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Nitrogen fertilizer: How much is left?

Agriculture.com Staff 06/04/2007 @ 11:46am

Significant rainfall in parts of the Corn Belt over the past three weeks has many producers wondering how much of the nitrogen (N) fertilizer they applied is still available to the crop.

Several factors can influence the amount of N loss, according to a University of Nebraska report, including rainfall amount and intensity, soil texture, soil temperature, fertilizer source and application date. Loss pathways can include runoff, ammonia volatilization, denitrification and leaching.

Runoff
If fertilizer had been recently applied to the soil surface, without incorporation or gentle rain of 1/2 inch or more to move N into the soil profile, substantial N loss may occur in runoff associated with intense rainfall.

Ammonia volatilization
Nitrogen supplied from urea-based fertilizer (urea or urea-ammonium nitrate solution [UAN]) can be lost as ammonia gas to the atmosphere under certain conditions. The greatest risk is from fertilizer broadcast to warm, moist soils with heavy residue. Rainfall of 1/2 inch or more generally is adequate to move fertilizer into the soil and protect it from ammonia volatilization. Very light rains, of 1/4 inch or less, may only moisten the residue and soil, increasing loss rather than preventing it.

Denitrification
The primary N loss mechanism from saturated, fine-textured soils is likely denitrification. This is the process which converts nitrate-N into gaseous forms (nitric oxide, nitrous oxide, dinitrogen) using anaerobic bacteria present in the soil. These gaseous forms of nitrogen can be lost to the atmosphere. In fields where the majority of fertilizer N was applied before planting, likely four to eight weeks ago, much of the N may have been converted to nitrate by the microbial process of nitrification. This nitrate is then susceptible to loss via denitrification or leaching.

Leaching
If nitrogen existed in soil in the nitrate or urea forms, significant leaching loss can occur with heavy rains, more so on coarse-textured soils. Some of this N may have leached deep enough into the root zone to be unavailable to the crop, at least early in the season. Continued precipitation or irrigation may leach this N out of the root zone entirely.

Management options
Unfortunately, there are many variables interacting to influence the potential for N loss, making it difficult to estimate how much fertilizer N has been lost and to determine whether producers should apply more fertilizer. Research in Iowa suggests approximately four to five percent of nitrate-N can be lost via denitrification for each day soils are saturated.

For anhydrous ammonia applied six weeks ago, perhaps at least 50% of the N has been converted to nitrate. If soils were saturated for five days, as much as 10% to 12% of applied N may have been lost to denitrification, with additional potential loss due to runoff or leaching. Whether remaining N will be adequate to optimize yield potential depends on the initial application rate and growing conditions during the rest of the season.

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