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Don’t Remove Too Much Residue

Leave enough crop residue to protect soil and to ensure nutrient cycling.

When figuring the dollar value of harvesting corn stover for either livestock use or ethanol production, build a long-term hedge into the equation by first determining how much crop residue the soil can afford to lose.

“Think about residue removal in terms of the long-term impact on productivity and soil sustainability,” says Iowa State University (ISU) agronomist Mahdi Al-Kaisi. “We’re trying to think ahead to educate farmers and agronomists about potential implications of residue removal.”

In an eight-year study, ISU researchers examined how crop yield and soil quality were affected by varying levels of corn-residue removal and differing tillage systems. The results indicate that corn yields held steady after one to three years of total removal of residue for both conventional-till and no-till systems.

However, as little as 25% to 35% removal of residue immediately affected soil quality.

“Corn production in Iowa generally produces 4½ tons of residue per acre at harvest,” says Al-Kaisi. “To protect the soil, we found that 3 tons of residue should be left on the soil surface to maintain soil organic matter at no change. Anything below that causes the bulk density of the soil to increase and water infiltration to decrease.”

As bulk density increases, soil becomes compacted, causing soil erosion and water runoff.

With little residue on the surface, soil is particularly vulnerable to rainfall.

“In a normal rainfall, raindrops 6 millimeters in diameter hit the ground at 20 mph,” says Al-Kaisi. “The cumulative impact of raindrops can be incredible, dislodging soil particles and splashing them up to 5 feet away. The splashed particles clog soil pores, effectively sealing off the soil surface and leading to soil crusting and poor water infiltration.”

Besides sheltering soils from wind and water erosion, crop residues provide a critical source of soil carbon. Soil microorganisms consume the carbon, and their feeding activity leads to final-stage breakdown of plant material. This results in the formation of organic matter and the sequestering of some carbon from the atmosphere.

“The loss of residues due to removal for any use can accelerate the loss of soil organic matter and nutrients for plant growth and soil microorganisms,” says Al-Kaisi. “In another long-term study, it was found that corn stover removed vs. stover returned had reduced the total source of soil organic matter by 20% and corn-derived soil organic carbon by 35% in a 13-year period.”

Destructive changes in physical properties of the soil can take 10 years or more to reverse by implementing conservation practices.

While corn yields may not decrease in the short term as a result of annual removal of residue, the diminished level of nutrient cycling by soil microorganisms may soon result in the need for increasing fertilization rates.

“Possible short-term impacts of corn stover removal may include an increase in application of nitrogen, phosphorus, potassium, calcium, and magnesium nutrients. These are needed to replace the nutrients lost due to residue removal and potential deficiencies in the soil nutrients’ pool in the long term,” says Al-Kaisi.

“Seven years ago, one study estimated that these macronutrients’ replacement cost due to residue removal was approximately $10 per ton of harvested residue,” he says.

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