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Table 18. Crop yields from the Falkirk trench plots from <br />1979 to 1983 as affected by thickness of soil replacement <br />and overburden material, and from the undisturbed com- <br />parison plots. <br />Overburden Material <br />Gravelly loamy sand Clay loam Silty clay Undisturbed <br />Topsoil No loam plots <br />Thickness Subsoil Subsoil <br />Inches <br />Wheat 1979 <br />bushels /acre <br />9 10.3 15.2 16.1 <br />18 16.2 17.6 17.0 <br />27 18.4 19.2 17.7 <br />Barley 1980 <br />-- --•------ --- -------- • - - - -b u s h e I s /acre <br />9 4.1 7.1 6.8 <br />18 6.9 8.1 6.8 <br />27 8.6 10.1 10.3 <br />Corn Silage 1981 <br />- •tons /acre <br />9 5.1 9.1 7.2 <br />18 6.9 9.4 9.2 <br />27 9.0 10.2 10.4 <br />Wheat 1982 <br />- - - - - - -- bushels /acre <br />9 13.0 21.3 21.7 <br />18 16.3 21.2 22.1 <br />27 20.7 23.4 23.9 <br />Com Silage 1983 <br />tons /acre <br />9 2.8 6.6 6.5 <br />18 3.5 6.8 6.5 <br />27 5.6 7.7 7.2 <br />13.5 <br />19.0 <br />15.3 <br />7.4 <br />12.1 13.8 <br />10.6 <br />8.1 <br />12.3 <br />9.3 <br />22 4 <br />24.2 <br />23.5 <br />7.4 <br />7.6 <br />7.1 <br />174 <br />10.7 <br />25 6 <br />6.0 <br />depths for the other three treatments were not dif- <br />ferent. On all treatments, yields tended to increase <br />as the topsoil depth increased except for the 27 -inch <br />topsoil depth over silty clay loam, where higher <br />yields were consistently obtained at the 18 -inch <br />depth. <br />Highest yields for topsoil depths of 18 or 27 <br />inches were equal to or better than yields from the <br />undisturbed comparison plots each year, except for <br />trenches filled with gravelly loamy sand (Table 16). <br />Over gravelly loamy sand without subsoil, yields <br />with 27 inches of topsoil were about 85 percent of <br />yields on the undisturbed plots. <br />Beginning in the first year of the experiment, <br />uneven settling (differential subsidence) occurred <br />on most of the trenches (Wollenhaupt and Richard- <br />son, 1982). On one of the trenches filled with silty <br />clay loam, the center of the plot, covered with 18 <br />Inches of topsoil, subsided more than the outside <br />plots covered with 9 or 27 inches (Figure 7). Conse- <br />quently, rainfall and snowmelt tended to run off the <br />outside plots and accumulate on the center plots. <br />The difference in elevation between the outside and <br />center of the trench was about 1 foot, with short <br />slopes (25 feet or less) up to 6 percent. Wheat yields <br />for 1979 and 1982 (Figure 7) were higher in the con- <br />cave center plot of the trench than on the convex <br />outer plots. Yield differences were relatively greater <br />In 1979 when moisture stress was greater than in <br />1982 when moisture conditions were more favorable. <br />These results explain why yields were higher with 18 <br />inches of topsoil than with 27 inches on the silty clay <br />13 <br />25 <br />® 1979 <br />18 <br />Topsoil Depth (inches) <br />18 <br />Topsoil Depth (inches) <br />® 1982 <br />27 <br />27 <br />Figure 7. Yield of wheat in 1979 and 1982 as affected by <br />microtophography (elevations) of different topsoil depths <br />in the trench filled with silty clay loam overburden. <br />loam treatment. They also emphasize the effect of <br />topographic location on crop yields within relative <br />short distances and with small changes in elevation. <br />Since yields for the first three years of the experi- <br />ment were low due to both low rainfall and con- <br />tinuous cropping, a trickle irrigation system was in- <br />stalled in 1982 and 1983 on part of the plots to Obtain <br />treatment comparisons at higher moisture levels. A <br />total of 3 inches of water was applied to wheat in <br />1982 and 6 inches to corn in 1983; the amounts ap- <br />plied were not adequate for maximum yields so that <br />treatment comparisons would not be masked. The <br />substantially higher yields with irrigation for both <br />crops (Table 17) generally followed the same trends <br />as yield without irrigation (Table 16). However, when <br />27 inches of topsoil was placed over gravelly loamy <br />sand, yields tended to be lower in comparison with <br />highest yields on the other three treatments when ir- <br />rigation was applied (78 percent as compared to 84 <br />percent). This may indicate that a greater thickness <br />of soil needs to be replaced to obtain maximum <br />yields under higher moisture conditions. When the <br />first irrigation was applied in both years, however, <br />crops growing on treatments with gravelly loamy <br />sand without subsoil were already under greater <br />moisture stress than crops on the other treatments. <br />Final yields may have been related more to the <br />degree of moisture stress before irrigation than to <br />the total amount of water available during the grow- <br />ing season. <br />