Rush hour in a farm field is more like “crush hour,” with some equipment weighing 30 tons/axle.

Heavy equipment, wet soils and impatience all contribute to soil compaction. The bad news is that deep compaction often cannot be remedied by tillage, experts say.

Equipment weighing at least 20 tons/axle generally causes deep soil compaction extending below 12 in. into the soil profile, says Jane Johnson, soil scientist with USDA-ARS North Central Soil Conservation Research Laboratory, Morris, MN.

Computer models have determined that axle load is the main cause of deep soil compaction. “Reducing vehicle weight, then, is the best way to avoid deep subsoil compaction,” says Randy Raper, ag engineer and lead scientist, USDA-ARS, Auburn, AL. This is in contrast to surface compaction, which can be mostly eliminated with disk ripping or chisel plowing the compacted layer.

An Ohio study found that 20 tons/axle caused measurable changes in soil compaction to 18 in., says Randall Reeder, Extension agricultural engineer, Ohio State University (OSU).

Two studies show the huge variability represented by soil types and soil moisture in compaction.

For example, on a Webster clay loam, Waseca, MN, corn suffered 9-30% yield declines after 10- and 20-ton/axle loads in a Minnesota study. Yet on a dry Ves soil, Lamberton, MN, there was little compaction, and grain yield was reduced by just 6% the first year after high axle loading.

Just one pass of a 20-ton axle load can cause long-term yield losses of 5-6% annually, according to a five-year Minnesota study on clay loams.

Soybean yields tended to be more sensitive to surface compaction (under 4-ton/axle wheel tracks) than to subsoil compaction (10-20-ton/axle load), says Johnson.

Compaction's reduced soil aeration and root surface area reduces uptake of all nutrients and water. “An extensive, fibrous, well-developed root system is especially important for maximizing phosphorus uptake,” says James Camberato, Purdue University associate professor of soil fertility and plant nutrition.

Surface compaction is still harmful, but at least it can be remedied. Axle loads of “only” 4 and 6 tons reduced corn yields on silt loams by an average of 7.5% over two years in a University of Minnesota study.

Sixteen years of OSU research on Hoytville silty clay loam showed that compaction from a 20-ton/axle load stole 18% of both corn and soybean yields. These yield losses resulted from three years of intentional compaction with a 600-bu. grain cart after harvest. No additional compaction was created for 16 years.

All plots were chisel-plowed to minimize surface compaction or subsoiled to remove deep compaction. Subsoiling increased yields, but their yields still averaged 10% below control plots.

“This shows that subsoiling helped the situation, but could not return a compacted field back to ‘normal,’?” says OSU's Reeder. On identical plots with 10-ton/axle loading, yields returned to normal within five years.

“Not all soils are affected by compaction,” Reeder says. “OSU has research with 20-ton axle loads on two other soils with minimal yield reduction, if any.

“I try to persuade farmers to adopt what I consider an ideal cropping system: continuous no-till with controlled traffic, using RTK auto-steering,” Reeder says. “Use a crop rotation and cover crops for maximum benefits. This seems to be the best advice to avoid the risk of compaction because we have no research data for most soils on how compaction affects yields.

“Because farmers usually treat every part of a field the same, they don't know whether compaction is reducing yield or not,” Reeder adds.

“Deeply rooted cover crops, such as oil seed radish, cereal rye and annual ryegrass (as cover crops) seem to be good tools to penetrate compacted soil, perhaps equal to a subsoiler. Plus, cover crops protect against erosion and build soil quality.”

HOW TO UNDO COMPACTION

Equipment weighing less than10 tons/axle generally causes compaction in only the top foot of soil. Surface compaction can be largely removed by disk ripping or chisel plowing. “Subsoiling helps but has an important downside,” says Ward Voorhees, a USDA-ARS North Central Soil Conservation Research Laboratory soil scientist who has researched compaction extensively.

“If you drive over your subsoiled field with ordinary farm machinery you will recompact it, sometimes even worse than before the subsoiling,” he says. “So if a farmer has deep compaction and has to do deep tillage, it's important to control the field traffic afterwards or he's taking one step forward and two steps back.”

