Reducing Compaction with Roots

Brian Doughtery, a Understanding Ag consultant, says compaction is a sign of a poorly functioning soil. Soil compaction is not a natural occurrence, it comes from too much equipment (heavy axle loads, too much tillage), not enough biology (lack of roots and living organisms), and excess nutrients.

Well aggregated soils look like black cottage cheese; soil crumbles easily in your hand and water and gases move freely in soil. Well aggregated soils have 50% solid materials made up of ground up rock, minerals and water with the other half being pore space by volume. Compaction just means the pore space is reduced or eliminated resulting in denser compacted soil.

For good plant growth, roots need to penetrate the soil to get water and nutrients and have adequate gas exchange. Roots need oxygen produced and released by the leaves to move down into the soil to break down the food produced by the leaves. Roots give off carbon dioxide after the oxygen breaks down the food in the roots. It’s a natural cycle, but it requires good soil gas exchange which is dependent on well aggregated soils and good soil structure. Reduced oxygen and carbon dioxide exchange limits plant growth and yields.

Interconnected pore space allows water, nutrients, and gasses to flow through the soil. Roots can easily grow into these spaces and gasses are exchanged and water and nutrients are absorbed. Roots help make these interconnected pore space along with earthworms and other organisms. The roots give off sugary root exudates to hold nutrients in those aggregates and the fungi give off glomalin to glue it all together. Aggregates are constantly forming and being destroyed. When aggregates are destroyed, they release nutrients and most only last about 4 weeks. However, too much destruction results in excess nutrient release and compacted soils. The microbes start consuming the glues (glomalin) and soon the whole soil gets hard and dense. If soils go more than 4 weeks without living roots, the soil starts getting dense and compacted. “No living roots, no glues, no aggregates!” says Dougherty.

On equipment, deep tillage is only a short-term fix followed by a downward spiral in soil health. Deep tillage initially increases water infiltration, but then the aggregates disappear and the compaction situation gets worse. Water infiltration slows down, gas exchange decreases, and soon aggregates are being excessively destroyed. The soil gets hard and dense and with no live roots, soil erosion increases. “Deep tillage only produces a yield gain about 25% of the time according to long-term University trials.” says Brian Dougherty.

Using cover crops increases root mass year-long, forming aggregates continually. However, it is a long-term game, it may take 3–5 years or longer to heal the soil. Cover crops improve water infiltration and soil structure down to 36 inches but it takes time. Generally, with good growing conditions and early cover crop planting, cover crops may penetrate up to 12 inches of compacted soil per year. Cover crops add organic matter, protect the soil surface from erosion, and also promote biological activity and nutrient retention and release.

“On equipment, try to keep your loads as light as possible or no more than 10 tons per axle load on well aggregated soils to reduce soil compaction. Keep your tire pressure low and as close to 10 psi as possible (depending upon the tire) to reduce soil compaction.” says Doughtery.

According to Randall Reeder, retired OSU ag engineer, about 75% of the increase in soil bulk density (compaction) and 90% of wheel sinkage occurs on the first pass after a field is tilled. The longer the equipment stays on that soil (the dwell time), the greater the compaction. University studies say that controlled traffic reduces fuel consumption 25%, requires less horsepower, and improves nutrient and chemical application with the added benefit of less soil compaction.

Some nutrients are known to cause soils to compact when they are in excess. Excess Nitrogen (N), Magnesium (Mg) and too much Potassium (K) are known elements that make soils dense and hard. Adequate calcium is known to flocculate soil which means the soil becomes less dense. Keeping these nutrient in balance is essential to keeping soils well aggregated.

Earthworms require a lot of calcium and they can till the top 6 inches of a soil in about 5–20 years. Earthworm slime and earthworm castings (earthworm poop) has 5–10 more soluble plant available nutrients than regular soil. Earthworm holes can increase water infiltration by 70%. Doughtery says, “Use cover crops to increase live roots year-round to increase soil aggregation, improve organic matter, increase water infiltration and to reduce soil compaction.”