Soil strength may influence root growth as much as soil bulk density and matric potential. Aerial plant growth and harvestable crop yield have been positively correlated with root growth under some circumstances. Farm machine traffic and tillage practices affect soil strength and mechanical impedance. Most field studies of soil strength, mechanical impedance, root growth, and tillage have been performed on relatively coarsetextured, loamy sand soils. This study was conducted to evaluate the effect of four continuous tillage systems on strength, mechanical impedance, and soil physical properties of a Nicollet clay loam soil, a fine-loamy, mixed, mesic Aquic Hapludoll.
Following 10 years of continuous tillage in a monoculture grain-corn (Zea mays L.) production system, soil bulk density (ρ), porosity (ε), water content (θ), and mechanical resistance (cone index, CI) were measured on four tillage systems. Replicated measurements of ρ, ε, θ, and CI were made as a function of both depth and position relative to the crop row. No attempt was made to distinguish trafficked and nontrafficked interrow areas because of lack of complete traffic control. Tillage systems included (i) fall moldboard plow, (ii) fall chisel plow, (iii) spring disk, and (iv) no tillage-slot plant.
Tillage treatment, soil depth, and depth by position interaction were sources of significant variation in CI. Differences in CI due to position relative to crop row were not significant. Significant differences in ρ, ε, and θ were attributed to depth and depth x tillage interaction.
Tillage and traffic pans were observed under all tillage systems except chisel plowing. Annual moldboard plowing and chisel plowing in the fall resulted in the least amounts of mechanical resistance. Maximum CI resulted from spring disking practices. Mechanical impedance following continuous no tillage was similar in distributional pattern to the spring disking system, although CI following no tillage was not as great. Continuous no tillage and spring disking resulted in pronounced increases in CI close to the soil surface (0–7.5 cm). These increases in CI were attributed to tillage, traffic pans, or to both. These zones of increased mechanical resistance may potentially be root-restricting layers.
Increased emphasis on reduced and no tillage systems of seedbed preparation and monoculture production systems may lead to an increase in root-restricting tillage pans in clay loam soils. If increased root growth is necessary to insure an adequate supply of water and nutrients, then periodic, relatively deep chisel or moldboard plowing may be necessary to eliminate or modify shallow tillage- and traffic-induced pans that could potentially restrict root growth.