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Crop Residue as a Soil Saving Strategy
Leaving crop residue on the surface of the soil is an investment. The near-term return is a savings in moisture with a payback in better yields. A long-term bonus is the building of organic matter in the soil.
“Residue is a building block of soil organic matter,” says Paul Jasa, University of Nebraska Extension engineer. “Under some conditions, residue can help soil build .1% of organic matter every year, eventually increasing the organic matter level in the soil by 1% every 10 years.”
Aside from building soil, residue also saves it from wind and water erosion. “Residue and growing vegetation protect the soil surface from erosion by absorbing the impact energy of raindrops, thus, reducing the movement of soil particles,” says Jasa. “Reducing the impact also minimizes surface crusting and sealing, enhancing infiltration and decreasing runoff.”
Besides sheltering soil from wind and rain, residue and crop canopies also shade soil from the sun. This reduces water loss through evaporation.
University of Nebraska research shows that the rate of evaporation from an irrigated corn crop with bare soil between the rows could be cut in half by no-till management, leaving 6,000 pounds per acre of flattened wheat stubble covering the soil surface.
“In the study, evaporation was reduced by 6.8 inches due to the crop canopy and another 3.8 inches due to the straw mulch,” says Jasa. “This savings of almost 4 inches of water in evaporation due to the crop residue represents only part of the story. Additional benefits of crop residue extend throughout the year and include runoff control and the capture of snow and rain. These other benefits could easily add 2 inches or more of soil moisture.”
The extra water saved during the growing season translates directly into better yields. Assuming a 12-bushel-per-acre corn yield increase for every inch of soil moisture captured, a growing season moisture savings of 3 to 5 inches could potentially increase crop yields by 36 to 60 bushels per acre.
Aside from saving moisture, residue’s shading of the soil and resultant cooling of the surface permits better root development for plants in the heat of summer.
“With stronger roots near the soil surface, corn, for instance, has better standability, and the plants are more able to use the light rain showers that don’t soak into the soil profile very far,” says Jasa.
Moist, cool soil makes a good habitat for the microorganisms needed to cycle residue into plant nutrients.
“The residue from one crop should break down while the next crop is growing,” says Jasa. “As that happens, the residue feeds carbon to the next crop, which uses it for growth.”
The decomposition of residue speeds up in moist soil and slows down when soil is dry. Thus, any water-conserving practice helps the cycling of residue.
Growing a cover crop, for instance, typically helps conserve soil moisture.
Cover Crops as Regulators
“Cover crops keep the sun and wind off the soil surface, thus, reducing evaporation,” says Jasa. “Many producers report that properly managed cover crops use less water than is lost through evaporation from an unprotected soil surface, particularly following low-residue crops.”
Cover crops can be used to regulate soil-moisture levels in fields where residue happens to conserve and capture more moisture than the soil can store.
“A cover crop can use some of that water to reduce its potential loss to downward percolation,” says Jasa. “The water use by the cover crop makes room for future rainfall events.
“For example,” he continues, “a sandy soil may only be able to store about an inch of water per foot, or 4 inches of water in a 4-foot rooting zone. If the precipitation is much more than that after harvest of one crop and before planting of the next, some water is lost to deep percolation and could have been used by a cover crop. This is especially true for wet falls and on soils that typically have excess water in the spring.”
While cover crops contribute their own residue to the soil, they typically speed up the decomposition of residue from the previous crop. This occurs because of the new population of biological soil life supported by the cover. In this way, the cover crop helps to regulate the buildup of surface residue.
“Producers sometimes say they have too much residue,” says Jasa. “But their perception of having too much residue indicates that there is not enough biological activity in the soil.”
Residue levels can be further regulated by rotational design. If high levels of residue are needed, growing high-carbon crops will help. Residue levels in the soil will increase with crops such as corn, wheat, oats, rye, and sorghum.
Because of the nitrogen in their tissue, legumes, on the other hand, will speed up the decomposition of residue. Brassica cover crops will also speed up residue’s breakdown.
Rotating diverse crops under no-till management fosters a self-regulating decomposition of residue.
“Growing continuous wheat, for instance, will not contribute much to soil biology,” says Jasa. “A corn-soybean-wheat rotation is much better for building biological activity, which would improve even further with still more diversity in the rotation.”
Efficient cycling of residue could lead to an eventual savings in fertility costs.
“Depending upon your fertility needs, the savings could amount to $20 to $50 an acre,” says Jasa.
Safeguard Soil Structure and Organisms
Improved soil structure is a long-term benefit of conserving residue and using no-till. Soil with good structure has stable pores extending from the surface to deep within the soil profile. These pores allow water infiltration, root penetration, and air exchange.
“In soil with good structure, a tillage operation will break up the existing soil structure, making it more susceptible to compaction,” says Jasa. “With soil structure weakened, future operations may compact the soil by squeezing out the pore spaces between the soil aggregates. Without structure and pore spaces, infiltration and internal drainage decrease, and problems with excess water arise, causing a lack of oxygen in the root zone.”
Tillage also interferes with the fungi, bacteria, and other soil microorganisms, as they process roots and residue to cycle nutrients and carbon through the soil system.
“Larger organisms such as larvae, bugs, and worms feed on the microorganisms, further cycling materials in the soil and providing additional channels through the soil profile,” says Jasa. “Without this biological life, soil structure suffers, and many of the nutrients are not as available for crop uptake.”