Tillage System and Crop Residue Effects on Surface Compaction of a Paleustoll

Concern of excessive compaction of soils under conservation tillage (CT) has persisted as CT practices gain acceptance in major agricultural systems. The objective of the study was to determine the cumulative effects of tillage systems [moldboard plowing (MT), stubble-mulch tillage (ST), and no-till (NT)] and rates of crop residues on bulk density of the near-surface zone under annual winter wheat cropping. In the fall of 1983, tillage plots were established on Bethany (fine, mixed, thermic Pachic Paleustoll) and Renfrow (fine, mixed, thermic Udertic Paleustoll) silt loams on a 2% slope near El Reno, OK. Bulk density and water content were measured to a depth of 0.3 m in 0.025-m increments using gamma and neutron attenuation techniques during 1991 and 1992. After planting, MT and ST soils had the lowest mean surface density; these soils consolidated as bulk density during winter wheat dormancy averaged 1.34 (±0.06), 1.30 (±0.06), 1.21 (±0.06) in 1991, and 1.31 (±0.06), 1.30 (±0.005) and 1.24 (±0.05) Mg m⁻³ in 1992 in MT, ST, and NT soils, respectively. Near the end of the season, surface density of ST and MT soils had increased by 1.6 and 4.8%, respectively. No-till soil showed a decrease of 5.4% during the same overwinter period and had the lowest bulk density in the end. The inherent reduction in field operation traffic in CT systems, and a decrease in tillage intensity proportionately reduced soil surface compaction under annual wheat cropping. Bulk density increased with soil depth in all treatments; however, NT resulted in decreased soil density in and below an old restrictive layer more effectively than ST or MT management. Removal of crop residues resulted in higher bulk density, primarily in the 0.05- to 0.125-m depth of MT soils, and in the 0 to 0.05-m surface layer of NT soils. Maintaining high amounts of wheat residues at the soil surface helps improve the macroporosity of near-surface zones of soils under conservation tillage. Severe compaction of CT soils did not appear prevalent under winter wheat production to cause physical constraints to crop growth in subhumid regions.