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w o rv rv '~'*-- --.. ~' ,--"--;-"4-.~'f/^, , '~"- ~,>x '.,:" '" ""','~,,'., . ~. ./'.' ,'" ~,,-,' ir:.'~ '~~ ..... _:1- ~ .........- , -. #~,~_.K Il " ~ ., ".....- , -.^ ., ...."" , .. t."~- -~,'~:'~;' ~" .J t , '" ~.=... - - r I 'It ~ Figure 8. Photograph of crystaline salt lenses in Mancos Shale in the irrigated area of the Grand Valley. A gravel and cobble layer has been found under some parts of the irrigated areas in the Grand Valley. It is believed to be ancient stream deposits of the Colorado River, laid down in recent geologic time, and serves as an aquifer for transmitting highly saline groundwater to the river. SOILS The physical features describing the project area are similar to the entire Grand Valley. The soils in the Valley were classified by the Soil Conservation Service (SCS) in cooperation with the Colorado Agricultural Experiment Station in 1955. Using these data a soil classification map of the Grand Valley's irrigated area is shown in Figure 9, The soil classi- fication symbols, along with a general description of each symbol and the relative percent of areal extent, are tabulated in Table 2. The dry desert climate of the area has restricted the growth of natural vegetation, and because of the lack of organic matter, the soils are very low in nitrogen content. The natural inorganic content is high in lime carbonate, gypsum, sodium, potassium, magnesium and other calcium salts. With the addition of irrigation, some locations have experienced high salt con- centrations with a resulting decrease in crop productivity, Although natural phosphate exists in the soils, it becomes available too slowly for use by cultivated crops, and fertilizer 24