Submitted to: Soil Science Society of America Special Publication Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 8/15/2009
Publication Date: 4/5/2010
Citation: Stewart, B.A., Baumhardt, R.L., Evett, S.R. 2010. Major advances of soil and water conservation in the U.S. Southern Great Plains. In: Zoebeck, T.M., Schillinger, W.F., editors. Soil and Water Conservation Advances in the United States. Special Publication 60. Madison, WI:Soil Science Society of America, Inc. p. 103-129. Interpretive Summary: Much of the land in the Southern Great Plains was controlled by American Indians until 1874. Then, ranching moved into the region. Farming began in the late 1800s but the 'Big Plowout' occurred in the early 1900s. These years were wetter than average and the favorable precipitation coupled with the newly plowed grassland soils led to abundant crop yields. As the soil organic matter levels declined rapidly and the years of abundant precipitation turned to years of drought, wind erosion became rampant and led to the infamous Dust Bowl of the 1930s. The popular theories for early-day water conservation included dust mulching to break capillary movement; deep plowing or sub-soiling to permit more water to enter the soil and enhance depth of rooting; and the belief that water rises from the water table or deep subsoil by capillary movement. Slowly but surely, many of these practices and theories disappeared. Because growing season precipitation is insufficient in much of the region for growing a crop every year, fallow periods that range from 11 to 16 months are widely used to increase plant available water stored in the soil profile as a supplemental water source that increases yields and reduces risk. Historically, only 15 to 20% of the precipitation occurring during the fallow periods was stored in the soil with the remainder lost by evaporation. Therefore, even though fallow was widely used and considered highly effective, it was not water efficient. In recent years, major advances have emerged in conservation agriculture that minimizes or eliminates tillage and leaves as many crop residues on the soil surface as feasible. These practices control wind and water erosion while maximizing soil water storage for subsequent use by growing crops through increased infiltration and decreased evaporation.
Technical Abstract: The U.S. Southern Great Plains comprise the broad expanse of prairie and steppe lands that lie east of the Rocky Mountains and cover parts of Colorado, Kansas, New Mexico, Oklahoma, and Texas where semi-arid dryland crop production merges into rainfed farming. Except for exposed Pliocene alluvial strata near rivers and Permian age residuum on the rolling plains, most southern Great Plains soils formed from an Aeolian loess mantle deposited during the Quaternary. Southern Great Plains farming developed in the early 1900s during years with above average precipitation that produced favorable crop yields; however, a return to normal and below normal precipitation limited crop yields and biomass production that, in turn, decreased soil stabilizing organic matter and aggregation. Subsequent erosion of the now fragile soil by wind produced the infamous Dust Bowl of the 1930s, but led to the development of emergency tillage and residue management practices that protected the soil and promoted rain storage as soil water. Because growing season precipitation is insufficient to sustain annual dryland crop yields, alternative cropping systems were developed to supplement growing season rain by introducing fallow periods that increased the amount of plant available water stored in the soil profile. Blade type sub-tillage technologies were explored to retain crop residue on the soil surface to reduce wind erosion. During fallow, this crop residue also decreased water lost by evaporation and increased the fraction of fallow precipitation stored as soil water from the historical 15 to 20% average with clean disk tillage to approximately 40% with minimum tillage. Continued research has improved residue management practices that minimize wind and water erosion while maximizing water storage for subsequent use by growing crops, accounting for up to two-thirds of the long-term yield increases.