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Map Unit No. 35 Aaberg Clay 20 to 30 percent slopes: <br />This steeply sloping, slowly drained soil is on rolling to hilly uplands. <br />Slopes are smooth to irregular. The native vegetation is mainly grasses, low <br />shrubs and fortis. Included are wheat grasses, Idaho fescue and sagebrush. Eleva- <br />tion is 6805 to 7120 feet. The average annual precipitation is about 17 to 18 <br />inches, the average annual air temperature is 41.5 to 43 degrees F., and the <br />average frost-free period is 63 to 90 days. Soil temperatures are in the <br />cryic soil zone. Individual areas of this unit are irregular in shape and <br />range from a few to many acres in size. <br />Typically the clay to silty clay surface layer is about 4 inches thick but <br />ranges from 3 to 8 inches thick. The subsoil is about 15-30 inches thick. It <br />is a coarse blocky, dark grayish brown (2.SYR 4/2-lOYR 4/2) clay or silty clay, <br />very hard dry, moderately friable moist and plastic wet and is calcareous in <br />places. The C horizon is a grayish brown, massive clay or silty clay with <br />weakly segregated lime in places. It is underlain by clay shale at depths of <br />30-60 inches or more. Electrical conductivity is low. Sodium absorption <br />ratios are low also. The pH ranges from 7.4-7.8. No minor element toxicity <br />is indicated by laboratory analyses. <br />Included with this soil in mapping are small areas of shale outcrop and <br />some areas where shale or siltstone fragments (2-5%) may occur in the B and C <br />horizons. <br /> <br />Water and air move through this soil at a very slow rate and surface run- <br />off is rapid on these steep slopes. Root development may be restricted below a <br />depth of about 6-B inches by the tough, swelling-type clays; wide cracks may <br />occur on drying. <br />This unit is not a good source for topsoil. Quality of topsoil is poor <br />because of high clay content which will require careful handling and the use of <br />large amounts of mulch to protect the surface. Quantity of suitable topsoil is <br />limited by an average depth of 4 inches to 8 inches in order to use that part . <br />having higher organic matter content and avoid the more plastic clays below. <br />Topsoil salvage operations are likely to be troublesome on these steep slopes. <br />The only other limiting factor for this unit is the high clay content, strong <br />shrink-swell properties and high plasticity of the montmorillinitic clays <br />-13- <br />