Brian Dougherty says growers combatting compaction in their fields should realize they have bigger problems than merely compressed soil, and compaction is a symptom of poor soil function.

“Compaction is not natural or inevitable in modern cropping systems and, to be blunt, it usually can be traced back to three factors: too much equipment, not enough biology and excessive nutrient application,” he says. “To deal with compaction in the long run, we need to address what is not allowing the soil to function properly.”

Dougherty is a field consultant for Understanding Ag, LLC, a firm dedicated to helping growers implement soil health improvement practices. He preaches the importance of well-aggregated soils which contain roughly equal volumes of minerals, water and empty pore space. Since minerals (soil particles) and water are non-compressible, when heavy equipment travels over a field, the only thing to yield in the soil is the pore space — which produces compaction.

“For proper plant growth, roots need a gaseous exchange and pore space is where that occurs,” Dougherty says. “If the soil is compressed and pore space is compromised, there’s little room for water and oxygen/carbon dioxide exchange. When those components are shorted, soil microbiology suffers and in the long run so does crop performance.”

The interconnected pore spaces so important for water infiltration and nutrient exchange with crop roots are protected by soil aggregates held together by a natural glue — glomalin — produced symbiotically by mycorrhizal fungi from carbon-rich exudates shed by adjacent living roots.

THE LONG GAME. It takes time — likely up to 10 years in some cases — to fully alleviate compaction in some fields, Dougherty says. The above graph illustrates how the effects of compaction in the topsoil and the upper part of the subsoil are temporary, whereas deep subsoil compaction is virtually permanent. Sjoerd Duiker, Penn State University

Click to enlarge

“Those aggregates are in a constant state of construction and destruction,” Dougherty says. “They break down about every 4 weeks and are continually being rebuilt by exudates of living roots. If you’ve gone more than 4 weeks without living roots you’re losing aggregation.”

He likens it to Jenga blocks stacked together and held firm by Elmer’s glue (glomalin) and notes how easy a Jenga tower without glue is toppled. 

“No living roots, no glue, no aggregates,” he says. “Living roots are absolutely fundamental if you want to be able to withstand compaction in the long run and to ultimately eliminate it.”

Equipment

Dougherty says he’s learned deep tillage is only a short-term fix for compaction and merely leads to more compaction in the long run.

“When comparing tillage to the use of cover crops to alleviate compaction, growers need to play the long game,” he says. 

“On the plus side, deep tillage changes soil structure to the depth of the tool immediately, and can provide a short-term improvement in water infiltration,” he says. “But such tillage leaves the soil more susceptible to even more compaction, and it does nothing to improve soil quality. It’s a short-term fix at best.”

University studies have shown deep tillage shows a yield benefit only 25% of the time with the remainder exhibiting no effect or reduced yields.

Over time, however, cover crops, improve soil structure down to 36 inches, increase water infiltration, protect the soil from surface compaction and erosion, add organic matter and nutrients, and promote root zone biological activity, Dougherty says.

Volumes have been written about the compaction-fighting benefits of low tire pressures on farm equipment and the use of very high flexion tires and Dougherty recommends keeping radial tire pressures near 10 psi to limit compaction within the plow zone.

He’s also a proponent of keeping equipment axle loads under 10 tons in well aggregated soils and under 5 tons in poorly aggregated soils to maintain the compaction within the top 6-10 inches of wheel tracks. While there are few pieces of equipment in modern agriculture with a less-than-5-ton axle weight, Dougherty suggests trying not to run full loads in wagons or grain carts on those fields — particularly in wet conditions.

“About 75% of the increase in soil bulk density and 90% of wheel sinkage is caused during the first field pass,” he says. “The longer the dwell time of a load on soil, the greater the increase in bulk density.

“Always try to limit the percentage of a field exposed to traffic,” Dougherty adds. “Concentrate that traffic in lanes and stage semis and grain carts near field exits.”

Studies show controlled traffic farming (CTF) increases overall yields, reduces fuel consumption by 25%, requires less overall horsepower, provides for more efficient fertilizer/chemical applications with less overlap, and enhances healthier, less-compacted soils. 

“CTF is expensive to set up and requires more management and planning, but it’s worth considering to prevent compacting fields,” Dougherty says.

Biology

“If you have compacted soils, your number one friend is the earthworm. You need to increase the worm populations in your fields,” Dougherty says. “The way you do that is to reduce soil disturbance and provide living roots throughout the year.”

Based on worm populations and how active they are, it’s estimated earthworms can turn over the entire top 6 inches of the field every 5-20 years.

“Earthworms can become your tillage tool, and you don’t even have to get any steel out of the shed necessarily,” Dougherty says. “If you’re worried about nutrient stratification, you need worms, you need living roots. You need cover crops to move those nutrients up and down the profile.”

Dougherty says worm channels also produce about 70% of a field’s water infiltration rate and they are loaded with slime that is high in calcium and contains 5-10 times more nutrients than the adjacent soil.

Nutrient Applications

It may seem counter-intuitive to blame nutrient applications for increased soil compaction, but Dougherty says too much fertilizer makes soils lazy, and lazy soils tend to lack aggregation.

“Regardless of the form of fertilizer, purchased nutrients or manure, they cause less root exudation because plants don’t need to produce those carbons to trade for nutrients,” Dougherty says. “In turn, soil microbes in these fields lack their food source and that leads to less aggregates formed. Microbes turn from consuming exudates to eating the biotic glue — glomalin — that stabilizes aggregates. This is particularly true when excessive nitrogen is present.” 

When this chain of events happens soil strength is lost and the soil becomes more susceptible to collapse and compaction.

“Soil texture and minerology play a part in overall soil health, too,” Dougherty says. “High magnesium and high potassium levels inhibit aggregation, but calcium is known as the ‘Goldilocks’ mineral that not only helps aggregate the soil but also improves oxygen diffusion within the profile.”

Dougherty says base saturations of 60-70% calcium (Ca), 12-18% magnesium (Mg), 3.5% potassium (K) is considered ideal, but cautions growers against “breaking the bank chasing a number on a soil test.”

He recommends the following ratios, based on ppm levels not base saturation: 

• 3:1 Ca:Mg ratio for sandy soils
• 5:1 Ca:Mg ratio for loamy soils
• 7:1 Ca:Mg ratio for clay soils.

Compaction Remediation

“Armor the soil and keep it covered with a living plant, is my best recommendation to begin remedying conditions that favor compaction,” Dougherty says. 

Cover crops between cash crops protect the surface from wind and water impact, build soil aggregates, break up compacted layers, add organic matter, reduce bulk density, improve water infiltration, and feed soil organisms that provide bio-tillage, he adds.

“Use plant diversity to your advantage,” he says. “Select covers with fibrous roots for topsoil compaction, and fibrous and tap roots for deep compaction, and realize it will likely take up to 10 years to accomplish your goals.”