Compaction is an issue that just won't go away. It affects the majority of farmers and can have severe impact on yields. Compaction can be caused in a myriad of ways, including rain, tillage, wheel traffic, and minimal crop rotation.
“I once heard if a field has been tilled, rained on, and had fertilizer or chemicals put on it, it has compaction,” says Bill Darrington, who farms in Persia, Iowa. “It's compacted because we have disrupted the natural environment. Nearly all soils have one natural compaction zone. But it is the other two or three that can be most yield limiting that we, as farmers, are responsible for.”
One way he sees compaction's impact is by scouting fields from the overview provided from the road. If you observe areas of extended ponding–or in dry times crops show leaf rolling and drought stress quickly–or growth is stunted, compaction is an issue.
Seeing compaction from the road is one thing, but it may not be the best way to go about it. A popular option is using a penetrometer. Darrington calls the tool a compaction confuser.
“You can go to most any field in the summer that has had plenty of moisture and it takes very little pressure to poke a pointed steel rod through moist soil,” he explains. “When the soil dries out, you need that deep, massive root system to provide water and nutrients. If you probe the soil, it quickly becomes a different story. It won't go down as far or as easily, and sometimes it's not possible to get the probe to penetrate more than a few inches.”
Instead, Darrington likes to dig pits in his fields using a wheel loader or a skid steer. Pits are dug about 3 to 5 feet deep at least once a summer. He usually tests two fields: one corn and one soybean. He looks for an area where things are looking good and an area where there appears to be some stress.
Once the pit is dug, he slides a 4-inch pocketknife into the sidewall of his root pit, from the bottom of the pit to where the bottom of the compaction zone stops the knife from moving easily up through the soil. To see how thick the compaction zone is, he pokes the knife into the soil wall to where it penetrates easily, and then he moves it down to where it once again stops the knife. This shows how thick the compaction zone is.
To find the uppermost compaction layer, he places the knife on the soil surface of the root pit and pushes down to find the top of another hard spot. “I have broken pocketknife handles doing this,” says Darrington. “The compacted areas will be easy to find.”
Also observe the growth patterns of the roots; they should not be growing in a zigzag manner. This is an indication they are trying to find the path of least resistance and should be growing down fairly straight. If you do not find any compaction, you may want to leave the root pit open a couple days to dry out.
Darrington says natural compaction layers are common around 17 to 20 inches deep. Generally, layers are 2 to 4 inches thick in areas he has been.