BU-07400     2000

To Order

SOIL MANAGER

Six steps to minimizing sompaction

1) Stay off wet soil.
2) Keep axle loads low (below 5 tons).
3) Reduce ground contact pressure.
4) Use a controlled traffic system.
5) Reduce trips.
6) Increase soil organic matter levels.

These are significant factors to consider as you try to reduce compaction in your fields. Different kinds of compaction problems have somewhat different solutions. (See page 10 for descriptions of types of compaction.)

Machines or cattle traveling on very wet soil will cause significant compaction, even if you have tracks, duals, or a light load (though light loads only create surface compaction). Letting soil dry for even one or two more days can greatly reduce soil damage. When you assess the moisture of your soil, check the soil at tillage depth as well as at the surface.

Staying off wet fields is rarely a simple decision. It requires weighing yield losses this year (caused by late planting or harvest) against losses for the next few years (caused by compaction). One way to reduce the frequency of this dilemma is to reduce the amount of field work that must be done during any window of time. Then you will have a better chance of being able to wait a couple more days for the soil to dry. Look at how your labor requirements are spread over the year on your farm. Compare this to the number of days that are typically available for doing field work. You might change your labor cycles by:

• diversifying crops so field operations are spread over time.
• using wider equipment so work can be finished more quickly. (With wide equipment, a smaller area of the field is driven over and compacted. Balance this with the disadvantage of using heavier equipment.)
• hiring extra help and equipment during key times.
• considering alternative tillage, planting, and fertilizer programs.

Subsoil compaction is rare with axle loads under 5 tons and highly likely with loads greater than 10 tons per axle. (Some surface compaction is likely with any load.) Although increasing the footprint of the tractor (such as by adding duals) reduces surface compaction, it does not change the axle load and will have little effect on reducing subsoil compaction.

Implement	100 HP tractor 14,000 lbs	4WD tractor 30,000 lbs	Empty 6-row combine 25,000 lbs	Full 6000 gal manure applicator 65,000 lbs	Full 750-bu single axle grain cart 50,000 lbs
Axle weight	3.5 tons/axle (w/o load)	7.5 tons/axle (w/o load)	10 tons on front axle	10-15 tons/axle (2 or 3 axles)	22 tons/axle

Ground contact pressure is the pounds per square inch exerted on the soil by equipment. It is determined by the axle weight and the size of the "footprint" (the area of the tires or tracks in contact with the soil). Increasing the size of the footprint can reduce surface compaction.

One way to control contact pressure is to reduce tire pressure. As a rough rule, tires put one to two pounds more pressure on the soil than their air pressure. For example, a tire inflated to 25 psi will put 26 to 27 psi of pressure on the surface soil. A tire inflated to 15 psi will only put 16 or 17 psi of pressure on the soil. Low pressures increase the risk of rim slippage and tube leaks, so talk to your implement dealer or other advisor about the appropriate pressure for your equipment and field situations. Radial tires tolerate much lower inflation pressures than bias-ply tires and may be worth the investment to prevent compaction.

4) Use a controlled traffic system, and 5) reduce trips

Using tires in tandem (one directly behind the other) or consistently driving in the same lanes has two benefits. 1) Most compaction is caused by the first pass of a tire. Subsequent passes do not increase compaction as much as the first, and traction is better. 2) A much smaller portion of the field is compacted so roots have plenty of non-compacted soil to explore.

Designing a system of traffic lanes used during all field operations requires that all equipment have the same distance between wheels. Controlling traffic and minimizing trips across the field is especially important in no-till situations where surface compaction is not regularly loosened.

6) Increase organic matter levels

At the same water content and machinery weight, a soil with more organic matter will suffer less compaction. Organic matter (and the associated biological activity) protects soil from compaction by "cushioning" aggregates and increasing the stability of aggregates.

Questions and Answers

How can I minimize compaction in the short term? (or: It’s been raining for two weeks and the corn is ripe. Now what?)

Equipment adjustments can be more important than what kind of tracks or wheels you drive on. Talk to an implement dealer, Extension educator, or other advisor about the optimum wheel size, inflation pressure, load size, ballast size, and ballast position for your equipment and field conditions.

If you must go out on wet soil, the effects of surface compaction can be reduced with duals, flotation tires, and tracks. They provide traction and reduce ground contact pressure.

Work the driest soils first to give the wet areas more time to dry. This may mean dividing large fields into smaller parts, and following contours instead of straight lines.

