of the level of management needed to avoid organic matter depletion. 'Organic matter depletion low' indicates soils that have features that are favorable for organic matter accumulation. These soils allow more management options while still maintaining favorable organic matter levels.”33
Fragile Soils Index
“Soils can be rated based on their susceptibility to degradation in the 'Fragile Soil Index' interpretation. Fragile soils are those that are most vulnerable to degradation. In other words, they can be easily degraded—they have a low resistance to degradation processes. They tend to be highly susceptible to erosion and can have a low capacity to recover after degradation has occurred (low resilience). Fragile soils are generally characterized by a low content of organic matter, low aggregate stability, and weak soil structure. They are generally located on sloping ground, have sparse plant cover, and tend to be in arid or semiarid regions. The index can be used for conservation and watershed planning to assist in identifying soils and areas highly vulnerable to degradation.”34 “Soils are placed into interpretive classes based on their index rating, which ranges from 0 to 1. An index rating of 1 is the most fragile, while a rating of zero is the least fragile.”34
These values are accompanied by interpretative classes that provide a more detailed evaluation of the susceptibility to erode and/or degrade.
Soil Surface Sealing
“Surface sealing is the orientation and packing of dispersed soil particles that result from the physical breakup of soil aggregates due to raindrop impact. Rapid soil wetting (in dry soils) and high exchangeable sodium percent can also cause aggregates to disperse. Sealing results when clay and silt particles get detached and/or dispersed and become suspended in the infiltrating water, which is moving downward through surface-connected pores. The pores become clogged with the fine particles, which become closely packed and create a surface seal. Surface sealing is the initial process in the formation of a mineral crust, which is a broader term for a surface feature that is dense, hard, or restricts infiltration. A seal is a more specific term and refers to a surface layer that inhibits infiltration (Heil, 1993).”35
Although surface sealing is an indicator of poor soil health and an undesirable condition, in arid areas where water erosion is not a concern, the propensity of a soil to develop a surface seal may be beneficial when it comes to wind erosion.
Reference cited as part of the above-quoted text:
Heil, J.W. 1993. Soil properties influencing hydraulic sealing of the surface on Alfisols in the Sahel. Doctoral dissertation, Texas A&M University. College Station, Texas.
Unpaved Local Roads and Streets
Web Soil Survey defines unpaved local roads and streets as “those roads and streets that carry traffic year-round but have a graded surface of local soil material or aggregate.”36 This interpretation evaluates soil suitability for building these type roads. Attributes include susceptibility to flooding, bedrock, low strength, shrink-swell, and dusty qualities. Each attribute is rated from 0 to 1, where 1 is most limiting. Additionally, a composite rating is compiled from all attributes that indicate whether the soil would be very limited, somewhat limited, or not limited.
Similar to Unpaved Local Roads and Streets, several recreational development scenarios are rated, including camp areas, off-road motorcycle trails, paths and trails, picnic areas, and playgrounds, with “dusty” being a primary feature evaluated. The importance of these recreational activities becoming very limited due to dust is underscored by realizing that these disturbed areas can serve as a catalyst to start the saltation process into adjoining areas, contributing to a larger suspension problem.
Soil Habitat for Saprophite Stage of Coccidioides
“Valley fever or coccidioidomycosis is caused by the soil-borne fungi Coccidioides immitis and Coccidioides posadasii which are endemic to the Southwest United States and a few other places in Central and South America. The symptoms of the disease range from none at all to mild cold or flu-like conditions in most people. However, some people experience the disseminated form of the disease, which can kill.
According to Kolivras et al (2001), the life cycle of fungus consists of a saprophytic and parasitic phases. The saprophytic phase lives in soil as entangled mycelia and hyphae. The hyphae grow and mature to produce generally rectangular arthrospores. The arthrospores are 1.5 to 4.5 microns in width and 5 to 30 microns in length. These spores move easily in air currents. The parasitic phase occurs in nature under dry, dusty conditions when a host mammal inhales airborne arthrospores. The fungus in this phase grows as spherules that mature and burst, releasing endospores that can grow into new spherules in the host lungs, inducing valley fever (Kolivras et al, 2001).”37
“Many prior maps of endemic areas are made from testing people for reactivity to coccidioidin and not the soil for presence of the fungi (Edwards and Palmer, 1957). The objective of the current study is to use the soil survey database to identify areas that are potentially habitat for this soil-borne fungus. This approach will allow habitat mapping at far finer spatial resolutions than has even been done in the past. This will allow habitat considerations to be targeted in the planning stage of any soil disturbing activity so as to proactively apply dust control methods when needed. The criteria mapped are as follows. The mean annual precipitation (about 230mm) and air temperature (about 20 degrees C) found in the Lower Sonoran Life Zone are used as the optima for habitat. For xeric areas, the rainfall can be somewhat higher and the temperature somewhat lower. Southerly slope aspect, moderate slope gradient, and low surface albedo are used to better capture extreme soil surface temperature effects. Electrical conductivity of over 4dS/m, soil reaction of at least 8.0, or the presence of gypsum in the upper 30cm of the soil are used to indicate an environment high in soluble salts. Some organic matter and water storage must be present in the soil for the saprophytic phase to grow. Soil components fitting all of those specifications, at least marginally, are considered possible habitat for the fungus. Variation in rainfall and temperature from year to year can increase or decrease the range of Coccidioides spp (Kolivras et al, 2001).”37
References cited as part of the above-quoted text:
Edwards P.Q., C.E. Palmer. 1957. Prevalence of sensitivity to coccidioidin, with special reference to specific and nonspecific reactions to coccidioidin and to histoplasmin. Chest;31(1):35-60
Kolivras, K. N., P. S. Johnson, A. C. Comrie, S. R. Yool. 2001. Environmental variability and coccidioiomycosis (valley fever). Aerobiologia 17:31-42
“Total range production is the amount of vegetation that can be expected to grow annually in a well-managed area that is supporting the potential natural plant community. It includes all vegetation, whether or not it is palatable to grazing animals. It includes the current year's growth of leaves, twigs, and fruits of woody plants. It does not include the increase in stem diameter of trees and shrubs. It is expressed in pounds per acre of air-dry vegetation. In an unfavorable year, growing conditions are well below average, generally because of low available soil moisture. Yields are adjusted to a common percent of air-dry moisture content.
In areas that have similar climate and topography, differences in the kind and amount of vegetation produced on rangeland are closely related to the kind of soil. Effective management is based on the relationship between the soils and vegetation and water.”38
Although range production cannot be considered as any measurement of wind erosion, it can be somewhat of an indicator of potential to erode, particularly during extended drought. Where production is low, the lack of vegetative growth may lead to increased potential for soil to blow. Of course, there are many other soil characteristics that may negate wind erosion, for instance- desert pavement, fragment percentage, litter, biological crust, etc.
32. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm.
33. Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Description – Organic Matter Depletion. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed August 23, 2019.
34. Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Description – Fragile Soil Index. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed August 23, 2019.
35. Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Description – Soil Surface Sealing. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed August 23, 2019.
36. Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Description – Unpaved Local Roads and Streets. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed August 23, 2019.
37. Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Report – Selected Soil Interpretation Interpretation name: Soil Habitat for Saprophite Stage of Coccidioides. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed August 23, 2019.
38. Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture. Description – Range Production. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed August 23, 2019.