Effects of Topsoil and Subsoil Thickness on Soil Water Content and Crop Production on a Disturbed Soil

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 productive 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 surface). Four crops—alfalfa (L.), crested wheatgrass (), native warm-season grasses (and), and spring wheat (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 management in the same county. In most instances, over 90% of the maximum yields observed was obtained when 70 cm of subsoil 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 surface. 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 situation.