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• soil thickness with a maximum occurring with the 16 inch to the 24 inch thick- <br />nesses. Cook, Hyde and Sims (1974) stated that topsoil should be evenly applied <br />and there should be a "plant growth medium" at least 18 inches deep, not neces- <br />sarily of topsoil. They felt that a depth of 18 inches is necessary to store <br />precipitation received during the fall and winter months. <br />Power, et.al. (1974) also showed that topsoil would increase infiltration and <br />reduce runoff from sodic mine spoils. Two inches of topsoil reduced runoff to <br />53% of that measured without topsoil. When the spoil was not covered by topsoil, <br />the cumulative intake was only 44% of that where 2 inches of topsoil had been <br />placed over the same spoil. Bauer et.al. (1978) concluded from topsoil studies <br />that 30 inches of topsoil material would be desirable in assuring infiltration <br />rates of not less than 4.7 inches/day and providing the minimum soil volume nec- <br />essary for adequate water storage. Gilley et.al. (1976) found that mechanical <br />roughening or tilling of sodic spoil before topsoiling didn't greatly increase <br />infiltration into the topsoiled spoil, since water movement into these materials <br />did not exceed 6 inches, i.e. the depth of the topsoil. <br />• All plants require a suitable rooting medium for maximum production and in short, <br />plant response can be expected to increase with increasing topsoil depth up to <br />some particular depth and then measurable incremental response will not occur. <br />Additionally, good tillage practices necessitate a fairly deep soil for croplands <br />to allow plowing without bringing spoil material back to the surface. <br />In order to provide site specific information on optimum replaced soil depths for <br />reclaimed lands at Trapper Mine, a topsoil depth study was initiated in the fall <br />of 1980 by Utah International Inc, and Colorado State University (CSU). Besides <br />evaluating species responses at 15, 30, 45, and 60 cm of replaced topsoil depth, <br />the study also looked at various mechanical treatments to spoil and replaced top- <br />soil, and the application of phosphorus to regraded spoil. <br />The study was divided into two sub studies, one study dealt with maximizing crop- <br />land production, while the ocher dealt with optimizing rangeland production and <br />species diversity. For alfalfa production on lands returned to cropland, the <br />• <br />3-53 <br />REVISED FEB 13 '87