Thomas is retired from the William H. Miner Agricultural Research Institute and is the president of Oak Point Agronomics, Hammond, N.Y. Mahanna is with DuPont Pioneer and an adjunct professor at Iowa State University.

As farms have grown in size, so has the equipment used in crop production. Field equipment is heavier, with the potential to cause more soil compaction at depths that make it difficult (in some cases impossible) to correct.

Farmers in the northern U.S. may assume that the winter freeze-thaw cycle will alleviate soil compaction, but research has shown that it's not very effective. Some people may think that soil compaction only occurs during wet field conditions when a crop must be "mudded out," but soil compaction also can happen in "normal" (and even dry) soils.

The difference is that deep soil compaction - that affecting the subsoil - is much more likely when using heavy equipment on wet soils. The key is to prevent soil compaction in the first place, but when it does occur, avoid corrective attempts that could make the situation even worse.

Reduced tillage and no-till have become so common in some areas of the U.S. that clean tillage (such as moldboard plowing) is the exception and not the rule. However, tillage opens the soil to allow water and air movement and can improve root growth. On dairy farms, tillage is sometimes needed to bury heavy amounts of manure, especially if it contains a large amount of bedding such as the manure from compost barns.

Incorporating manure and crop debris will improve soil fertility and add organic matter. It should be remembered, though, that tillage never builds soil structure - it breaks it down.

Crop losses of 15 percent
The impact of soil compaction on crop yields depends on the level of compaction and where it is in the soil profile. Heavier axle loads result in subsoil compaction which is much harder to correct. On dairy farms, liquid manure spreaders often are the biggest cause of soil compaction due to their heavy axle load. A large manure spreader can weigh 30 tons (spreader plus manure); research has found that an axle load of "only" 10 to 12 tons can result in soil compaction at least 20 inches deep with a first-year yield loss of about 15 percent. If the manure spreader has two axles, a 30-ton load results in 15 tons per axle.

Though it may be minimized with time, the impact of subsurface compaction on yield can continue for years, even if no additional compaction occurs. Soil compaction at least 20 inches deep isn't influenced by the freeze-thaw cycle, and subsoilers won't do much to break it up, so it can have a long-lasting hit on yield. Prevention is therefore the key. The only effective strategies are to avoiding working on wet fields and to reduce the axle weight, either by reducing the load or by using equipment with more axles. If possible, limit axle weights to less than 10 tons and preferably less than 6 tons per axle.

Alter tire pressure
There are several ways to avoid topsoil compaction. One of the easiest is reducing tire pressure to the minimum allowable level by the tire manufacturer. One trial found that tires inflated to 35 psi caused more compaction at 14-inch soil depth and resulted in ruts more than twice as deep compared with the same size tires inflated to 12 psi. However, there was no difference in subsoil compaction between the two inflation pressures because the load per axle was the same. Flotation tires, radial (versus bias ply) tires, duals, tracks and 4WD (versus 2WD) equipment all can help in preventing or reducing surface compaction. So can reducing the number of trips across the field.

The extent and location of soil compaction can be measured with a soil penetrometer, which is a metal rod that's forced through the soil profile. A meter records the resistance required to force the rod through the soil. If resistance is very high - over 300 psi - that's an indication of a compacted layer. If resistance falls after the rod penetrates a compacted area, that's an indication that specialized tillage may be able to reduce the compaction.

The most common implement used to shatter a compacted layer is the subsoiler. Heavy-duty shanks are pulled through the compacted area, shattering the soil. However, if the subsoil is wet, the subsoiler shank can smear the soil next to it, doing more harm than good.

Tillage research on silty clay loam soil in central N.Y. found that when corn was planted on compacted soil that had been subsoiled, the yield was lower than when corn was planted on similarly compacted soil that had been moldboard plowed with no subsoiling.
It takes a lot of horsepower to operate a subsoiler - at least 40 HP per shank - which is why other implements have become somewhat popular. Paraplow shanks are angled sideways and take less power than straight-leg subsoilers.

The shanks also result in less surface disturbance, allowing surface debris to remain in place. Heavy-duty chisel plows are also used, sometimes with every second shank removed, but these are more effective at breaking up compaction that lies just under the plow layer, not that which is a foot or more deep.

This article appears on page 648 of the October 10, 2013 issue of Hoard's Dairyman.