The Truth About Compaction

Soil compaction isn’t always a visible problem. Still, you’ll want to prevent and correct compaction, even if you can’t see it.

Soil has large and small air pockets that help it function properly. When soil particles are squeezed as heavy equipment travels across, the soil becomes compacted.

“Those big air pockets are really important for water infiltration,” says Marla Riekman, soil management specialist with Manitoba Agriculture.

This compression slashes soil’s water-holding capacity. It also restricts root growth, which impacts nutrient uptake.

Following are Riekman’s management recommendations  not only to overcome compaction but also to return soil to a healthy state.

How to spot compaction

One way to measure the extent and depth of compaction is with a diagnostic tool called a penetrometer. The device looks simple, but it’s a little tricky, she says.

Instead, Riekman’s tool of choice is a shovel – the cheaper and easier option.

“Everything can be figured out with the shovel, in my world,” she says. “If you go out with a shovel and you start seeing things like restricted root growth or roots growing on a plain, you have a problem.”

On sandy soil, wheel tracks will be better performing than the rest of the soil because of forced water-holding capacity.

“Compaction will force water to pond a little bit more,” she explains. “When you do want water to pond during those dry seasons, it can be beneficial.”

What’s the cost?

In a wet season, the crop response will be negative. In a dry season, you might see a yield boost from a bit of compaction, says Riekman.
 
“Things like compaction are harder to predict in terms of overall yield impacts,” says Riekman. “The University of Minnesota did a study where they looked at rutted and unrutted areas where corn was planted. They found there was a height difference but no significant plant population difference.”

Growth stage was significantly behind in the rutted, compacted areas.  

“There was about a 17% yield reduction,” says Riekman. “If you take those ruts and the rows that are rutted and compacted, and you multiply that across the entire field – every trip, for every area where you have wheel tracks – that could be quite significant across total yield.”

The Causes

There are two causes of compaction. The first type you have no control over – that’s natural compaction.

“These soils have poor structure,” says Riekman.

Normally, these types of soils would be put back into pasture because they don’t function well.

Human-induced compaction is the second type. It is caused by excessive or untimely tillage, wheel traffic, and axle load.

“I know people who’ve been no-till for 30 years, and they still have an existing hardpan,” she says. “This is not something that disappears quickly.”

Over time, it heals itself.  

“The problem is, we continue to till and fluff up the stuff at the top,” says Riekman. “Our tillage implements push down and compact the lower layer, while the topsoil fluffs back down on it. It becomes compacted on itself.”

If plants can find a crack in the soil or an old root channel, they will start to grow into it, and over time, they will break up soil compaction.

“You can’t plant something that will immediately break up compaction – it doesn’t work that way,” she says. “Plants need time to work their way through easy-access channels. From there, you start to see weathering and breaking up of that solid block of concrete.”

The Solution

The top 6 to 8 inches are shallow compaction, says Riekman. That’s caused by pressure on the soil from wheel traffic.

“Make sure you run tire pressure for field conditions and not road conditions,” she says. “Tire pressure for roads will be a lot higher than the pressure you want in your field.”

A misconception she’s heard is that tracks are better than tires for compaction.

“There’s no real difference if your duals are properly inflated,” she says.

Deep compaction, below 8 inches and as much as 3 to 4 feet deep, is related to your axle load.

Riekman says 80% of compaction happens in the first pass. “You need to think about when you are moving across the field and under what conditions.”

When the soil is dry, it’s not going to be affected.

Ditto for wet soils. “When your soil is fully saturated and you’re running on it, you’re not really compacting the soil heavily,” explains Riekman. “You’re messing with structure because you’re sliming the soil all over and creating ruts. Deeper down, if there’s a lot of water, the water is protecting and holding those pores in place.”

The issue is when soils are moist or near field capacity. “This is when your aggregates are lubricated,” she says. “The biggest pores are filled with air because they’ve been allowed to drain.”

Those pores will collapse when driven over. Often, fieldwork occurs in this moist situation. Mother Nature will fix it with the freeze-thaw cycle, right? Wrong.

“The freeze-thaw cycle, the idea that it breaks up and fixes compaction, only really works at the surface.”

Prevention is the first step. Running equipment full might be a problem if you are in moist conditions.

“Stay off the field when conditions are poor,” she says. “Check your tire pressure and reduce your axle load when you can by emptying the combine earlier and running the grain cart half full.

Riekman also recommends that you minimize trips across the field, remain in the same tracks when possible, and reduce tillage.

These aren’t easy options, but in a high-risk situation, you need to consider them, she says.

Another option is to rotate deep-rooted, water-loving crops. That includes cover crops. Moisture is part of this problem; suck up as much as you can, she says.

“Cover crops can alleviate compaction and reduce susceptibility to compaction because having the roots there helps stabilize soil. Research shows that a cover crop will allow you to be a bit above field capacity before maximum compaction,” she says.

Another benefit? The cover crop soaks more moisture out of the ground in the fall when you’re running heavy equipment.