With high prices of commercial fertilizer, you should take a second look at how feedlot manure fits into your farming operations, says Gary Hergert, soil fertility specialist at the University of Nebraska Panhandle Research and Extension Center in Scottsbluff.
In the past two years, the price of nitrogen has tripled, and the price of phosphate has quadrupled. As of early October, average Nebraska 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 says, 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 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 says, 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. Thus, 25 tons of manure per acre would satisfy a corn crop's phosphorus requirements for four years. But depending on the crop, this amount of manure might not meet a crop's nitrogen needs.
Hergert stresses the importance of having manure analyzed at a laboratory to know its nutrient content. Unlike commercial fertilizers' standardized content, manure's 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, because its effects vary with several biological processes in the soil, which 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 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 says.
During the second year, he adds, 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, although 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 in the as the soil warms up in the spring.
During the third and fourth years, N requirements will depend on the crop.
"All in all, using manure is positive," Hergert says.
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.