Subsoils are naturally compact, and what looks like a soil compaction problem might actually be a case of erosion removing the top, soft layer of soil and exposing the dense subsoil.
“People assume they have a soil compaction problem, but exposed subsoil is often misdiagnosed as a soil degradation issue,” says David Lobb, a professor in the Department of Soil Science at the University of Manitoba.
Subsoils are naturally compacted at depth, with a higher bulk density than top soils.
“Sometimes in areas where glaciers have been moving over the surface you get a lodgement till, resulting from massive pressure compacting the soil. Some soils will have bulk density of 1.7 to 1.8 grams per centimetre cubed. That’s natural,” he says.
“But once you have thousands of years of bugs and worms and roots mixing the soil, you get soil that’s loose at the surface. That’s what we think of as normal and ideal, that’s what plants think too.”
A relatively dense subsoil is natural and fairly consistent across the Prairies, according to Lobb, but problems with bulk density can occur more easily in soils on hilltops, where the top soils are thinner and subject to erosion.
Lobb says true soil compaction caused by traffic is usually a result of too much activity on the soil when it is too wet to resist the pressure. “For the most part, when soil is relatively dry, it has good strength and resists compaction by driving,” says Lobb. “That’s when you want to do field work, when the soil is dry enough to resist compaction.”
But often, field work is done at what Lobb calls the “shoulders” of the growing season, when there is a risk of rainfall events. “That means that soil has less ability to resist compaction, so bulk density goes up and you get problems.”
Lobb says there are telltale signs of true soil compaction. First, there will be poor plant stands and spotty growth, he says, which is conspicuous in two places: at headlands and field entrances, which see the most traffic during planting and harvesting.
“That’s where you look to see if there’s a crop impact,” he says. “That’s where people should be looking. I hear people say they’ve got soil compaction in the field, but if you do not see these issues near headlands or entrances, you don’t have a problem with compaction caused by traffic.”
If soils are heavily compacted, roots will begin to grow laterally rather than into the subsoil.
One way to deal with traffic-caused compaction is to change the axel design to isolate compaction to distinct rows, says Lobb, rather than dual or triple wheel rows. “You’re sometimes better off keeping the zone of compaction low,” he says.
“This becomes really important for liquid manure applicators which carry tremendous weight. Minimize it by localizing it.”
Other obvious strategies include minimizing traffic at headlands and field entrances, and confining traffic to dry periods as much as possible.
One controversial method of correcting subsoil compaction is “ripping” the subsoil with deep rippers that penetrate 10 to 16 inches.
“The only time that ever works really well is to do it when the soil is really dry,” says Lobb. “You’re giving up a cropping year. But it also doesn’t last very long — that de-compaction is lost within months to a year or two. If you’ve got that problem and that’s the choice you’ve made to deal with it, you’ll be burning lots of fuel to deal with that issue.”
Long-term solutions to soil compaction include the use of deep-rooting crops like alfalfa, which can break up the subsoil over time and create better infiltration. Practices that build up organic matter improve the soil’s structural strength and resistance to compaction.
Some compaction can naturally correct itself over time. “We have a lot of freeze-thaw in the Prairies,” says Lobb. “Compaction is a problem where you don’t have natural alleviation. Freeze-thaw can go down fairly deep and cause a lot of heaving and cracking and de-compaction.”
With regard to subsoils exposed through erosion, Lobb says there is no way to get rid of natural compaction. There are only two ways to deal with this issue — by reducing erosion, and moving soil back to the exposed areas.
“Properly diagnosing the problem is important,” he says. “If you’ve got compaction due to erosion versus compaction due to traffic, the solutions are different.”