2002-07-08_GENERAL DOCUMENTS - M2002004 (4)

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Even though yields for the first year (1974) were satisfactory (but less than optimum) when 12 inches of topsoil was applied, yields declined as the experi- ment progressed. Part of this decline can be at- tributed to drought in 1976, but soil analyses in- dicated that part of the yield depression was due to upward movement of sodium from the spoil into the overlying topsoil (Sandoval and Gould, 1978). Merrill et al. (1980a and 1983a) have shown that upward movement of sodium into overlying soil materials tends to increase with the degree of dispersion of the spoil; dispersion in turn increases as adsorbed sodium increases. These initial experiments in- dicated that even though 12 inches of topsoil was in- itially adequate to reestablish grass cover on highly sodic spoils, productivity could not be maintained. Yields of Wheat and Corn Silage: An experiment (Pole et al., 1979) was initiated in 1974 at the Knife River Mine in which 2, 6, 12, and 24 inches of sandy loam topsoil were placed over moderately sodic spoil material (SAR 12). Corn and wheat were rotated on two series of plots so that each crop was grown each year. Yields of all crops for the first cropping year (1975) were extremely low due to the dryness of the recently - applied topsoil, and were low in 1977 and 1980 because of drought. Wheat yields were not obtained in 1977. Yields of wheat (Table 4) indicate that highest yields for the first two years were obtained when 24 inches of topsoil was applied, but highest yields in 1979 and 1980 were obtained on plots to which 12 inches of topsoil was applied. Yields in 1978 and 1979 were similar on plots which received either 12 or 24 inches of topsoil. Table 4. Wheat yields at Knife River Mine as affected by depth of topsoil applied over moderately sodic spoil (SAR 12, Pole et al., 1979, and unpublished data). Topsoil Wheat Yield depth 1975 inches - 2 1.7 6 2.1 12 3.1 24 3.6 1976 1978 1979 1980 bushels /acre 11.2 17.3 16.2 3.0 13.7 19.4 18.9 4.8 14.1 21.1 20.0 6.1 15.3 22.4 20.0 5.1 Average 9.9 11.8 12.9 13.3 Corn silage yields (Table 5) followed the same general trends as the wheat yields, except that after two years, yields from plots which received 12 inches of topsoil were equal to or higher than yields from plots which received 24 inches. Soil samples taken in 1978 did not indicate any significant upward movement of sodium, but sodium movement 3 to 6 inches above the soil /spoil inter- face was detected by intensive resampling in 1981. 7 Table 5. Corn silage yields from 1975 to 1980 at Knife River as affected by depth of topsoil over moderately sodic spoil (SAR 12, Pole et al., 1979 and unpublished data). Topsoil Corn Silage Yields depth 1975 1976 1977 1978 1979 1980 Average inches tons /acre 2 1.0 1.7 1.2 4.9 5.4 1.3 2.6 6 1.3 2.3 1.5 5.4 7.7 1.6 3.3 12 1.8 2.9 1.7 6.1 8.1 2.4 3.8 24 2.7 4.2 1.4 6.1 7.8 2.3 4.1 Results from another experiment adjacent to this site indicated that much of the sodium which moved upward into the topsoil was periodically leached downward into the spoil (Merrill et al., 1983b). TOPSOIL AND SUBSOIL DEPTH EXPERIMENTS The results of the preceding experiments indicted that the depth of soil needed to restore optimum pro- ductivity would require more soil material than was available from the A horizon (topsoil or plow layer) of the premine soils. Subsoil (B and C horizon) materials, when available of suitable quantity and quality, would also need to be replaced. Therefore, additional experiments were initiated with thicker depths of soil replacement using both topsoil and subsoil materials. Stanton Wedge Experiment: The details of an experiment constructed in 1974 at the Glenharold Mine at Stanton have been reported by Power et al. (1981). Subsoil was shaped into a wedge varying in depth from 0 to 7 feet over sodic spoil. Topsoil depths of 0, 8, and 24 inches were spread over the subsoil wedge so that each top- soil treatment extended over every depth of subsoil. A fourth treatment consisted of subsoil and topsoil mixed in a 3:1 ratio during construction of the sub- soil wedge. Construction details are shown diagram- matically in Figure 3. Topsoil was mostly from the A horizon and subsoil from the B and upper C horizons of an undisturbed Temvik - Williams silt loam. Proper- ties of the topsoil, subsoil and spoil materials are given in Table 6. The spoil (SAR 25) was a poor medium for plant growth and exhibited severe sur- face sealing when exposed. The subsoil was slightly saline (EC 4) and somewhat sodic (SAR 6) but within the minimum suitability criteria for subsoil (second lift) material under current North Dakota stripmine reclamation regulations. As reported by Power et al. (1981), alfalfa yields tended to increase as the subsoil depth increased to 28 or 36 inches (Table 7). Highest yields were obtain- ed when topsoil was placed over subsoil, but no in- creases in yield were noted when depth of topsoil in-