By Mick Lane (re-printed with permission of Wallaces Farmer)
A pound of nitrogen in manure isn’t quite the same as a pound of nitrogen in commercial fertilizer. Farmers wonder about that, especially when they don’t quite get as much yield response from manure application as from commercial fertilizer.
“Nitrogen from different sources should be considered differently with respect to corn production,” says Iowa State University agronomy professor Fred Blackmer. “If you have organic materials like manure and crop reside, the nitrogen is usually tied up by carbon in various forms. The N must be released by bacteria in the soil.”
Ground corn applied as a fertilizer? “Think of it as unused hog feed,” says Tracy Blackmer, director of research for the Iowa Soybean Association.
ISA helped fund these studies with a combination of soybean checkoff dollars and matching funds from other sources, including USDA’s Natural Resources Conservation Service and Environmental Management Solutions.
The soybean residue and ground corn were applied to the plots at rates sufficient to provide 125 lbs. of nitrogen. Commercial nitrogen fertilizer in the form of solid urea was applied to other plots at the same time and rate.
Manure plots were established to compare yields with no added nitrogen, 135 lbs. of nitrogen from injected liquid swine manure, and 50, 100 or 150 lbs. of nitrogen as a 32% urea-ammonium-nitrate fertilizer solution.
“We had intended to put on enough manure to give us approximately the same amount of nitrogen as from 150 lbs. of commercial fertilizer,” explains Fred. “We determined the actual N by analysis of samples of the manure collected during application.”
Yield Results Show Difference
Each increase in rate of commercial fertilizer applied resulted in higher corn grain yields. However, the 150-lbs. rate produced only 4 bushels more than the 100-lbs rate.
Applying the ground corn grain increased yields, but it appears that nitrogen from this was only 44% as effective as that in the fertilizer solution. The hand-harvested results showed soybean straw actually decreased grain yields.
“The reason yields were lower with the grain and straw than with commercial nitrogen fertilizer is because of the higher carbon content of these two “organic” materials,” says Fred. “The carbon in the straw consumed the added nitrogen plus some soil-derived nitrogen that was available to plants in the other plots.”
The chemical bonds between carbon and nitrogen in the grain were broken down more rapidly than in the straw. This demonstrated that quality characteristics or organic materials should be expected to influence the rate of nitrogen release and the effects on plant growth.
The injected liquid swine manure also increased yields, but not as much as commercial nitrogen. The results show the nitrogen in manure was only 5.8% as effective as N in the fertilizer solution.
“It isn’t possible to determine exactly why the nitrogen in manure was less effective,” says Fred. “Likely reasons are some chemical bonds between carbon and nitrogen were not broken rapidly enough, and some organic materials prompted immobilization and/or denitrification of nitrates.”
“Manure is a very good source of nitrogen,” adds Tracy. “But, as this study shows, the nitrogen portion is not as readily available to a corn plant as commercial nitrogen fertilizer. Less than two-thirds of the manure nitrogen was available to the crop in the first year at these four locations. Actual availability of nitrogen from injected hog manure varies, but is usually less than the 100% figure commonly used in manure management plans.”