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hlappin~ Unit Number 27 - Deep (more than 40 inches to bedrock), dark colored, moderately well • drained. moderately coarse textured (less than 1S percent clay but finer textured than loamy sand), weakly developed soils having no distinct soil horizonation other than a dark surface horizon (A horizon) occurring in parts of the study area where the mean summer soil temperature. (June, July, and August mean temperature measured at 20 inches) is 59°F or warmer. Classification of Maior Components -The components listed below comprise the major part of the mapping unit: A. Pachic Haploboroll (Chernozemic Regeosol). 1. Coarse-loamy, mixed, mesic. (Breese Series) Extent and 1\lajor Areas of Occurrcncc -Mapping unit number 27 has a small total acreage in the study area and occurs on the ^oodplains of the North Fork of the Gunnison River in the northern part of the study area. Characteristic Tonocranhv -These soils occur on nearly level to gently sloping floodplains adjacent to major streams. Slopes range from 0 to about 6 percent in the study area. Physical Properties -The soils of mapping unit number 27 are immature, having no genetic soil horizonation other than a darkened surface horizon (A horizon). Soil structure is weak and relatively unstable. However, their moderately coarse textures (sandy looms) insure good permeability and aeration to these soils or the materials derived from them without additional aggregation. Consequently, the soils oC this group can be disturbed, transported, and relogted for new seedbeds without serious loss of good physical condition. • The soils of this group are subject to some packing if continuously traversed by heavy machinery, and form a structureless mass i( handled near saturation. These conditions can be corrected with conventional tillage. All physical qualities indicate that these soils are good sources o(seedbcd material; however, their water tables present severe problems to their accessibility. The soils of mapping unit number 27 have very low shrink/swell ratios. Total potential extensibility ranges from 0 to O.S inch fora 40-inch section. No evidence of mass movement was noted and the potential for such movement is negligible. In their natural state, the soils of this unit have low susceptibility to either wind or water erosion except for bank cutting where they border the North Fork of the Gunnison River. if native vegetation is destroyed or soils disturbed, susceptibility to wind erosion increases to a moderate level, but because of their Iceel topoeraphy, water erosion is low. The possibility of accumulation of sedimenu from Hood waters is moderate in tither condition. Chemical Prenerties and Natural Fertility -These soils are immaturely developed, hayring no genetic horizonation other than darkened surface horizons (A horizons). Normally they lack horizons of secondary carbonate accumulation. The control section (10 to 40 inches) is neutral to mildly alkaline while the C horizon is neutral to moderately alkaline and, while normally noncalcareous, may contain some free carbonate in a few localities. Under such conditions the cation exchange complex (portion of the soil having an ability to hold and exchange cations) is normally base saturated with calcium being the predominant cation, but there is not enough free calcium and magnesium carbonate to seriously depress the availability of other plant nutrients. Natural fcnility levels arc high in the upper horizons of other plant nutrients. Natural fertility levels arc • high in the upper horizons of the soil but gradually decrease with depth as organic content decreases. No evidence of deficiencies of essential plant nutrients or of trace elements is present.