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APPENDIX 2.05.4(2)(d) -1 Effects of Topsoil and Subsoil Thickness on Soil Water Content and Crop Production on a Disturbed Soil'. J. F. POWER, F. M. SANDOVAL, R. E. RIES, AND S. D. MERRILL ABSTRACT Data by which we can quantify effects of soil depth upon productivity from controlled experiments are essentially lacking for semiarid regions. In connection with mined land — reclamation research in North Dakota, an experiment was established in which soil was reconstructed by building a wedge with produc- tive subsoil (B and upper C horizon) on top of leveled sodic mine spoils derived from shale. Thickness of the subsoil wedge ranged from 0 to 210 cm. Topsoil (A horizon) was then spread over the subsoil wedge to provide a topsoil either 0, 20, or 60 cm thick. A fourth treatment consisted of mixing subsoil and topsoil within the wedge in a 3:1 ratio (no topsoil on the sur- face). Four crops — alfalfa (Medicago saliva L.), crested wheat - grass (Agropyron desertorum), native warm - season grasses (Bou- teloua gracilis and Bouteloua curtipendula), and spring wheat (Triticum aestivum L.) — were grown each year on these plots from 1975 through 1979. Yields of all crops increased as total soil thickness (topsoil plus subsoil) increased to the 90- to 150-cm range. Highest yields equaled or exceeded yields that would be expected in these years on similar undisturbed soil types under good man- agement in the same county. In most instances, over 90% of the maximum yields observed was obtained when 70 cm of sub- soil plus 20 cm of topsoil covered the sodic spoils (SAR = 25, clay = 38%). Yields from 60 cm of topsoil were similar to those from 20 cm of topsoil. With no topsoil, only native grama grasses produced over 75% of maximum, but all crops except wheat produced at least 90% of maximum with at least 90 cm of the mixed subsoil. topsoil spread over spoils (wheat yields were only about 80% of maximum). Water was extracted from the upper 30 to 90 cm of spoils when the soil-spoil interface was within 90 cm of the soil sur- face. Thickness of topsoil had no influence on depth of water extraction. Alfalfa extracted water to about 135 cm if sodic spoils were within 90 cm of the surface and to about 175 cm where spoils were covered with at least 150 cm of soil materials. Depth of water extraction by crested wheatgrass under these two situations was about 120 and 150 cm; by native grasses about 80 and 120 cm; and by spring wheat about 75 and 90 cm, respectively. There was no evidence of any accumulation of soil water just above the soil -spoil interface under any situa- tion. Additional Index Words: reclamation, mined -land reclama- tion, water use, soil depth, reconstructed soil. Power, J. F., F. M. Sandoval, R. E. Ries, and S. D. Merrill. 1981, Effects of topsoil and subsoil thickness on soil water con- tent and crop production on a disturbed soil. Soil Sci. Soc. Am. J. 45:124 -129. S OIL THICKNESS IS RECOGNIZED as an important param- eter in determining soil quality and productivity. Thickness of the genetic A and B horizons is an im- portant criterion in the classification and management of natural soils, and plays a major role in determining land values. Earlier research showed that the presence of a few centimeters of topsoil helped restore productivity to cut areas resulting from land - leveling (Carlson et al., 1961). Likewise, some of the first research conducted on sodic mine spoils resulting from strip mining in North Dakota gave similar results (Power et al., 1975). In recent years, Legislation has been enacted in the 124 United States requiring original soil material to be saved and replaced on mine spoils after shaping. Even with this knowledge and current legislation, available literature generally lacks quantitative data on the effects of soil thickness upon crop production. In most regions soil thickness may influence plant rooting depth and the quantity of soil water avail- able for plant growth, thereby affecting crop growth potentials. This relationship is especially important when the underlying material is a poor medium for plant root activity, as are sodic mine spoils (Power et al., 1975). The research reported here was initiated to rovide quantitative data on how (B upper C -horizons) and topsoil (A horizon) thickn_ffect the amount of w ater extracted and the de•th of extraction, and subsequently upon growth anityield of several crops important in semiarid agri- culture. METHODS Over 2 ha of sodic mine spoil at the Glenharold mine near Stanton, North Dakota, was leveled to 0.5% grade in the sum- mer of 1974, about I year after mining was complete. Spoils consisisted primarily of soft shale from the Tongue River mem- ber of the Fort Union geologic group and were relatively high in both exchangeable Na (Na adsorption ratio of about 25) and clay (Table 1). After leveling, subsoil (B and upper C horizons) was taken from an unmined Temvik silt loam (fine - silty, mixed Typic Haploborolls) and spread with motorized scrapers over the lev- eled spoils in a wedge that increased from 0 to 210 cm in thick- ness. The wedge of subsoil constructed was 52.5 m wide from toe (0 cm of subsoil) to top (210 cm of subsoil) and 240 m long giving the surface a 4% southerly slope. Thickness of sub- soil increased about 4 cm from each 1 m of distance upslope. The wedge was subdivided into 12 main plots, each 20 m wide and 52.5 m long. Three of the plot treatments consisted of covering the subsoil wedge with either 0, 20, or 60 cm of top- soil (A horizon from Temvik silt loam) uniformly from toe to top. The fourth main plot treatment consisted of thoroughly mixing subsoil and topsoil in a 3:1 ratio during construction. Each of these four main treatments was replicated three times. Properties of the subsoil and topsoil are listed in Table 1. Ap- proximately 20,000 m' of soil material was needed for construc- tion. Four subplots (4.2 m wide and 52.5 m long) and an alley 3.2 m wide were established within each main plot. The alleys prevented soil mixing between train plots during construction and were also shaped to control runoff water (usually from snow melt) between main plots. Subplots were seeded at recom- mended rates and depths to four different crops in the spring of ]975 —hard red spring wheat ( Triticum aestivum L.), alfalfa (Medicago saliva L.), crested wheatgrass (Agropyron deserto- rum), and a mixture of native warm- season grasses consisting of blue grama (Bouteloua gracilis) and side oats grama (Bou- teloua curtipendula). Before seeding, 35 kg P /ha as triple super- phosphate was broadcast on all subplots, and all but alfalfa sub- plots received 55 kg N /ha broadcast as ammonium nitrate. No K was required. Access tubes were installed to the 2 -m depth 7.5 and 37.5 m from the toe of each subplot (30 and 150 cm subsoil, respectively, plus topsoil), and soil water content was determined to the 180 -cm depth with a neutron probe. At maturity, samples of wheat were harvested from 4.2- by "Contribution from the USDA-SEA-AR. Received 1 May 1980. Approved 15 Aug. 1980. Soil Scientists, Range Scientist, and Soil Scientist, respective- ly, Northern Great Plains Research Center, Mandan, ND 58556. Dr. Power is now in the Soil & Water Conservation Research Unit, SEA -AR, Univ. of Nebraska - Lincoln, Lincoln, NE 68583.