Depending on how you direct traffic across your fields, you may have tire prints on 40% or more of the field area. The compaction that results can cut yields big time.

"There is no question that wheel traffic can reduce yields," says Kirk Wesley, with Key Agricultural Services, Macomb, IL. "How much yield is lost depends on the soil type, how wet fields are when heavy equipment runs over them, and how many different tracks you make in the field."

Wesley, an agronomist and independent crop consultant, has studied compaction and its effect on crop yields for several years. He uses a computer-driven penetrometer to check compaction in soil profiles up to 50" deep, then compares yields in fields with varying amounts of compaction.

The ideal composition of soil is 50% mineral, 25% water and 25% air.

"But soils are not all created equal," says Wesley. "Sandy or silty upland soils are less susceptible to compaction damage, and compaction doesn't affect these soils as deeply. Gumbo and high-clay-content soils can become dense and compacted to greater depths. All soils are more subject to compaction when wet."

When the tires of a heavy implement roll over the surface, the air spaces or pores in the soil are squeezed closed. Water can no longer penetrate readily, and plant roots may not be able to grow through a compacted layer, either.

And the compacted zone does not just occur directly under the tire.

"There is considerable soil compaction to the side of the tire track, too," says Wesley. "For example, tractor duals not only compact soil beneath the track, but also in the area between duals."

Wesley calls these between-duals areas "pinch rows." He has studied how densely compacted soils can become in pinch rows and how much yields are reduced.

"We've found penetrometer readings in pinch rows up to 200 pounds per square inch (the force needed to push the probe through the soil) and as much as a 50-bushel per acre yield decrease in corn," he says.

"Anyplace there is a tire footprint, you'll have some compaction to the side of the track. But the biggest yield losses come in the zone of tractor duals. If you don't absolutely have to have duals, don't use them."

This advice may fly in the face of conventional wisdom that says the more rubber on the ground, the better the traction and the less the compaction. But it gets support from Bill Casady, a University of Missouri ag engineer. Casady says the most efficient tires are large radials maintained at inflation pressures "just adequate to support the weight of the tractor."

Tires should be selected and managed to provide maximum contact area with the soil surface, says Casady.

"The pressure exerted by a tire on the soil surface near the lugs is roughly two pounds per square inch greater than the tire inflation pressure," he explains. "Lower-pressure tires transmit power to the soil through the largest area, so they provide good traction and minimum compaction."

Tractors with front-wheel- drive assist generally develop more traction in the rear wheels than those without front-wheel drive, he adds. The front wheels "pre-compact" the soil enough to give rear wheels a more efficient "bite."

However, the best way to avoid soil compaction is not to drive on it at all, and Wesley believes most farmers could eliminate a lot of compaction by giving a bit more thought to field traffic patterns.

"As much as possible, match field operations and track widths," he suggests. "Try to design a system with common-width equipment to reduce wheel traffic in the field."

In effect, try to establish a "tram line" system of farming. For example, a 30'-wide field cultivator; a 12-row, 30" planter; and a sprayer with a 60' boom all can utilize the same tractor tracks.

"Keep in mind where the tractor wheels will go and what you'll be pulling behind the tractor," he adds. "As farmers go to larger, wider equipment, they may need to consider track-type machines at some point. If you've got heavy machinery on the ground when the soil is most vulnerable - as in springtime, especially on clay soils - you're going to have compaction. It's better to limit that compaction to a relatively small area than to spread it all over the field."

On severely compacted soils, it may be necessary to deep-till every three or four years to break up zones of compaction. A few years ago, Wesley worked with a big-acreage farm near Havana, IL, where some soils were tightly compacted 24" or more deep.

"Corn yields averaged 99 bushels per acre when we started," he recalls. "Among the things we did, we deep-ripped a lot of that soil, and yields climbed to an average 168 bushels per acre.

"But deep-tilling isn't cheap; ripping can cost $40 per acre," he adds. "If you do it every three or four years, the costs add up."

Deep ripping needs to be done when the soil is dry, points out Wesley.

"Don't rip when the soil is wet; you'll do more harm than good," he states.