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How much manure to apply?

David Ostdiek
Panhandle Research and Extension Center

Prompted by spiraling commodity prices and input costs, many farmers have been sharpening pencils, recharging calculators, or updating spread sheets on laptop computers .

Farmers who apply feedlot manure to crops are among those who could benefit from re-calculating their math. They could take a fresh look at how manure meets the crop’s nutrient requirements. Specifically, instead of spreading enough manure to satisfy a crop’s nitrogen needs, they should focus on meeting the crop’s phosphorus requirements.

That’s the recommendation from Dr. Gary Hergert, soil fertility specialist at the University of Nebraska Panhandle Research and Extension Center.

With high prices of commercial fertilizer, producers are taking a second look at how feedlot manure fits into their farming operations, Hergert said. In the past two years, the price of nitrogen has tripled, and the price of phosphate has quadrupled. As of early October, average Panhandle nitrogen prices were 61 cents a pound (anhydrous ammonia) to 84 cents (urea ammonium nitrate solution), to 95 cents (urea solid form). Average phosphorus prices in the region were 80 cents a pound to $1.29 depending on the form.

When using manure to replace commercial fertilizer, Hergert said, the strategy commonly has been to fertilize on the basis of the manure’s nitrogen content. In manure and compost, the ration of nitrogen to phosphorus is usually 2:1 or 1:1. But most crops will uptake much more nitrogen than phosphorus (corn, 6:1, wheat, 5:1, beans, 9:1, sugar beets 7:1, alfalfa, 7:1), so Hergert points out that meeting a crop’s nitrogen need will at the same time far exceed its phosphorus need.

When manure is plentiful, it makes sense to apply enough to meet a crop’s nitrogen need, Hergert said. But at today’s prices, it makes more sense to base the application on phosphorus need – then, if necessary, fill the crop’s N requirements by supplementing with commercial sources of nitrogen.

Manure is typically applied every four years. So 25 tons of manure per acre would satisfy a corn crop’s phosphorus requirements for four years. (Research shows that 200-bushel corn removes 35 pounds of phosphorus per acre, and it would take 6 tons per year to supply this much phosphorus.) But depending on the crop, this amount of manure might not meet a crop’s nitrogen needs.

Hergert stressed the importance of having manure analyzed at a laboratory to know its chemical content. Unlike commercial fertilizers’ standardized content, manure’s chemical makeup will vary with the feedlot rations and moisture content.

Farmers who fertilize with manure need to watch their crop more closely than when they use commercial fertilizer, according to Hergert. Manure needs to be tracked more closely because its effects vary with several biological processes going on in the soil, and these processed can vary with the weather.

For example, not all of the nitrogen in manure is available to the crop immediately. Manure contains organic N, and nitrogen must go through a process called mineralization before it is in a form that plants can use. The rule of thumb is that, during the first year after application, about half of the manure’s nitrogen content will be mineralized and thus available to the crop. Still, that is likely to be more than enough for corn, Hergert said.

During the second year, Hergert said, the amount of N available to the crop is about half of what was available the first year. So a producer may need to supplement N during the second year, but he said that’s not automatic. It’s necessary to take a deep soil sample, then estimate how much additional nitrogen will become available to the crop through mineralization as the soil warms up in the spring.

During the third and fourth years, N requirements will depend on the crop, Hergert said. Dry beans might not need additional nitrogen.

Hergert pointed out that UNL recommends sugar beets not be planted on land where manure has been applied until at least two to three years after application, because of the effect of high soil nitrate on beet quality and SLM (sugar loss to molasses).

“All in all, using manure is positive,” Hergert said. Manure contains zinc, potassium, and other micronutrients as well. But the big value is in all the carbon contained in manure, which benefits the soil in several ways. It builds the organic content, improves soil quality, and improves the soil structure. This leads to less runoff.

“There are a lot of long-term benefits from properly applying and managing manure as part of your soil fertility program,” Hergert said.


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