Packing it in: Crops Yields Drop in Compacted Soils
Data for 2000 shows that corn yields on compacted plots were 11 bushels per acre lower than in control plots, and that soybean yields on compacted plots were 5.5 bushels per acre lower than in control plots. The yield differences were 7 percent and 12 percent lower than control plots for corn and beans, respectively. In the real world, farmers don't have control plots to which they can compare yields. Year-round equipment traffic may compact soils without the farmer realizing the damage in terms of yield loss, Reeder says. Instead, a farmer might think yields are normal, he says. Reeder's study on Hoytville silty clay loam soils at the northwestern branch of OSU's Ohio Agricultural Research and Development Center started in 1987, when he used 20-ton and 10-ton per axle loads to compact every inch of soil on research plots each fall for three consecutive years.
Those weights are similar to that of large farm equipment on Ohio farms. A big combine's main axle bears 15-18 tons, while extenders on its grain bin can increase weight capacity. A single-axle, 600-bushel grain cart provides 20 tons of pressure to the ground. A four-wheel drive tractor provides up to 10 tons of pressure per axle to soils. Reeder's 20-inch-deep measurements at Hoytville showed the compacted sites had higher levels of soil density compared to non-compacted sites.
"A farmer isn't going to knowingly and deliberately compact fields as we did, but after years of random traffic in a field, they will likely run over every square foot of soil with those heavy loads," Reeder says. Three years after the last intentional compaction, yields recovered to control levels on the 10-ton plots. But the 20-ton compacted plots showed a lot of yield variability over 11 crop years. For example, the 1999 yields were similar between compacted and control plots for both crops.
Why the erratic yields? Reeder says it could be the effects of weather in specific years, but he's not sure. "What this shows is you can't predict yields for the next year, but you can project over five years that yields are likely to be reduced in the compacted plots." Research on two other Ohio soils, Crosby and Kokomo, showed only minimal effects of deep compaction from heavy loads, Reeder says. Interestingly, northwestern Ohio farmers have found how to improve compacted soils. They use subsoiling implements to loosen the soil profile. Reeder says his research proves they're right.
In soybeans, compacted plots subsoiled every other year produced 9 bushels more per acre than non-subsoiled, compacted plots in 2000. Subsoiling also showed a benefit in non-compacted plots. In the latter case, non-compacted plots subsoiled every other year gave a 7 bushels per acre yield advantage over non-compacted, non-subsoiled plots. In corn, the subsoiled plots averaged about the same yield this year as those that were not deep tilled, Reeder says. In the nine years since subsoiling was added as a variable, the subsoiled plots have yielded about a 4 percent advantage. He says this increase would easily cover the cost of subsoiling every other year on the Hoytville soil. Reeder says the best way to limit compaction is to use controlled traffic. In that system, field traffic is confined to the same tire paths every year, which leave the rest of the field non-compacted.
The first step to a controlled traffic system is to decide on a single operating width for all equipment, Reeder says. For example, a farmer with a 12-row planter and corn head might settle on 30 feet. All future equipment purchases would be sized to fit that pattern, including a grain drill and grain platform for the combine. A sprayer would likely be 60 feet or 90 feet wide. The next priority is to reduce the total number of tracks, Reeder says. For a 30-foot width, getting down to four tracks would be ideal. That would leave about 75 percent of the field non-compacted. For a 20-foot width, two tracks are possible. But even if there are more tracks, controlled traffic is beneficial.
The next-best solution is to change to wider tires, more tires, or rubber tracks to distribute the equipment's weight over a larger area, thus reducing the pressure per square inch of ground. Of course, if a farmer could avoid driving in the field, compaction wouldn't be a problem, Reeder says. "But I doubt we'll see the day when an Ohio farmer will be pulling a planter or harvesting corn with a helicopter."