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The effect of annual compaction (1987-1989) of 9 and 18 Mg axle loads, and subsoiling for a corn/soybean rotation was investigated on a Hoytville silty clay loam (very poorly drained) soil. The effect on soil physical properties was also examined.

Deep tillage is often prescribed to alleviate subsoil compaction, but how long do the benefits last? Subsoilers from Deere, DMI, Kewanee, Tye, and Yetter were operated at a 280-mm depth on a compacted 25-ha field in autumn 1990. The different subsoiler designs were evaluated by draft, crop residue cover, soil physical properties and yield measurements over the subsequent two growing seasons. Air porosity and cone penetrometer measurements showed continuing benefits, from all subsoil operations, two years after tillage in areas not trafficked. However, two passes of a tractor recompacted the soil. Controlled traffic is, therefore, considered essential to obtain long-term benefits from subsoiling. Compared to check strips, soybean yield increases for the five subsoilers ranged from 3.0 to 6.9% in 1991 and corn yield increases ranged from 1.5 to 3.0% in 1992. Economically, the first year yields paid for the subsoiling, second year increases were profit.

Changes in soil physical properties resulting from multiple passes of an 18 m3 single-axle grain cart with a 15.2 t axle load were measured one year after the traffic occurred. Air permeability, air-filled porosity, and dry density data were analyzed to determine how the soil responded to grain cart traffic and pre- and post-traffic tillage. Four repeated passes of the cart decreased air-filled porosity and permeability by as much as 50% in the 200- to 400-mm depth range when compared to the effect of a single wheel pass. With pre-traffic chisel plowing to a depth of 200 mm, most of the compaction in the 100- to 200-mm depth occurred on the first pass. However, four repeated passes had a greater effect on soil properties in the plots that received no tillage compared to the plots that were chiseled before trafficking.