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Cover Crops Improve No-till Corn

No-till corn is a tough sell to most farmers. Farmers are tilling ahead of corn planting because  they typically see a yield increase with tillage that covers the added costs. Often corn yields  drop slightly after switching to no-till, perhaps 5-10-15%. Corn benefits from tilled soils due to  the release of nutrients from soil organic matter. Tilling the soil injects oxygen into the soil  which stimulates bacteria and other microbes to decompose the organic residues and releases  nutrients faster. Every 1% soil organic matter in top 6 inches holds about 1000 pounds of  nitrogen (N) and 100 pounds each of phosphorus (P) and sulfur (S). However continuous tillage oxidizes or burns up soil organic matter and soil productivity declines with time. Thus, tillage  results in poorer soil structure and declining soil productivity.

Long-term research reveals that continuous no-till produces higher yields than conventional tilled fields but it takes 7-9 years to improve soil health by getting the microbes and soil fauna back into balance, and start to restore the nutrients lost by tillage. In the last 50 to 100 years, our soils have lost 60-70% of the original soil organic matter due to tillage. Using cover crops with continuous no-till reduces the transition period to 2-4 years while the soil microbe numbers and soil organic matter levels increase in the soil. No-till corn yields are often 10-15% lower during this transition period for several reasons. First, initially fewer nutrients are being released from surface residues. Tillage allows surface residues to decompose faster,
releasing nutrients. Second, as the biological activity increases in no-till soils, more nutrients are tied up by the microbes (especially N and P). This initially deprives corn of N and P because the microbes feed first and tie up soil nutrients. Third, the soil is building humus organic matter which requires N to stabilize the organic residues. Every 1% soil organic matter requires 1,000 pounds of N, so the N may not be initially available to the corn in the short-term. Fourth, soil compaction from tillage and heavy equipment causes denitrification on heavy clay soils resulting in a lost of as much as 40-60% of soil available N. Sandy soils may not lose as much N from denitrification, but due to lower organic matter levels, the N may be lost through leaching.

To reverse these problems, First cover crops are grown to reduce soil compaction and improve carbon inputs and nitrogen recycling. Second, as the soil organic matter levels build, more N and P are efficiently recycled and released to the soil through increased microbial populations.

Third, denitrification loses decrease as soil compaction decreases due to improved water infiltration. The end result is increased soil nutrient storage, increased water infiltration, improved soil structure and soil tilth. This process may take 2-4 years to restore the soils ecological balance. Farmers can help to compensate for initially lower N availability by
increasing N fertilization and front loading N to no-till corn. As soil organic matter levels increase, no-till soils tend to stay moist and friable, which increases crop production during dry periods.

For farmers converting to no-till corn production, they often complain about cold wet soils. Living cover crops can significantly alter soil temperatures. Cover crops decreased the amplitude of day and night temperatures more than average temperatures resulting in less variability. Cover crop mulches protect the soil from cold nights and slows down cooling. This may be a benefit in hot regions, but may slow growth in cooler regions. Winter cover crops moderate temperatures in
the winter. Standing crops have higher soil temperatures than flat crops. Row cleaners help m0anage residues and improve soil temperatures in no-till fields. Corn responds to warmer soil  temperatures so strip tilling in a 4-6 inch band by moving the top soil residue may increase stand establishment and corn growth initially when converting from conventional tillage to no-till.
Long term no-till farmers who use cover crops say that their soils are not cold for three reasons. First, in the transition from conventional tillage to no-till, soils tend to be compacted, keeping the soil wet and saturated. Water holds the heat and cold longer than air. Thus cold soils tend to be wet and insulated from warm air by residue on the soil surface. Cover crops in a no-till rotation allow rain to infiltrate the soil and warm air into the soil. Grass cover crops can typically remove about 12 inches of soil compaction per year, so it may take 1-3 years to remove soil compaction that is several feet deep. Second, as organic residues are added to the soil surface, the soil color changes from lighter to darker colors as organic residues decompose. Black organic residues absorb sunlight and heat, warming the soil. This process may take another 1-3 years to occur. Third, as organic residues accumulate on the soil surface, the intensity of the biologic activity on the soil surface increases. Biologically active organic matter, like compost piles, give off heat as the microbial decomposition intensifies, warming the soil. In order  for this last sequence to occur, a thick layer of residue needs to accumulate on the soil surface.

(Source – http://www.ctic.org/media/pdf/Cover%20Crops/Country%20Journal%20Using%20Cover%20Crops%20to%20convert%20to%20No-till%20corn%20100709.pdf)


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