Soils in the Planning Area are diverse and highly variable. Soil characteristics can differ over relatively short distances, reflecting differences in parent material, position on the landscape, elevation, aspect, and climatic variables such as precipitation and temperature. The plant communities supported by such a wide diversity of soils are equally diverse, ranging from sparsely vegetated desert saltbush and sagebrush-bunchgrass communities to forests and alpine meadows. More than 60 ecological sites have been identified in the Planning Area. Low annual precipitation, salinity, alkalinity, and shallow depths have the greatest effect on soil productivity and the plant communities they support.
The Washakie County soil survey is the only published soil survey for the Planning Area (NRCS 1983). Soil data for Hot Springs, Big Horn, and Park counties have been compiled from earlier inventory efforts and are available in digital format. A soil database allows soil data to be applied for use and suitability interpretations. This database is adequate for most soil interpretations.
Soils in the Planning Area formed from a wide variety of geologic material. Variation in parent material, along with variable climate, topography, and vegetation, has resulted in soils with diverse characteristics and textures.
Soils commonly found in the Planning Area include soils with moderately fine to fine textures (clay loam, sandy clay loam, silty clay loam, sandy clay, silty clay, clay) that formed over shale or were influenced by shale parent material. Soils in the Planning Area that formed over sandstone or were influenced by sandstone parent material generally have medium to moderately coarse textures (sandy loam, fine sandy loam, very fine sandy loam, loam). Coarse-textured soils (loamy sand, sand) in the Planning Area are generally associated with windblown soils derived from sandstone parent material. The soils characterized by reddish hues often are referred to as red bed soils. The formation of these highly productive soils was strongly influenced from the red sandstone common to the Chugwater formation. These soils have high gypsum content and generally have medium textures (very fine sandy loam, loam fine sandy loam). As a result, they are highly susceptible to erosion following surface disturbance or vegetation reduction.
Biological soil crusts, often referred to as cryptobiotic, cryptogamic, and microbial soil crusts, are found on all soil types throughout the Planning Area. Biological soil crusts are an intimate association between soil particles and cyanobacteria, mosses, lichens, microfungi, and algae (Rosentreter et al. 2007). The presence of biological soil crusts increases soil stability and the soil’s resistance to wind and water erosion, and by forming stable soil aggregates, allows for increased water infiltration. They also add carbon to the soil surface, convert atmospheric nitrogen to bio-available nitrogen, and increase bio-available phosphorus. The distribution and extent of biological soil crusts have not been well documented in the Planning Area largely due to the age of the soil survey data. Rangeland health surveys are documenting the presence of biological crusts using the 17 indicators of rangeland health. As the soil survey dataset is updated, key data collected will relate to biological crusts.
The Planning Area lies within two MLRAs: the Northern Intermountain Desertic Basins – 32 (5- to 9-inch and 10- to 14-inch precipitation zones) and Central Rocky Mountains – 43B (15- to 19-inch and 20+-inch precipitation zones) (USDA 2008). The following paragraphs provide an overview of Planning Area soils by MLRA.
The dominant soil orders in the Northern Intermountain Desertic Basins are Entisols and Aridisols. These soils have a mesic temperature regime, an aridic soil moisture regime, and mixed mineralogy. They generally are shallow to very deep, well drained, and loamy and consist of Torriorthents formed in alluvium on alluvial fans and floodplains (Apron and Kishona series) and in residuum and colluvium on hills and piedmonts (Chipeta, Greybull, Persayo, Shingle, and Worland series); Torrifluvents (Lostwells and Youngston series) and Natrargids (Uffens series) formed in alluvium on floodplains, alluvial fans, and stream terraces; and Ustorthents (Spearfish series) formed in residuum and colluvium on hills.
The dominant soil orders in this area are Inceptisols, Alfisols, and Mollisols. These soils have a frigid or cryic soil temperature regime and an ustic, udic, or xeric soil moisture regime. Soils on mountain side slopes and ridges are formed in colluvium, residuum, and glacial till and have mixed mineralogy. Areas of rock outcrop and rubble land are on ridges and peaks above timberline. Most of the soils are skeletal and are medium textured to coarse textured.
There has been no comprehensive analysis of the current condition of soils and soil health in the Planning Area. There have been qualitative assessments throughout most of the Planning Area using the 17 indicators of rangeland health found in Technical Reference 1734-6, Interpreting Indicators of Rangeland Health (BLM 2005e). Of the 17 indicators of rangeland health, ten are used to assess soil and soil site stability. Qualitative assessments using the ten indicators show an apparent upward trend in the overall health of the soil resource. The ability of the watersheds to capture and slowly release water without excessive erosion is expected to continue to improve.
Past land uses in the Planning Area have resulted in a network of incised gullies extending into the uplands, often replacing what are thought to have been broad grass-covered swales. This gully network is not restricted to any particular ecological site or plant community and is present throughout the uplands in the 5- to 9-inch precipitation zone and 10- to 14-inch precipitation zone. As a result, peak runoff discharges are of greater intensity and shorter duration, and water is not being retained on the watersheds as it appears to have been in the past. Based on qualitative rangeland health assessments, most gullies are in the process of healing and stabilizing. However, a few gullies still continue to creep farther into the uplands.
