Deep-Plowing Experiment

W hat constitutes “long-term” research may be as apparent as collectable art; although many are quick to identify the plots in Rothamsted-UK, started in 1843, as the gold standard of long-term research. When researchers conduct experiments in reference to a hypothesis that determines treatment application, the test duration usually lapses after a few “climatically representative” years. Mr. O. Reggie Jones initiated his deep plowing experiment in September of 1971 to test the hypothesis that deep plowing a Pullman soil to break-up dense subsoil layers would increase dryland crop yield through increased rooting that expands the amount of soil explored for water. Any question of treatment durability or the sustainability of a management practice constituted an additional hypothesis. Nevertheless, sustained benefits of deep plowing depend on processes of soil consolidation and redevelopment of the dense subsoil layers, and these processes could span a growing season or, possibly, decades. The summer of 1971 was also my first time to work at the Bushland Research Center as a student, armed with a nearly new driving permit. I helped collect soil and plant samples, and I quizzed practically everyone about the assortment of nearby experiments. These included on-going groundwater recharge experiments using excavated basins and efforts to quantify soil profile modification and deep plowing effects on the growth and yield of irrigated crops. One recurrent concern about some of these tillage practices was that the effects may be short lived. My professional appointment with the USDA-Agricultural Research Service at Bushland began in September of 1997. Agency leaders stressed that research conducted by ARS scientists should P recipitation on the semiarid Southern Great Plains, during an average year, provides approximately 25 percent of the potential evapotranspiration (ET) for crop water use. Dryland cropping systems rely on stored soil water to supplement precipitation during dry years. The Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) is the dominant soil found on of the Southern Great Plains region, occupying 1.5 million ha (Unger and Pringle, 1981), and features a very slowly permeable subsoil layer from 0.15 to 0.7 m depth. This layer may impede infiltration of rain for crop use and delay internal drainage of ponded water. Eck and Taylor (1969) proposed a onetime deep, > 0.4 m, plowing modification of the soil profile to eliminate the dense subsoil to increase infiltration of rain and irrigation. Several studies at Bushland evaluating the effects of soil profile modification have consistently reported decreased bulk density (BD) and increased infiltration for a period of up to 26 years (Eck and Taylor, 1969; Eck, 1986; Eck and Winter, 1992; Unger, 1993). Another benefit of disrupting dense subsoil layers with deep plowing was decreased penetration resistance (PR) and, consequently, an expanded volume of soil explored by roots of irrigated crops. There is little information documenting the longevity of deep plowing effects on soil physical properties or growth and yield of dryland crops.