Many years ago, I was the consulting agronomist for a soil compaction study funded by a large Northeastern U.S. power company. The objective was to determine how much soil compaction was caused by the heavy equipment involved in electric power line construction, and compaction's impact on crop yields. Two crop fields were chosen that were close to the construction area, both on operating dairy farms. One was in the Finger Lakes region of Central New York, near Watkins Glen. (Close enough that while working at the site we could hear the cars running practice laps at the Watkins Glen International Raceway.) The other site was next to the Canadian border in northeastern New York. One site was clay loam, the other silt loam.
We first determined the extent of soil compaction at the construction sites by use of a soil penetrometer. Then the test site was compacted (in one case by a dump truck filled with stones) until soil compaction in the test fields was the same as the nearby construction sites. Then we attempted to "fix" the soil compaction by using a standard subsoiler and another deep tillage implement called a Paraplow. We subsoiled as deep as the huge tractor would permit, getting about 18 inches deep, while we ran the Paraplow at about 12-inch depth. The control treatment was a moldboard plow, which was the primary tillage implement used on both farms. Compacted areas at both sites weren't tilled or cropped at all. The following spring we remeasured soil compaction under all treatments, and then planted corn with several replications of each treatment.
Our first surprise: Even where there wasn't any tillage, the freezing and thawing action of the New York winter considerably reduced soil compaction. We were perhaps fortunate that at both sites the winter was open enough (not a lot of snow) that the soil froze early in the winter, probably to a greater depth than during a snowy winter, then thawed and refroze several times. That spring we disked all treatments, which was the typical secondary tillage on both farms, and then planted corn. Our second surprise: During almost the entire growing season the worst-looking corn at the central New York site was in the subsoiled plot. There wasn't any visible difference at the northeastern New York site. When we harvested the corn that fall, the central New York subsoil treatment produced the lowest yield. We never identified the reason for the lower yield, but concluded that at least in this trial subsoiling certainly didn't pay. A trial in Waseca, Minn., found similar results: Subsoiling failed to increase either corn or soybean yields, and decreased corn yield one out of two years.
Timing makes a difference
For subsoiling to be successful in reducing soil compaction below the plow layer, the subsoil must be dry enough to fracture. Pulling a subsoiler through moist or wet subsoil just makes a trench that quickly fills back in, with little if any positive effect - essentially a waste of time and diesel fuel. The reason I'm writing this article now is that late summer is often the ideal time to be subsoiling hay fields to be rotated to corn, because that's when the subsoil is most likely to be fairly dry. (Late summer, obviously, isn't the time to subsoil cornfields since the corn is still in the field.) A farmer in northeastern New York made the practice of giving up the last harvest in the field's final year of alfalfa production, subsoiling in August. The alfalfa, with its deep taproots, apparently helped pull moisture out of the subsoil, and when he ran the subsoiler it was obvious that he was working relatively dry soil to a depth of at least 15 inches. Did it work? I don't know for sure, but the farmer sure thought it did - and he was a top grower with consistently high field and forage crop yields.
Other deep-tillage options
A subsoiler isn't the only deep-tillage option. Any tool that operates a few inches below the normal plow layer has the ability to break up what farmers commonly refer to as a "plowpan." This is the compacted area just under normal moldboard plow depth, and it differs from the deep compaction caused by heavy field equipment including combines, forage trucks, etc. At Miner Institute, we have a chisel plow that can be used in this manner by removing half the chisel shanks and running it 10 to 12 inches deep rather than our normal 7 to 8 inches. Why remove half the shanks? Because it takes a whole lot more horsepower to chisel 10 to 12 inches deep. When we did this, it was obvious that we were breaking up at least some of the plowpan because we could see the compacted chunks of soil. Other deep-tillage options have found favor on some farms, but talk to your farm equipment dealer before making any purchase. Also, talk to farmers in your area who are using the same implement you're considering buying. Even better, if possible, rent one to use on a field or two to see if this technology has a place on your farm.
Ways to minimize soil compaction
You can't prevent all soil compaction, but you can limit it. If at all possible, stay out of wet fields, and if you have to run on wet land, don't use equipment that's heavier than is necessary. With in-furrow plows the right-hand tractor tire runs in the moldboard furrow, while with on-land plows all tires are on as-yet unplowed soil. The tractor tire running in the furrow can result in deeper compaction, that's why many farmers prefer on-land moldboard plows. Compared to single tires, dual tires can reduce deep compaction by about 50 percent. More surface area is exposed to compaction, but the compaction isn't as deep and is more likely to be corrected by normal tillage.
Tire pressure makes a big difference: The lower the tire pressure, the less soil compaction. An empty pickup truck exerts pressure on the soil surface of 25 pounds per square inch, while the front axle of a 200 hp four-wheel drive tractor produces less than 8 pounds per square inch. A 125-pound woman in stiletto heels produces about 8,000 pounds per square inch! (Logical conclusion: Keep women in high heels out of your crop fields.)
Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has a written our Forage column for over 13 years and has been an expert contributor on a number of other topics.