Where native plant communities have retained a healthy stand of perennial grasses and shrubs, the ten indicators of soil and site stability reflect that the soils are relatively intact and stable. There is little evidence (e.g., water flow patterns, pedestals/terracettes, bare ground, and gullies) of past or current erosion, and water is being captured and safely released. The upward trend in overall soil resource health is most pronounced in these plant communities and is expected to continue. Where incised gullies are present, they are expected to continue to heal and stabilize; however, they will continue to channel runoff from the uplands at an accelerated rate.
In areas where the plant communities have shifted to a blue grama sod plant community or a Gardner’s saltbush/bare ground plant community, damage to the soil resource is evident. Bare ground is excessive and often interconnected. Loss of the nutrient-rich A horizon is common in bare areas and runoff and erosion exceed the expected rate for the site. However, these plant communities appear to be static, showing neither improvement nor further degradation. The incised gully network in these more degraded sites is expected to slowly improve or remain static.
In areas where the plant communities have shifted to annual grassland dominated by cheatgrass, there is little evidence of damage to the soil resource and runoff and erosion indicators are almost absent. These areas are often characterized by dense stands of cheatgrass, with excessive litter creating an oxidized layer of thatch. Little change to soil and soil site stability is anticipated in these communities. Ongoing research is revealing that cheatgrass-dominated sites undergo biogeochemical changes that alter soil evolution and plant succession.
Wildland fires are occurring more frequently and are becoming larger, and burn with greater intensity over longer periods. When viewed from a soils and watershed perspective, these larger fires lead to increased soil erosion. In many situations, as in the case of cheatgrass monocultures, entire plant communities are shifted as a result of wildland fire.
Despite some evidence that water is not being retained on the landscape as it once was and that soils are being affected in some areas, the soil resource remains capable of producing forage for wildlife and livestock. It is also proving capable of maintaining a balance between infiltration and runoff, thus protecting watershed condition. The soil resource should be capable of sustaining increased demands without long-term impacts. Surface-disturbing activities are likely to be the greatest demand on the soil resource. In the arid climatic conditions common to the Planning Area, long-term soil loss exceeding 2 tons per acre per year could adversely affect the soil resource.
Position on the landscape, slope, physical properties, and most notably, surface texture and structure and chemical properties, contribute to susceptibility of soils to wind and water erosion. Slopes greater than 25 percent have a high water erosion potential, whereas slopes from 10 percent to 25 percent are considered to have a moderate water erosion potential (Map 50). Runoff potential is increased if plant communities are disturbed. Many other soils have naturally high runoff potential (Hydrologic Group D) due to high clay content and their tendency to swell when wet.
Susceptibility to water erosion is a function of slope and soil surface texture. As a rule, slopes greater than 25 percent are considered to be highly susceptible to water erosion, particularly after surface disturbance. Management of slopes of 10 percent or greater requires an emphasis on runoff and erosion control. Map 50 shows the percent slope in the Planning Area. Table 3-5 summarizes the number of acres susceptible to water erosion.
Table 3.5. Soils with High Water Erosion Potential in the Planning Area
| BLM-Administered Surface | Federal Mineral Estate | All Land Ownership | |||
| Acres | Percent of BLM-Administered Surface | Acres | Percent of Federal Mineral Estate | Acres | Percent of Lands within Planning Area |
| 464,538 | 14.6 | 671,825 | 16.0 | 824,245 | 14.6 |
Source: BLM 2009a
| BLM | Bureau of Land Management |
Using the USFS web-based Water Erosion Prediction Project (WEPP) erosion model, surface-disturbing activities have the potential to increase annual soil loss to levels far greater than 5 tons per acre (WEPP 2008). Site-specific mitigation measures, including timely reclamation, are needed to minimize soil erosion and protect long-term soil productivity. WEPP erosion predictions consistently show that erosion rates following surface-disturbing activities return to background levels within 3 to 5 years following full reclamation.
Wind erosion is not widespread in the Planning Area. Where high winds occur, the soils with sandy surface textures (sand, loamy sand, fine sandy loam, sandy loam) are highly susceptible to wind erosion. Existing soils data is not adequate to make a realistic determination of acres susceptible to wind erosion or to produce a meaningful map of their locations.
Management challenges for soil resources in the Planning Area stem largely from surface-disturbing activities. Development of mineral resources, including road building, well pad construction, pipeline installation, and vegetation treatments all impact the soil resource. Other actions that affect soils include a variety of surface uses that loosen topsoil and remove vegetation or other ground cover, such as grazing and browsing by animals, off-highway vehicle (OHV) use, development of trails and campgrounds, rights-of-way (ROWs), fire-suppression activities, and prescribed fires. Soil compaction resulting from surface-disturbing activities and associated development can reduce infiltration, increase runoff, and hamper reclamation.
Other challenges include implementing improved reclamation techniques, control of invasive species, and establishment of native plant communities on disturbed sites. In addition, areas where plant communities have shifted to blue grama sod or Gardner’s saltbush/bare ground offer a unique management challenge. The BLM applies restrictions and implements mitigation measures and BMPs to protect soil resources (Refer to Appendix H and Appendix L).