While many strip-tillers actively think about the health of their soil, they likely aren’t thinking about its strength.
Jodi DeJong-Hughes, a regional educator for the University of Minnesota Extension based in west central Minnesota, defines soil strength as a measure of the soil’s capacity to withstand stresses without giving way to those stresses by collapsing or becoming deformed.
A well-aggregated soil is more resistant to compaction because it is stronger. Aggregates act like mini columns to hold up the weight of equipment traveling across the surface. DeJong-Hughes says an ideal soil is 50% sand, silt, clay and organic matter, and 50% pore space that’s equal parts water and air.
“Pore space is like highways,” DeJong-Hughes says. “The Interstate system makes up less than 2% of all the roads in the U.S., but it conducts about 25% of all traffic. That’s the same with large pore spaces and small pore spaces. There aren’t a lot of the large ones, but they conduct the most water.”
The simple definition of compaction is a reduction in pore space in the soil. Wet soils are vulnerable to compaction, and when soil collapses, remediation can be a lengthy, costly and complicated business. But DeJong-Hughes says strip-tillers have choices in when and how they choose to address compaction — without making it worse.
Minimize Axle Loads, PSI
Axle loads determine the depth of compaction in subsoil. Compacted subsoil can affect plant roots’ ability to reach water and nutrients, potentially causing yield losses for many years.
Axle loads have “gone way up” over the decades, says DeJong-Hughes. Today’s farm equipment can range from 10-12 tons per axle for a 4,200-gallon manure tanker to 35-40 tons per axle for a loaded 1,200-bushel grain cart.
“You want to stay between 5-10 tons per axle load,” DeJong-Hughes says.
For equipment heavier than that, controlling traffic is critical. A 2,000-bushel grain cart, for example, can easily reach 70-76 tons per axle.
“I don’t care if you have tracks or tires on — that is pushing compaction 3-4 feet into the ground,” she says. “If you have a 2,000-bushel grain cart, park it on the side of the field during harvest. Don’t haul that down your field. That is the heaviest thing out there.”
“Make sure you do zone tilling in a very dry fall…”
Heavy loads push compaction into the soil. DeJong-Hughes estimates that every 10 tons per axle pushes compaction about 1 foot into the soil.
“But if you have really poor soil, even 10 tons an axle is too much, and 5 tons an axle is recommended” she says. “If you have started to build up your soil health, you can go up to 10 tons per axle.”
Tire PSI, or pressure per square inch, controls the intensity of that compaction, and it is a critical factor that strip-tillers can control.
“Fuel use goes up when you have over-inflated tires, and your tractor does not perform as well,” DeJong-Hughes says.
PSI on the road can range from 50-100, depending on the load, but DeJong-Hughes recommends keeping PSI on the field to around 10.
“Tires that are overinflated have the worst compaction,” she says. “They exert the highest pressure on the soil, losing your porosity.”
Going up one tire size can cut the PSI in half. Adding an axle or changing the tire configuration can also help redistribute the load and reduce PSI. But controlling traffic remains critical no matter. Up to 80% of compaction happens on the first pass, DeJong-Hughes warns.
Impact of Wheel Traffic on Soybean Yields
Wheel Traffic Yield: 45 bushels per acre
No Wheel Traffic Yield: 59 bushels per acre
Total: -14 bushels per acre due to compaction
She offers an example of a farmer who was taking out a windbreak and opted to drive his tractor in only one area of the field instead of all around the windbreak. The soybeans planted in the area where he drove the tractor experienced a 24% yield reduction and 14-bushel difference compared to the rest of the field. While 24% in the controlled traffic area was a lot, DeJong-Hughes says it was better than spreading the compaction and losing yield across a larger area.
Repairing deeply compacted soil is not easy, DeJong-Hughes says, but it is possible. One natural remediation strategy is using cover crops, which stabilize aggregates, create pore space for root growth and infiltration, and break through plow pans.
It can take several years before cover crops starts to make an impact, especially where compaction has affected the subsoil.
“It takes a while for the cover crops to work, but they work really well,” DeJong-Hughes says. “Not only are they taking care of compaction, but they’re also building aggregation, so the soils aren’t as susceptible to compaction later on.”
PROCEED WITH CAUTION. While zone tillers can break through subsoil compaction while maintaining surface residue, Jodi DeJong-Hughes says zone tilling has several downsides and risks, including leaving deep knife slots in soil too wet for ripping (L) and smeared soil (R).
One study conducted by the Agricultural Research Service in South Dakota and Minnesota found that soil strength significantly improved in cover cropped systems vs. no-till or conventional tillage systems without a cover crop. The best results in the study came from fields planted to a hairy vetch cover crop.
Farmers planning to start strip-tilling or no-tilling can also consider mechanical remediation before they make the switch to minimal tillage, says DeJong-Hughes.
One option is a zone tiller, an aggressive subsoil tillage operation that can tackle compacted layers as deep as 20 inches. With a straight shank, a zone tiller can break through hardpans while maintaining surface residue and temporarily increasing infiltration.
DeJong-Hughes says use zone tilling should be used with great caution as it has several downsides and risks. There’s a huge horsepower requirement at 30-50 hp per shank, and soil can recompact after just a few years with ongoing traffic. Yield bumps are minimal, and there can be smearing in wet clay soils.
“It is difficult to know if it’s too wet to do tillage 4 inches in the soil — how are you going to find out if it’s too wet 20 inches down?” DeJong-Hughes asks. “Make sure you zone till in a very, very dry fall.”
“You want to stay between 5-10 tons per axle load…”
At that increased depth, drag increases exponentially, and fuel costs rack up quickly. Farmers also have to be careful to avoid power lines buried below 18 inches and hooking up rocks.
“If you’re in a rocky area, don’t do this,” DeJong-Hughes says. “And remember that the more you till, the more you break apart your hard-earned aggregates into individual soil particles, which can be moved very easily with water and wind.”
If producers do opt to use a subsoiler, there are strategies to minimize risk. DeJong-Hughes says first find the compacted layer and set shanks to 1-2 inches below it. Use the least invasive shank to prevent dragging subsoil and rocks to the soil surface, and don’t drive on it again.
“You took out the structure in the tilled area,” she says. “If you drive on it, you’ll recompact it. Restrict subsoiling to the area where you’re going to plant your crop the next year. That way you’re not driving on it.”
Why Compaction Happens in Your Soils
In an online-exclusive article, Jodi DeJong-Hughes goes into more detail about how increasing equipment weight and reliance on “myths” to alleviate compaction cause soils to harden and negatively impact yields. Click here to read more.
DeJong-Hughes recommends farmers — regardless of tillage type or compaction remediation strategy — reduce the number of passes they make on the field.
“If you can’t quite jump into strip-till yet, change out the points and shanks on your chisel plow to be less aggressive,” she says. “Get used to having more residue out there, and don’t use a disk. Shallow up the implements that you have. They don’t have to run at the max depth. Rotate your tillage to match your field conditions. And if you don’t have a lot of residue out there, you can do less tillage.”