If you’re driving alongside your field before your crop comes up this spring, it will likely be very easy to see the paths your grain carts and combine drove last fall. Look a little closer and you might be able to see the lines your sprayer and even your seeder drove months before harvest.
The tracks — caused by and clear evidence of soil compaction — can in a single year cover 50 per cent or more of your field. Farmers should be concerned by the tracks criss-crossing their fields: compaction results in all kinds of negative, long-term impacts to crop growth. And, because as much as 80 per cent of compaction occurs in the first pass, more tracks mean more compaction. Though a growing crop soon masks the lines, know that compaction will affect your crop all season long.
“You might feel better once you can’t see the issues from the road. You might think the crop evens everything out. But if you send a drone up over the field, you can visually see the downsides of compaction right through until harvest and into subsequent years,” says Jodi DeJong-Hughes, a regional extension educator with the University of Minnesota. “Wheat, for example, suffers at every stage, from germination to tillering right to yield. It’s not a small problem.”
A healthy, well-structured soil’s total volume is made up of only about 50 per cent sand, clay, silt, organic matter, etc. The other 50 per cent is pore space, half of which ideally is filled with water, the other half with air. These pores allow oxygen exchange, promote the movement of water within the soil, create good growing conditions for microbes and roots and much more.
When soil is compacted, the air is squeezed out, causing a whole range of problems.
Without pockets of air throughout the soil structure, water has much more trouble filtering into the soil. That translates into cooler, wetter ground in spring, which brings poor seeding conditions; inconsistent emergence; a higher incidence of root, seedling and crop disease; variable crop growth throughout the season; higher fuel costs and greater equipment wear and more. Saturated soil also results in denitrification: in fact, microbes in over-wet ground can convert between two and four kilograms per acre of plant-available nitrogen to greenhouse gas each day.
Less air space might mean too much water at the beginning of the season, but it generally means too dry soil as soon as summer’s heat kicks in. Without small air pockets, soil lacks water-holding capacity. As compacted soil dries and hardens, root growth is limited, decreasing a plant’s ability to find nutrients and water. And, because roots need water to access nitrogen and potassium effectively, a dry compacted soil translates to thirsty, nutrient-starved plants, even when nutrients are close at hand.
Healthy, well-structured soil sticks together in clumps. Compacted soil breaks down more easily into fine particles, which can be washed away by wind or water.
Compaction primary causes
The first step to minimizing compaction is understanding its primary causes.
The single biggest culprit for compaction is running equipment in wet soil. “While sometimes you can’t help it and you just have to get into the field, other times you could probably wait a day or two,” says DeJong-Hughes. “The bad news is that last year’s wet harvest undoubtedly caused a lot of compaction on many farms.”
Tillage, which breaks apart soil clumps into individual particles that then collapse in on each other, is another major cause of compaction.
Not surprisingly, equipment weight is factor No. 3: the heavier the weight of field machinery, the deeper, wider and higher the intensity of compaction. Just take a look at the numbers: at 4.4 tons per axle, soil compacts to a depth of 12 inches. At 6.5 tons per axle, compaction deepens to 16 inches. At 16.5 tons per axle, soil compacts two full feet.
“Ideally, you want to stay below 10 tons per axle,” says DeJong-Hughes. “We’ve been going bigger and bigger but that’s not always better. With a combine, we don’t always have a choice. But maybe with a tractor, we do.”
For example, she says, consider two similar tractors with different horsepower. A four-wheel drive, 200 horsepower tractor has an axle load of 7.5 tons. Up the horsepower on that tractor to 590 and axle load jumps to 18 tons.
Though axle weight directly correlates to compaction, overinflated tires can worsen the impact by reducing the area that weight is spread across.
No. 1 defence against soil compaction
A farmer’s best remedy for compacted soils comes directly from nature: practice planting diversity. Crops with strong taproots can help break through hardpan, while crops with fibrous roots can build structure. Cover crops and perennial crops are effective soil structure builders, so consider them even when a wheat-canola rotation seems to make more immediate economic sense.
“Soil structure is the No. 1 defence against soil compaction. You want to do everything you can to build and maintain soil structure. If you can add a pasture, cover crops or alfalfa into your rotation, that can go a long way to supporting healthy soils,” says DeJong-Hughes.
In severely compacted soils, some farmers opt to till. For best results, DeJong-Hughes recommends first digging a pit one metre deep to assess compaction depth. Then, set shanks two to five centimetres deeper than the compaction layer. Working only when the soil is dry and using the most non-invasive, deep, straight shank, till only to the depth needed. Recognize that ripped soil is extremely vulnerable to further compaction and consider driving in controlled traffic lanes to protect tilled soils.