Soil compaction has a big — and ongoing — impact on yields, and the problem is getting worse as equipment gets bigger, says an expert from the University of Minnesota’s extension branch.
Heavy packing reduces the size of pore spaces in the soil and can result in uneven seeding depth, surface crusting, increased seedling disease, smaller root volumes, and plants that appear to be suffering from nutrient deficiencies.
“Working when the soil is wet, especially if it’s at or near field capacity, is the main cause of the problem,” says Jodi DeJong-Hughes. “Getting out on the field as soon after a wet spell as you can travel leaves you with compaction problems.”
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A little compaction can actually be a good thing, but only a little, she says.
“Soil moisture clings to the soil particles,” says DeJong-Hughes. “Roots can access moisture faster if the soil particles are closer together. But when compaction is greater, it cuts yields in both wet and dry soils.”
DeJong-Hughes says she has seen whole field yield losses of six per cent a year from moderate compaction. She also mapped areas compacted and rutted in a wet fall, and found average corn yields in those areas the following year were 17 per cent lower — and up to 37 per cent lower in rutted areas. And things didn’t improved the following year, with soybean yields in the compacted areas 15 per cent lower than in comparable undamaged fields. It was only in the third year following the compaction that corn yields were no different from undamaged fields.
“The No. 1 defence against compaction is to reduce tillage,” she says. “Fluffing up the soil is not helping it. You’re putting air into the soil — what’s the carrying capacity of air? The more tillage you do, the more you need to do.”
Healthy soil has its own defence against compaction, says DeJong-Hughes, because it has enough organic matter to support a large and diverse microbe population that, in turns, builds strong soil structure. Tillage destroys that structure, she says.
“Healthy soil structure is based on soil particles that are less than the size of a pea,” she says.
“Those are the soil particles that resist compaction and erosion. Anything bigger than a pea is getting into clods. We need to manage for soil health with diverse rotations, including fibrous and taproot crops, pulses and biomass.”
If you must cultivate, DeJong-Hughes recommends using the least aggressive tools — narrow points on straight shanks — and working the soil when it’s dry. Before trying costly deep tillage, she suggests digging some holes, even a few post-holes, to see whether there is a “restrictive layer” that limits root growth. Be careful not to mistake a natural soil horizon that might affect the abundance of roots for a restrictive area, she says, because you’re looking for a soil layer where roots turn and grow horizontally, not just thin out.
“Don’t think there’s a crop-limiting zone because you feel a change in the soil bulk density,” says DeJong-Hughes.
If there is a compacted soil layer limiting root growth, then subsoiling is more likely to pay off. In South Dakota, deep ripping where there was no restrictive layer produced one extra bushel of corn over 64 site years and the same advantage to wheat. But where there was a restrictive layer, deep-ripping produced an extra 10 bushels of wheat and 14 to 18 bushels of corn an acre. Once you’ve deep-ripped, DeJong-Hughes advises going to controlled traffic farming, using roadways to limit soil compaction.
“You have to commit to matching your equipment to your lines, but it may be worthwhile,” she says.
DeJong-Hughes aims for a maximum of 10 tons per axle and 10 psi or less, but she says, there’s no chart that shows yield losses for compaction.
“Everything affects crops in compacted soil,” she says. “The amount and the type of clay particles in the soil, fertilizer placement, amount and timing of rains, everything. If it’s very dry the following year, or if you have some well-timed rains, you may see no effect. But other years, you can be badly affected.”
Some equipment just cannot meet the compaction specialist’s target, with grain carts a particular concern. She has a problem with one of the newest models that holds 2,000 bushels of grain.
“That’s two semi-loads, 75 tons on each axle,” she says. “Even putting it on tracks won’t spread the load enough. It compacts soil down to four feet. No amount of tillage can correct that.”
