There are primarily two forms of N in manure: inorganic (ammonium) N and organic N. The ammonium N is initially present in urine as urea and may account for approximately 50% of the total N in manure. Urea in manure is no different from urea in commercial fertilizer. It hydrolyzes rapidly to ammonium. In principle, all of the ammonium from urea in manure is available for plant growth. However, parts or all of it may be lost because ammonium is rapidly converted to ammonia, especially at higher soil pH and as manure dries. Atmospheric exposure of manure on the barn floor, in the feedlot, in storage, or after spreading on fields increases N loss. Thus, an analysis of the manure is useful to determine how much inorganic N may be conserved before spreading.
The more stable organic N is present in the feces and is slowly released to plant-available forms. The organic N in manure consists of proteins, peptides, etc. of undigested feed, scurf from the digestive tract, and so on. The decomposition of stable organic N to a plant-available inorganic form occurs at different rates. The less resistant organic N decomposes during the year of application, and the more resistant organic N decomposes very slowly in future years. Repeated application to the same field results in an accumulation of slow release manure N.
Click through the following diagram for a review of the basics of manure N availability.
Consider the following questions about nitrogen in manure.
Click to the next page to see how the basics are applied.
To review the N Cycle and Crop N Guidelines, read the Nitrogen Guidelines for Field Crops in New York (sections 2 and 3)