Ohio State research supports this point. “When we switched our compaction plots from chisel plow to no-till six years ago, and recompacted every three years, subsoiling reduced yields rather than increasing them,” says OSU's Randall Reeder. “Firm no-till soil resists compaction better than deep-tilled soil.”

The first trip across loose soil causes about 85% of the total compaction.

HOW TO MINIMIZE COMPACTION

  • LIMIT TRAFFIC TO 20-30% of field area by designating traffic lanes. All farm equipment is the same width and traffic is confined to specific lanes year after year. This preserves 70-80% of the soil uncompacted by heavy traffic.

    Besides limiting soil compaction, separating the areas used for root growth and those used for vehicle traffic is a very useful form of limiting soil compaction, says USDA's Raper. Controlled traffic reduces input overlaps and enables earlier field access, since compacted traffic lanes are more tractable.

    Without traffic lanes, up to 70% of a conventionally tilled field is trafficked by farm equipment.

  • LIMIT MACHINERY WEIGHT to 10 tons/axle, says USDA's Voorhees. Research here and in Europe found weights above that to cause compaction deeper than the upper 8-10 in.

    “You can remedy surface compaction with tillage, but deeper than that, tillage isn't always practical,” Voorhees says. “Limit your heavy grain carts to the headlands or controlled traffic lanes. It's a tradeoff between time saved harvesting and reduced yields from compaction.”

  • AVOID WORKING WET soil, and reduce secondary tillage passes, says Peter Thomison, OSU Extension agronomist. Sometimes even waiting a day or two can let soil dry out to better handle a heavy load.

    “Shallow compaction created by excessive secondary tillage can reduce crop yields,” he says. Also, deep tillage should only be used when you've identified a compacted area and soil is relatively dry.

  • REDUCE TIRE PRESSURE to reduce surface compaction. “On the average farm, the majority of tires are over-inflated, which reduces the tire footprint, increasing compaction,” says OSU's Reeder.

  • IF NECESSARY TO remove traffic's effect, use in-row or bentleg subsoilers that minimally disturb the soil surface.

  • IMPROVE field drainage.

  • USE RADIAL TIRES, and minimize their inflation pressure. Rubber tracks are somewhat helpful in dispersing the weight.

  • SOME WONDER WHETHER tracks reduce soil compaction over conventional farm tires or radials. Tracks and flotation tires may enable traffic on wetter soils, allowing more severe compaction, says Minnesota Regional Extension Educator Jodi DeJong-Hughes.

    “An advantage of tracks is their large footprint and low ground-contact pressure. This footprint is spread front to back instead of side to side (as are duals or triples), so less of the field is compacted,” she says. “The disadvantages are that the weight of the tractor is on a given volume of soil for a longer period of time, and the vibration of the track may increase surface compaction.

    “Some companies suggest that tracks reduce surface compaction over wheels. However, they may not reduce deep compaction. They may seem to have lower axle weights because the total weight of the tractor is spread over a larger area, but the guide wheels act as pressure points in the soil raising the track's average pressure on the soil to two to four times as high,” DeJong-Hughes says.

  • DUAL TIRES HAVE also been used as a method of spreading the load while maintaining constant axle loads. One study found that duals reduced the pressures by about 50% throughout the soil profile to a depth of 23 in. But duals' second tire increased the surface compaction.

    “Duals essentially traffic twice the width of the vehicle track and, depending upon the crop and cropping system, may cause excessive surface compaction,” says Raper.

  • CONSIDER ADDING deep-rooted crops like alfalfa to your rotation.

  • VARY THE DEPTH of tillage. Continual tillage of the same depth can cause a very thin layer of soil immediately under the tillage tool caused by tillage implements, not wheel traffic.

  • FREEZE-THAW CYCLES do not remedy soil compaction caused by machinery deeper than 5 in. in the soil, according to nine years of university research at Lamberton, MN. Studies there also show compaction from wheel tracks extended down to 18 in., despite freeze-thaw cycles.