Dr. Dan Talks Agronomy

QUESTION: "I enjoyed your article on baling stalks, but was wondering if one should also take into account the nitrogen that is removed with the bales. Although it may be tied up until it is mineralized, would it not ultimately be available to the plants after mineralization occurs? Also wondering about the potential loss of micronutrients in calculating the budget for the baling enterprise."

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I enjoy getting reader letters and the question above came as a result of a December article exploring the cost of removing cornstalks including labor to harvest and concentrated on phosphorus and potassium. There is a cost for removal and depending on the price per ton for the stover, it can more than cover nutrient removal costs. On our family farm, I calculated last fall that harvesting six bales or 4 tons removed per acre at a price of $65 per ton gave us enough profit to make it worth the effort, expense and wear on machinery. Baling up stalks is tough on rakes and balers.

Cornstalks contain about 21 lbs. nitrogen (N); 5 lbs. phosphorus (P); 25 lbs. potassium (K); 11 lbs. calcium (CA); 8 lbs. magnesium (Mg) and 3.5 lbs. sulfur (S) per ton of dry corn stover (minus the ear).

P and K removal is easier to measure and account for in fertilization plans. Things get a little more complicated with N, S and micronutrients and organic carbon -- all have less to do with the short term (next season), but have long-term consequences if stalks are continually removed.

NITROGEN CALCULATIONS

Using my farm example above, baling 4 tons of stalks removed 84 lbs. of N (one ton equals 21 lbs. N) with nutrient replacement costs of $0.43 per lb. nitrogen. DTN reported the average fertilizer prices the last week of December at $0.43 a pound for ammonia; urea at $0.50; UAN28 at $0.57 and UAN32 at $0.57. Total nitrogen cost for removal equaled $36 (84 lbs. x $0.43 per lb.)

Here's where it gets tricky. Removing cornstalks can actually save nitrogen and improve corn yield. A study at the University of Missouri sponsored by Pioneer showed a 23 bushel corn yield gain when stalks were removed compared to no-till corn. Another study at the University of Illinois demonstrated that the optimum N rate for no-till corn-on-corn was 47 lbs. per acre less when half of the cornstalks were removed and 65 lbs. per acre less when all of the cornstalks were removed. Information on these two studies is available at: https://www.pioneer.com/….

So what would have been the fate of the 84 lbs. of N if I had not removed the stalks?

We know that cornstalks have a carbon to nitrogen ratio of 60-70:1. That means it takes additional nitrogen for microbes to digest the stalks. A rule of thumb I follow is 10 lbs. of N per ton of aboveground residue in the field -- be it cornstalks or wheat stubble. By removing 4 tons (I left about a ton per acre of surface material in the field), I saved not having to provide 40 lbs. of N to drive decomposition.

Had I left the cornstalks, eventually the 84 lbs. of nitrogen tied up in the organic matter would have become part of the microbial biomass and released as ammonium back to the soil. That ammonia would convert to nitrate and be available for plant uptake or be vulnerable to loss from leaching or denitrification.

However, much of that nitrogen release occurs later in the summer and after corn's rapid period of nitrogen uptake. As a result, it is not of much benefit to the next season's crop. It's also the reason soybeans are an ideal choice in a rotation -- they don't require additional nitrogen. And the later nitrogen is mineralized, the less likely it is to slow nitrogen fixation -- nitrate inhibits nitrogen fixation.

Because mineralized nitrogen is not available early enough for the next season's crop and because nitrate is vulnerable to loss during the summer and fall, no one accounts for the loss of nitrogen in a fertility plan.

MICRONUTRIENT MATH

A ton of cornstalks contains only trace amounts of micronutrients and over the short term isn't a major consideration. According to the International Plant Nutrition Institute, removing 5 tons of cornstalks (200-bushel corn equivalent) extracts 1.50 lbs. of manganese, 0.50 lbs. of iron, 0.30 lbs. of zinc and 0.06 lbs. of boron and copper.

SOIL CARBON CONSIDERATIONS

Maintenance of soil organic matter is important to carbon cycle. Nutrient mineralization contributes to soil physical properties that are critical to soil tilth, water infiltration and water-holding capacity. Soil organic matter is maintained by decomposition of plant biomass returned to the soil. Both aboveground and belowground plant tissue contribute to soil organic matter.

To maintain soil organic matter levels, 2 to 3 tons an acre of crop residue should be left in the field annually. Soybeans leave less residue and continual soybeans will lead to a drop in organic matter levels. A corn and soybean rotation will maintain and build organic matter under no-till practices. Greater residue amounts are necessary with more aggressive tillage practices because tillage accelerates decomposition of both new residues and existing soil organic matter. Planting continuous corn will build organic matter and actually accumulate on the surface and can become a nuisance under no-till -- compelling some growers to consider removing it.

Dan Davidson can be reached at AskDrDan@dtn.com

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