Long‐Term Effects of Tillage Method on Soil Tilth Independent of Wheel Traffic Compaction

Abstract Field experiments were conducted on a Nicollet silty clay loam (fine‐loamy, mixed, mesic Aquic Hapludoll) in southwestern Minnesota to measure and evaluate the direct effects of tillage method on soil tilth independent of wheel‐induced soil compaction. The controlled wheel traffic concept was used on plots that had two forms of primary autumn tillage—either moldboard plow or conservation tillage (chisel plow or tandem disk). The degree of soil tilth was evaluated by measuring bulk density, clod density, and aggregate size distribution over a period of 9 y. Wheel‐induced compaction increased the density of the soil and the mean diameter of aggregates < 9 mm in diam. More importantly, wheel traffic essentially eliminated differences in tilth due to tillage method. When tilth parameters were measured in the nonwheel‐tracked soil, tillage method did have an effect on tilth, and this effect changed with time. Initially, moldboard plowing produced a more porous soil in the 0‐ to 15‐cm depth than did conservation tillage. This difference gradually changed with time, and after 3 to 4 y, conservation tillage produced a higher porosity than did plowing. A similar effect was measured in the 15‐ to 30‐cm layer, but about 7 y of conservation tillage were needed to produce a soil with porosity equal to that of plowing. Continuous conservation tillage produced larger diameter aggregates and more porous clods than did continuous moldboard plowing. These changes in soil tilth with time illustrate the need for long‐term experiments. More importantly, these studies show that wheel traffic of normal farming operations can negate or mask the true effects of specific tillage methods.