2012-06-20_PERMIT FILE - C2010089 (60)

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128 SOIL SCI. SOC. AM. J., VOL. 45, 1981 Table 7 —Water used during the 1978 growing season as affected by crop and soil thickness. Topsoil thickness Subsoil Crop thickness 0 cm 20 cm 60 cm and subsoil cm Alfalfa 30 27 32 150 25 27 L.S.D. (0.05) = 3.0 Crested 30 24 27 wheatgrass 150 23 26 L.S.D. (0.05) = 1.9 Native grasses 30 29 33 150 27 29 L.S.D. (0.05) = 3.5 Wheat 30 29 28 150 29 32 L.S.D. (0.05) = 2.4 Mixed topsoil 30 27 26 25 25 26 27 26 33 29 31 29 29 28 31 33 (Bond et al., 1971; Power and Alessi, 1970). Total growing season water use for the various treatments was calculated each year, and data for 1978 are pre- sented in Table 7. Water used was calculated by add- ing growing season precipitation to the change in soil water content to 180 cm between spring seeding and harvest. There was no evidence of runoff or deep per- colation during this period. Generally, subsoil thick- ness had little or no consistent effect upon total water use, but water use was often greater where the crop was grown on either 20 or 60 cm of topsoil. There was very little difference between crops in amount of water used. Thus, crop yields measured in 1978 were poorly related to water use. A similar conclusion can be made from data for other years also, probably be- cause precipitation was seldom adequate to wet soil material more than 100 to 120 cm. Figure 2 depicts soil water content in the spring of 1975 and after harvest of the 1978 crop. Since these two data sets represent approximately the wettest and driest soil conditions encountered, differences between these two curves represent maximum water withdrawal. Depth of water withdrawal by dryland crops is usually an indicator of depth of root activity (Bond et al., 1971; Haas and Willis, 1962; Power and Alessi, 1970). Data in Fig. 2 illustrate that depth of root activity, as determined by water withdrawal depth, was usually only 15 to 30 cm greater where 150 cm or more of permeable soil material was present than where relatively impermeable mine spoils were within 30 to 90 cm of the surface. Data are presented only for the 20 -cm topsoil treatment because thickness of (or even the absence of) topsoil over the permeable B- horizon subsoil material had no appreciable influence on depth of root activity. For all crops and topsoil treatments, significant amounts of water were ex- tracted 30 to 90 cm into the highly sodic spoils where they were covered with only 30 cm of soil material. Alfalfa generally extracted water to the greatest depth (about 135 cm for 30 -cm subsoil treatments and about 175 cm for the 150 -cm subsoil treatments). Crested wheatgrass was the next most effective, followed by the native grasses and spring wheat —the latter ex- tracting water to only about the 75- and 90 -cm depths for the 30- and 150 -cm subsoil treatments, respectively. It is conceivable that because of the decrease in permeability at the soil -spoil interface, there might have been a tendency for water to accumulate above 40. 80- t1 120 c 160 to 2 0 a. 0 40- 80- 120- SOIL WATER CONTENT, cm / cm 0 .25 .50 0 .25 .59 0 .25 .50 0 .25 .50 30 cm of SUBSOIL 1 1 AUGUST1 1978 1 • MAY 1975 I MAY I / 1975 AUGUST - 1978 1 i ALFALFA 150 cm of SUBSOIL 160- 1► CRESTED NATIVE GRASS WHEAT WHEATGRASS Fig. 2 —Soil water content at beginning of experiment (spring 1976) and after harvest of third crop (fall 1978) as affected by crop and soil thickness. this interface almost to the point of saturation. From data presented in Fig. 2 and from approximately monthly soil water profiles, there is no evidence of a buildup of soil water for any treatment at any time. The results of this study indicate that a minimum thickness of about 90 cm of soil material is required to obtain maximum production of most crops under the conditions encountered. For production of crops such as spring_ wheat, topsoil and subsoil need to -- be segregated to Obtain maximum yield. This require- ment may possibly be related to a greater tendency for crusting, compaction, and related phenomena that could affect emergence and stand establishment and seedling vigor in the absence of topsoil. Unfortu- nately, no data are available on such factors from this experiment. Because unprotected disturbed soil ma- terial is highly erodible, and because upward salt mi- gration may occur, possibly somewhat more than 90 cm of soil material should be used in reconstructing a soil profile to ensure that at least 90 cm of good quality soil is available several years later. It should again be emphasized that these results apply when sodic, impermeable materials underlie the soil ma- terial. Similar data from experiments on spoils less hostile to plant growth are not yet available. ACKNOWLEDGMENTS We thank Basin Electric Power Cooperative and the Consoli- dation Coal Company, who jointly provided the manpower and equipment required for the earthmoving operations in- volved.