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

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Table 9. Average native grass yields (1976-79) as affected by thickness of soil materials at Stanton wedge experi- ment (Power et al., 1981). Subsoil Depth inches 4 12 20 28 36 48 60 72 inches 4 12 20 28 36 48 60 72 Topsoil Depth (inches) Mixed Subsoil - 0 8 24 topsoil 0.04 0.20 0.30 0.33 0.36 0.35 0.32 0.33 6.3 18.7 21.0 25.4 25.2 24.2 23.5 22.1 tons /acre 0.31 0.26 0.42 0.30 0.47 0.39 0.48 0.47 0.41 0.41 0.39 0.37 0.45 0.40 0.42 0.35 Table 10. Average spring wheat yields for 1975 and 1978 as affected by thickness of soil materials at the Stanton wedge experiment (Power et al, 1981). Subsoil Topsoil Depth (inches) Depth 0 8 24 bushels /acre 21.8 26.6 29.4 28.8 31.9 31.6 31.8 33.5 33.8 34.1 33.2 31.8 31.0 33.1 32.8 30.3 0.01 0.07 0.29 0.38 0.37 0.43 0.40 0.41 Mixed Subsoil - topsoil 7.0 19.6 27.5 27.6 26.7 27.2 28.0 28.3 For each topsoil - subsoil treatment and crop, the three highest yields (as given in Tables 7 through 10) were averaged; for each crop, relative yields were determined as percentages of highest average yield (Table 11). Highest relative yields (100 percent) were always obtained for topsoiled treatments, and dif- ferences between the 8 and 24 -inch depths were not significant. Yields for subsoil without topsoil were from 64 to 76 percent of maximum yields, and when subsoil and topsoil were mixed, yields were from 83 to 94 percent of maximum yields. This indicates that at least 8 inches of topsoil must be replaced to restore optimum yield levels with subsoils of the quality used in this experiment. Table 11. Average relative yields for each of the four crops for the various topsoil and subsoil combinations at the Stanton wedge experiment (calculated from Power et al., 1981). Crop Crested wheatg rass M ixed native grasses Alfalfa Spring wheat grain Topsoil Depth (inches) Mixed Subsoil - 0 8 24 topsoil percent 74 92 100 94 76 100 98 90 64 100 94 84 74 100 98 83 9 The yield data in Tables 7 through 10 indicate that yields obtained when 8 and 24 inches of topsoil were replaced did not differ greatly for the same subsoil depths. However, yields at both topsoil depths tend- ed to increase as the subsoil depth increased to about 28 or 36 inches. This was true even when an additional 18 inches of topsoil was applied, and even though highest yields were always obtained on top - soiled treatments. However, yields obtained with 0, 8, or 24 inches of topsoil can be compared at equal total soil depths of 28 and 36 inches (Table 12). At equal total soil depths, yields were consistently higher with 8 inches of topsoil than with 24 inches except for crested wheatgrass at 36 inches. The top- soil was sandy loam in texture, with an available volumetric water - holding capacity of approximately 15 percent; the subsoil was clay loam with an available water - holding capacity of approximately 26 percent (Power et al., 1981). Thus, the available water - holding capacity of the subsoil was nearly twice that of the topsoil, and the higher yields ob- tained with 8 inches of topsoil are likely related to this high water - holding capacity. Table 12. Average yields for topsoil depths of 0, 8, and 24 inches at total soil depths of 28 and 36 inches at the Stan- ton wedge experiment. Data calculated from Tables 7, 8, 9, and 10. Total Soil Depth inches Topsoil Depth (inches) 0 8 24 Alfalfa tons /acre 28 0.67 0.88 0.63 36 0.58 1.00 0.79 Crested Wheatgrass tons /acre 28 0.98 1.13 0.96 36 0.92 1.08 1.14 Native Grass tons /acre 28 0.33 0.47 0.26 36 0.36 0.48 0.30 Wheat bushels /acre 28 25.4 31.9 26.6 36 25.2 31.8 28.8 As noted in the preceding discussion, yields for each subsoil - topsoil combination tended to reach a maximum at subsoil depths of 28 to 36 inches and then decline as the subsoil thickness increased. The slope of the completed wedge was about 5 percent, and higher yields at the midpoint of the slope are at- tributed to higher moisture levels due to accumula- tion of runoff water from the summit of the slope and to accumulation of snow at the midpoint of the south - facing slope. The results of this experiment can be briefly sum- marized as follows: