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Map Unit No. 10 Binco Clay 15 to 30 percent slopes. <br />This stronglysloping, very slowly drained soil is on rolling to hilly <br />uplands. Slopes are irregular to smooth. The native vegetation is mainly <br />wheatgrass, needlegrass, big sagebrush, rabbitbrush, and fortis. Elevation is <br />6750 to 7350 feet. The average annual precipitation is'about 17 to 18.5 inches, <br />the average annual air temperature is 41 to 43 degrees F., and the average <br />frost-free period is 79 to 91 days. Soil temperatures are in the cryic soil <br />zone. Individual areas of this unit range from a few to several acres in size. <br />Typically the dark olive brown to dark grayish brown (2.5YR 4/2-lOYR 4/2) <br />clay surface layer is about 6-7 inches thick but ranges from 4-7 inches thick. <br />The subsoil is about 20-24" thick. It is a coarse blocky to coarse prismatic <br />clay olive brown to alive gray (2.SY 5/4-SY 5/2) in color, very hard dry, plas- <br />tic wet, calcareous in places and cracks widely on drying. The C horizon is <br />light brownish gray to light olive brown (2.SY 6/2-4/4) massive clay commonly <br />calcareous and with few or no roots. It is underlain by clay shale at depths <br />of 36 to 60 inches. <br />Included with thi: soil in mapping are small areas of clay shale outcrops <br />on slope breaks and eroded ridges. <br />Water and air move through this soil at a very slow rate and surface run- <br />off is rapid because of texture and slope. Root development maybe restricted <br />below a depth of about 8-10 inches by the dense clay subsoil although a few <br />roots penetrate to 60" or more. <br />This unit is not a good source for topsail. Quality of topsoil is rated <br />poor because of texture and will require careful handling to reduce compaction; <br />use of heavy mulches may be necessary. Quantity of best suitable topsoil is <br />limited by an average depth of 6-8 inches in order to use that part having <br />higher organic matter and better structure. Fertilization and mulching are needed <br />to assist revegetation. Topsoil salvage operations are not only limited by the <br />clay texture but by slopes of 15-30 percent and presence of shale outcrops in <br />olaces. The most limiting factor for this unit is the high content 60-70% of <br />strong shrink-swell clays. <br /> <br />-17- <br />