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Let's Talk Stalks
Monday, November 24, 2014 11:35AM CST

By Daniel Davidson
DTN Contributing Agronomist

Huge amounts of cornstalks that seem resistant to decay require creative solutions and trial and error. Weather plays a role in whether a strategy works. Tillage doesn't always work, nor does spraying on nitrogen, sulfur and carbon (sugar and humates). Microbes and enzymes don't always work to our satisfaction either.

Southern portions of the Corn Belt have a bit of an advantage when it comes to natural decay. Air temperatures stay warmer longer in the fall and warm up more quickly in the spring. Corn is often harvested earlier, beginning in late August and early September. Early harvests coupled with warm temperatures yield more decay. This is very evident after 30 or so days when the material begins to blacken, indicating it is decaying naturally. Throw in some tillage and products and you can enhance nature's process.

I wrote a DTN article earlier this fall as an overview of tactics to get cornstalks to break down. A number of readers posted more specific questions about the challenge of residue decomposition and the benefit to the next crop.

Question: "Could you address the availability of nutrients at some point in the future to help feed another crop? I would assume that those nutrients are available sometime."

Answer: Two-hundred-bushel corn produces 4 to 5 tons of above-ground biomass, and that carries a lot of nutrients that are valuable to the soil and crop. Research from Iowa State University reports that corn stover contains about 6 pounds of phosphate per ton and 25 pounds of potash per ton. Five tons of above-ground material will contain about 30 pounds of phosphate and 125 pounds of potash per acre. Below-ground biomass will be similar in mass and nutrient content on an acre basis. Corn stover also contains nitrogen, calcium, magnesium and sulfur and micronutrients.

The potassium leaches out of cornstalks relatively quickly after a few rain showers in the fall and will remain in the soil profile. However, nitrogen and phosphorus are tied up in the organic matter and only released through mineralization next spring and summer. Cornstalks do provide mineral nutrients and their contribution needs to be considered in a fertility program and their loss factored in if you remove the residue.

Question: "If one gets the cycle going after a couple of years, do they still need to be adding N every fall?"

Answer: Crop residue contains a lot of nutrients; however, release depends on the ability of microbes in the soil to degrade the material and recycle the nutrients back to the soil as plant-available forms as ammonium, phosphate and potash. Microbial activity depends on soil conditions (electroconductivity, moisture, pH, temperature and aeration), availability of microbially active carbon and the carbon-to-nitrogen ratio. If you have a low carbon-to-nitrogen ratio (25 or less), there is plenty of nitrogen in the system to drive decomposition. If the ratio exceeds 50-to-1, there is not enough nitrogen in the system. The carbon-to-nitrogen ration depends on the previous crop: wheat (80:1 or greater); corn (50:1 greater); soybeans (20:1 or less), alfalfa (27:1). The ideal ratio for microbial activity is 25:1.

Soil fertility can be maintained, to a degree when nutrients are efficiently recycled by additions of residue and microbial activity. The question is, can you do without additional nitrogen? If one is in a corn-corn, corn-soybeans, wheat-soybeans or wheat-soybean-corn rotation, there will not be enough nitrogen maintained in the system that would keep this cycle operating at its highest recycling level. However, when you add in more legumes and cover crops to the rotation, you will be adding nitrogen in the system to lower the carbon to the nitrogen ratio in the soil to keep nutrients recycling without additional nitrogen beyond what a grower applies to his crop. For example, rye straw has a carbon-to-nitrogen ration of 82:1. However, rye as a cover crop, terminated during vegetative growth will have a carbon-to-nitrogen ratio less than 30:1, similar to soybean residue, and will feed nitrogen into the recycling process.

Question: "Will applying nitrogen in the fall help break down wheat straw? We grew a 125-bushel winter wheat crop on a field this year and I would like to get rid of the straw without tilling it. I ran a heavy harrow (to mix in some soil), but still have lots of residue to get rid of before next fall."

Answer: Wheat has a carbon-to-nitrogen ratio of 80-100:1, even greater than corn, and contains only 0.25% to 0.40% nitrogen. For rapid decomposition, additional nitrogen needs to be available to supplement the low nitrogen content of the straw. With normal temperatures and adequate moisture, fertile soils usually will provide enough nitrogen from nitrification to meet the requirements for decomposition for a 40- to 60-bushel yield. However, when you are harvesting 100- to 125-bushel wheat yield, you will be producing 4 to 6 tons of straw per acre. This will require 10 pounds of nitrogen per acre to bring down the carbon-to-nitrogen ratio. Consider the soil can provide 10 to 20 pounds. Therefore, you need to add back 20 to 40 pounds of nitrogen per acre. Keep in mind that applying nitrogen on top of residue isn't always reliable. Throw on a bit of soil with light tillage, add in warm temperatures and rainfall, and it you will get a more consistent response.

Question: "If we spray 10 gallons of 28% UAN on our corn stalks, should we work the ground right away or let it lay for a while? And should we wait for a certain temperature to apply? We do vertical tillage on all our acres."

Answer: Spraying on 10 gallons of 28% UAN (28 pounds per acre) seems to fall within the guidelines of 30 to 40 pounds per acre often recommended. UAN contains ammonium and that will volatilize laying on the surface, exposed to the atmosphere. Also, cornstalks are covered with a waxy layer that is relatively impenetrable, and it's not the nitrogen that breaks down the residue. The organisms that feed on the cellulose, hemicellulose and other carbon compounds need nitrogen to do their work. The best strategy is to spray nitrogen on the residue and then immediately do vertical tillage to cut and fracture the residue while throwing on some soil with light tillage. It's the soil that introduces the microbes that feed on the carbon and nitrogen, and light tillage cuts and fractures the cornstalk surfaces. As for temperature, spray the cornstalks and vertically till as soon as possible after harvest because the decay process will begin sooner with more warm days remaining in the fall.

Question: "In your article, you mention several ways to speed up stalk breakdown. In my situation, I plant corn-on-corn in southeast South Dakota and I do fall tillage. What would you recommend is the best practice to help with this scenario, and do I apply it before or after tillage?"

Answer: Corn-on-corn is where growers complain about the build-up of residue and accumulation over time, and many take aggressive action to prevent the building. As you know, it is possible to plant corn-on-corn and make it work if you manage it correctly. However, the residue buildup and nitrogen immobilization that results the next spring is a challenge. My recommendation is to plant earlier-season corn and harvest wetter to gain more time and warm temperatures in the fall to drive degradation. Use a chopping corn head to chop the stalks and spread them evenly behind the combine. Spray on some nitrogen (starting at 30 pounds nitrogen per acre) and sulfur (7:1 N-to-S ratio) on the cornstalks and lightly throw some soil on the residue to tie it down and introduce microbes. If you use a regular corn head, spray and try vertical tillage or light conventional tillage. As for applying a biological spray (microbial cocktails or enzymes), I recommend some testing to decide what will work best and consider it an additive on top of processing residue. Stimulating soil health is important because it is an important part of residue decomposition and nutrient recycling. Find a way to add cover crops into your rotation.

Dan Davidson can be reached at AskDrDan@dtn.com


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