"Deep tillage can't fully restore soil to its 'pre-compacted' state, but it can introduce space for air to enter the soil, which over time helps increase pore space," he said. "It also helps break up the physical barrier that compacted soil produces as it dries out."

Several types of equipment can be run deep enough to penetrate compacted zones. The most basic are "deep rippers" that are capable of ripping to a depth of 12 to 18 inches.

Either straight points or sweeps can be used, with sweeps requiring more horsepower, and typically running a little shallower. However, they provide more "soil shatter," which is helpful in relieving compaction.

Because the deep-ripping operation is expensive in terms of equipment wear, fuel, and time, it has been common practice to use these implements only when standing water or other signals point to the need to use them in individual fields or parts of fields. This fall, Nafziger said this might mean most parts of most fields.

Many farmers use combination disk-rippers, which can bury some of the residue and leave soil surfaces more level than deep rippers.

Another primary tillage tool is the heavy disk harrow that tills with large, notched blades and can be adjusted to bury the desired amount of residue.

Regardless of the implement used to relieve compaction, Nafziger said there are a few basic principles to remember during these operations.

• Running implements to depth to relieve compaction does little good if soils are not dry enough to shatter well. Tillage implements and the tractors that pull them are heavy, which means you can cause as much compaction as you relieve.

Water use by the corn crop ended early this season, and soils without active crop roots and with corn residue after harvest dry slowly as temperatures drop. So if you're turning up wet soil or if the implement pulls easier than expected, it's possible that it's not as dry as it should be.

• Tillage operations can only relieve compaction to the depth at which they are operated. A heavy axle load can cause compaction to a depth of 18 inches or more.

While implements like the offset disk are often blamed for causing compaction, the real drawback is that they typically aren't run deep enough to relieve compaction to the desired depth. The moldboard plow often has a similar limitation, though the "modified mini-moldboard" can, if operated to a depth of 12 inches or so, effectively relieve compaction.

• Primary tillage needs to leave some residue on the surface to minimize soil loss before next spring.

Typically, corn fields that will go back into corn are the highest priority for relieving compaction, Nafziger said. However, questions are arising this year about the need for deep tillage following soybean harvest in preparation for corn in 2011.

"Soybean fields often have a mellower surface in the fall than corn fields, but deeper compaction caused during the 2009 season is still present," he said. "At minimum, soybean stubble fields should be checked for compaction this fall. Because soybean fields have less residue than corn fields, we need to retain as much residue on the surface as possible during the tillage process."  FG

—Jennifer Shike, University of Illinois, ACES-news