At the annual meeting of the Conservation Districts of Iowa, which was held in Ames last week, many soil and water conservation district commissioners were talking about the increased amount of tillage they've seen in fields across the state this fall. "A lot of farmers want to apply nitrogen this fall, to avoid higher prices next spring," says Wilbert Knobloch, who drove to Ames from Lester - a tiny town in the far northwest corner of the state. "So they've knifed a lot of their bean stubble fields to apply anhydrous ammonia nitrogen this fall. That's disturbed and destroyed a lot of the soybean stubble.

"There has also been a lot of corn-on-corn ground this year, which produces more stalks, so farmers are doing tillage on those fields, too," he adds. "They want to avoid a build-up of cornstalks. These stalks from these new Bt corn hybrids don't deteriorate as fast as the old corn hybrids did."

To till or not to till, that's the question

The decision to till this fall or at any time needs to be carefully considered, says Mahdi Al-Kaisi, an Iowa State University Extension soil management specialist.

When soil conditions are near field capacity for soil moisture, soil aggregates are "lubricated" by water and readily reposition themselves through the air spaces, especially when heavy harvest or tillage equipment is used, he explains.

In addition, equipment operators need to remember that soil compaction can occur during the application of manure or anhydrous as well when soil moisture exceeds field capacity (maximum amount of moisture retained by the soil). "Under wet conditions, the use of heavy equipment, such as tractors, grain carts, and combines, can significantly change soil structure and cause soil compaction," he says. "Operating in wet conditions and especially doing extra tillage will increase fuel use per acre as well."

Don't create soil compaction

Compaction near the surface, within the top 3 to 6 inches of the soil, is generally associated with the amount of surface pressure. Compaction below that is primarily associated with axle weight, says Al-Kaisi.

For example, if soil a foot below the surface is at field capacity and the tractor's axle load is 7 to 8 tons or greater, compaction can occur at this depth, despite lower surface pressures. This is true especially at this time of the year, when significant amount of tillage preformed around the state.

Although it's tempting to consider tillage to loosen up soil that may have been compacted by fall harvest traffic, soil below the surface a couple of inches in most cases is still holding significant amounts of moisture from mid-October rainfall. Living plant roots are not present to remove infiltrated rainfall and soil moisture is at or near field capacity, making it too wet for good soil fracture from tillage operations, says Al-Kaisi.

The potential damages to soil, such as side wall compaction, potential soil erosion, loss of organic matter, etc., exceed the perceived benefits of deep ripping or any conventional tillage system. "Tillage studies have shown deep ripping or any other conventional tillage systems typically have no economic return advantages in both corn and soybean performance in these types of conditions," says Al-Kaisi.

The cost associated with conventional tillage over no-till in these ISU tillage trials ranged between $25 to $35 per acre. Before rushing to till their fields, farmers should ask the following question: What is the objective for tilling my field? "Your decision should be based on your field's performance and on sound economic returns--considering the input cost for the tillage operation," he says.