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• i <br /> n <br />Outside of the sodic spoil of the Northern Great Plains, the San Juan Bas <br /> of New Mexico appears to be the second largest area of the west having <br /> large areas of sodic mine spoil. Gould et. al. (1975a) and Miyamoto et. al. <br /> (1975) briefly addressed this research, but more detailed accounts of these <br /> studies are presented in the reports by Gould et. al. (1974, 1975b, 1976). <br /> These topsoiling depth studies were unlike all of those conducted and <br /> described previously for the Northern Great Plains. The major differences <br /> being that the New Mexico studies included treatments of mixing a thin layer <br /> of topsoil into the sodic spoil followed by application of the desired thickness <br /> of respread topsoil. Virtually all of the studies conducted in the Northern Great <br /> Plains simply compared the effects of various thicknesses of respread topsoil <br /> directly over regraded sodic spoil. Thus, the New Mexico studies eliminated <br /> the abrupt interface between the spoil and topsoil found in the topsoiling <br /> depth studies that have been performed in the Northern Great Plains. Such <br /> a practice is now required for all mines in New Mexico. Frst year resuRs <br /> (Gould et. al. 1974) established that the best plant growth was produced on <br /> a treatment consisting of 4 inches of topsoil incorporated into the spoil (SAR <br /> averaged 32) followed by the application of 8 inches of respread topsoil. This <br /> 12 inch thickness of respread topsoil produced 18 percent higher stand <br /> ratings than the straight 12 inch topsoil thickness and 23 percent higher than <br /> the 18 inch topsoil thickness. In 1975, three new areas received the same <br /> thicknesses of respread topsoil (Gould et. al. 1975b). On Area V-A, the <br />• highest stand ratings occurred on the 4 inches of topsoil incorporated into the <br />spoil overlain by 8 inches of respread topsoil. On Area V-B, the highest stand <br /> ratings were obtained on the straight 8 inch topsoil plots. In Area V-D, the <br /> highest stand ratings were obtained on plots having only four inches of <br /> topsoil incorporated into the spoil with next highest stand rating produced by <br /> the four inches of incorporated topsoil overlain by eight inches of topsoil. <br /> Averaging the stand ratings for all three sites, the highest stand ratings were <br /> associated with the straight eight inch topsoil treatment but no significant <br /> differences existed in any of the topsoil treatments. <br /> In an experiment designed to test the effect of various amendments <br /> believed necessary to correct the sodicity of the spoil, Gould et. al. (1976) <br /> and Gould (1982) compared the effects of 2.5 cm, and 20 cm of respread <br /> topsoil to spoil on plant growth. The greatest growth after six weeks, and at <br /> the end of the second and third growing seasons was reported to occur on <br /> the 20 cm thickness of respread topsoil, while the spoil had the lowest <br /> growth. The 2.5 cm treatments yielded slightly higher values than the straight <br /> spoil. <br /> In Colorado, several extensive research studies have addressed the <br /> revegetation concerns of the highly sodic processed oil shale material of the <br /> Piceance Basin. These materials normally have SAR values ranging as high <br /> as 29 and contain signficant amounts of sodium and other soluble salts. <br /> Numerous topsoiling experiments have been attempted on these materials, <br /> ony a portion of which will be summarized. Berg et. al. (1983) summarized <br /> the most extensive work that has been conducted on salt migration on <br />• processed oil shale. On a site receiving 30 crn of respread topsoil over sodic <br /> shale, it was reported that after six growing seasons there was a net <br /> downward movement of sags. The failure of upward salt movement was <br /> attributed to the relatively coarse fragment content of the respread topsoil that <br />12 <br />