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How do I accurately credit nitrogen from manure? Photo courtesy of Rick Koelsch.
How do I accurately credit nitrogen from manure?

Tips offered for estimating nitrogen credit from manure

Best method for crediting manure nitrogen is quick calculation of farm’s manure sample, manure source and soil incorporation practices.

Manure is a valuable source of nutrients that offers agronomic and soil health value, Rick Koelsch in the University of Nebraska's department of biological systems engineering noted in a recent edition of the university's "BeefWatch" newsletter.

Most manure nutrients (e.g., phosphorus) can be managed successfully with traditional soil analysis. However, nitrogen in manure requires some simple advance planning to ensure that it is given proper credit for offsetting commercial fertilizer inputs, Koelsch said.

Manure nitrogen comes in two forms: ammonium-nitrogen (or NH4+-N) originating from urine, and organic-nitrogen originating from the feces. Soil samples typically only measure nitrate-nitrogen (NO3-N), although ammonium-nitrogen can be requested, he said. Ammonium-nitrogen from manure will convert to nitrate-nitrogen in the spring as soils warm.

Organic-nitrogen from manure is a slow-release nitrogen that converts to crop-available nitrogen through the warmer summer months of the growing season. A late-fall soil sample will completely miss any manure nitrogen, and a spring soil test may only partially credit manure’s ammonium-nitrogen while not crediting any of the organic-nitrogen, Koelsch said.

Crop-available nitrogen from manure

Crediting manure’s nitrogen replacement value requires identification of an availability factor for both ammonium- and organic-nitrogen, Koelsch said. Ammonium-nitrogen availability is dependent on the method of application and how quickly it is moved into soil. For example, Koelsch said if solid beef feedlot manure is applied and incorporated within one day, estimate that 50% of the ammonium-nitrogen will be credited.

Organic-nitrogen availability is dependent upon on the source of the manure. For the solid beef manure example, estimate that 25% of the organic-nitrogen is available to this year’s crop and 15% available to next year’s crop, Koelsch explained.

To illustrate the simple calculations for estimating crop-available nitrogen, assume that 25 tons of solid beef manure are applied per acre and a manure analysis reports that there are 16 lb. of organic-nitrogen and 4 lb. of ammonium-nitrogen in the manure. If the beef manure is incorporated within 24 hours, how much crop-available nitrogen is there? Assume that no manure has been applied to this field in prior years.

In this example, Koelsch said the manure application rate offers an offset of 150 lb. of nitrogen per acre of commercial fertilizer.

He said the take-home message is that the best method for crediting manure nitrogen is a quick calculation of an individual farm’s manure sample, manure source and soil incorporation practices.

Additional considerations

If manure application is used to accurately offset fertilizer, research has demonstrated lower nitrogen losses to surface and ground water. Manure is the original slow-release nitrogen, with the organic-nitrogen fraction becoming available closer to the nitrogen requirements of a corn crop.

Koelsch said some research has demonstrated that peak yields are observed for manure application at a rate replacing roughly 75% of the crop nitrogen requirements, with supplemental commercial fertilizer used for the balance. This is especially important for manure with minimal ammonium-nitrogen (such as open-lot beef manure or broiler barn litter) or those that are surface applied.

Since organic-nitrogen is a slow-release nitrogen, a fertility program that supplements manure organic-nitrogen with inorganic fertilizer (possibly a starter fertilizer) can assist with early-season crop vigor, he added. Manure with significant ammonium-nitrogen content (and injected) may need less supplemental fertilizer for early-season vigor.

As a general rule, manure application timing should follow the same best practices as commercial nitrogen fertilizer, with application recommended after soil temperatures drop below 50°F. Manures with high organic-nitrogen levels are a little more forgiving, he said, but a late-summer application of manure nitrogen on wheat stubble may lose some of the organic-nitrogen by the next spring’s corn crop.

Listen to a discussion of the content in this article on this episode of the "BeefWatch" podcast.

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