What difference do soil amendments make?

It is unlikely that any soil amendment will directly loosen compaction. In fact, fertilizer and irrigation can increase compaction problems by reducing a plant’s incentive to grow deep roots. Root growth may be an important tool for controlling compaction. Anything that assists plant growth - nutrients, lime, water - will help crops overcome compaction in the short run, but if it does not enhance the extent of root growth, it may make the problem worse in the long run.

Organic amendments help prevent and loosen surface compaction by promoting biological activity and by raising the soil organic matter levels. Soils with higher levels of organic matter resist compaction more effectively.

Should I buy tracks or wheels?

The decision whether to use a tractor with tracks or wheels is not simple. Either can cause significant compaction if misused or improperly adjusted. Any system will cause more compaction and be less time and energy efficient if it is not properly adjusted and ballasted for the load and soil conditions.

Tracks and flotation tires may make it possible to drive on wetter soils, allowing a farmer to create more severe compaction.

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. 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 increases compaction.

Studies suggest that tracks reduce surface compaction compared to using wheels. However, it is not clear whether they reduce deep compaction. They may seem to have lower axle weights because the total weight of the tractor is spread over more than two axles, but the axles are close together and may not act like separate axles when applying pressure to the soil.

Does compaction increase under reduced tillage systems?

The reduction or elimination of tillage does not necessarily mean that surface compaction will become a serious problem, but compaction is managed differently in low-till systems and deserves monitoring.

The increasing levels of near-surface soil organic matter help soil resist compaction. Increased surface residue encourages the activity of organisms that burrow through the soil and enhance soil aggregation, but it may take a few years for biological activity to increase substantially. Because there is no regular tillage to loosen soil, traffic lanes become more important for preventing surface compaction.

How To Treat Compacted Soils

Wait

If you stop compacting the soil, yields will probably return gradually to pre-compaction levels. The process of rehabilitating compact soils is not well understood. Researchers have seen yield recovery even when physical measurements (such as bulk density) have not returned to pre-compaction levels. Medium to heavy soils are more susceptible to severe compaction and take longer to recover.

You cannot rely on frost action to relieve compaction each year. It may have some effect on surface compaction, but does little to ease deep compaction. Subsoil usually goes through only one freeze/thaw cycle each winter.

USDA researchers compacted soil at the Waseca Experiment Station in 1981 by using ten-and twenty-ton axle load equipment. For the first 2 to 3 years, corn yields on the compacted plots were less than those on the non-compacted plots. During the next decade, the one-time compaction event caused reduced yields during years of very high or low rainfall, but not under optimal moisture conditions.

Loosen the soil

Normal tillage relieves surface compaction.

Roots are invaluable for loosening soil. Deep tap-rooted plants will grow through hardpans (such as tillage pans) and deep compaction. When they die, the root channels become conduits for water and other roots. There has been minimal research linking yield improvements with crops used to loosen compaction, so there is little guidance about which plants to use and how to grow and manage them. The strong tap roots of alfalfa and sweet clover are thought to be useful in treating subsoil compaction. These plants may need three or more years to be effective. Other useful antidotes to deep compaction may include red clover, lupines, soybeans, reed canarygrass, burdock and dandelion. Any grass (especially quackgrass) loosens surface compaction.

Earthworms and other soil invertebrates loosen soil and create channels for root growth and water movement. Attract earthworms by regularly adding organic matter.

Subsoiling (or deep tillage) is sometimes helpful, but has not been found to be a reliable cure for deep compaction in the upper-Midwest. If you think it might help on your farm, consider these guidelines.

• Be sure compaction is the problem. Explore your fields with a shovel or probe to learn where it is compacted and how deeply.
• It may be most cost effective to deep-till only the worst areas, such as headlands.
• Be sure soil is dry so it fractures. Cutting through moist soil will have little effect.
• Plan to avoid future compaction. One pass with a heavy implement will wipe out your investment in deep tillage.

Compensate for decreased nutrients and water

If you know what is causing a yield loss, it may be possible to partially compensate by increasing fertilizer or irrigation inputs. For example, it may be necessary to use split applications of nitrogen to minimize denitrification losses, or to use row applications of phosphorus and potassium to improve availability. Keep in mind that these short-term fixes may aggrevate compaction problems. They increase traffic and may reduce the extensiveness of root growth.

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