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• square meter) of air dry forage per acre, reapplication of 40 cm (16 inches) <br />of respread topsoil would yield an average forage production of 170.1 grams <br />per square meter. As has been pointed out previously, this comparison is <br />biased because it averages the higher values from the thicker depths of <br />respread topsoil associated with the first few years of plant establishment and <br />assumes that these values remain constant over time. Since this comparison <br />ignores the pronounced shift in production being favored by the thinner <br />depths of respread topsoil with time, it is somewhat of a 'tivorst case' <br />estimate. <br />Using the site specfic yield data collected from the established reference <br />area and the 1984 reclamation from the refuse pile it can also clearly be <br />determined that the four foot cover requirement is excessive. The 1987 yields <br />from the reclaimed portion of the refuse pile averaged 229.3 grams per <br />square meter. This corresponds to reference area yields in 1987 of 74.1 <br />grams of biomass per square meter. Assuming a linear correlation between <br />yields and topsoil thickness, yields sufficient for bond release on the <br />reGaimed refuse pile could be obtained with only 15.5 inches (39 cm) of soil <br />cover. This assumption is documented by the equations of Barth (1984). <br />This comparison is remarkably similar to the findings reported by Doll et. <br />al. (1984) for the Zap Double Wedge Experiment in North Dakota, where it <br />was reported that crested wheatgrass yields were highest on a coarse <br />textured subsoil and for the harsh south facing slopes no yield differences <br />• were evident between 16 (40 cm) and 52 inches (130 cm) of respread topsoil. <br />On the more favorable north facing slopes, maximum yields were achieved <br />at a thickness of respread topsoil of between 20 inches (50 cm) and 32 <br />inches (80 cm) for the coarse textured subsoil, while for the finer textured <br />subsoil materials, the maximum yields were associated with the 16 inch (40 <br />cm) depth. Upon correlating the textural properties of North Dakota subsoils <br />with those of the Southfield Mine refuse materials, which contain an average <br />clay content of 18.2 percent it is evident that these materials fall midway <br />between the fine textured 'B° and coarser textured "C' topsoils used in the <br />North Dakota experiment. Assuming that the Southfield Mine refuse materials <br />have similar response patterns to the North Dakota materials, it can be <br />concluded that maximum yields from the Southfield Mine refuse materials <br />would be obtained using approximately 14 to 16 inches (35 to 40 cm) for <br />south facing slopes and approximately 16 inches (40 cm) for the north facing <br />slopes. It seems more than a coincidence, based upon the studies of Doll <br />et. al. (1984) and Barth, (1984) that amount of respread topsoil necessary for <br />reclamation of these apparently sodic materials is basically identical. <br />Therefore, upon assuming that the Southfield Mine refuse materials are <br />indeed sodic and are similar to the sodic spoil studied in the Northern Great <br />Plains, ft is obvious that only 16 inches (40 cm) of respread topsoil are <br />necessary to maximize forage yields on these materials. <br />Another point to consider in the determination of amount of cover <br />necessary for reclamation of the Southfield Mine refuse materials deals with <br />• the amount of cover necessary to sustain the proposed post mining land use. <br />This concept is best emphasized in the discussion of Doll et. al. (1984) <br />wherein they state: 'the depth of soil materials that must be replaced to <br />restore soil productivity is dependent upon the chemical and physical <br />properties of the underlying spoil.' This concept is partially acknowledged by <br />42 <br />