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OSU Extension

College of Food, Agricultural, and Environmental Sciences

April 27, 2015 - 1:47pm -- Anonymous

During some of the fertilizer applicator certification meetings this past winter I had some questions about the use of nitrogen inhibitors.  As we begin our 2015 corn planting season an appropriate question might be; should you consider the use of a nitrogen inhibitor?   The reason we have this discussion is because nitrogen can be lost by leaching through the soil profile, through ammonia volatilization or through denitrification.   To understand those losses and when they occur we have to know a little about nitrogen and soil chemistry and how our crop plants absorb nitrogen.

A common chemical process with nitrogen fertilizers such as anhydrous ammonia, urea and urea-ammonium nitrate (UAN) is the conversion of ammonium nitrogen, NH4 to nitrate nitrogen, NO3.  This conversion process is called nitrification and it occurs by the presence of certain species of bacteria in the soil.  It is significant because ammonium nitrogen has a positive charge and nitrate nitrogen has a negative charge.  The plant can absorb or uptake nitrogen in either the ammonium or the nitrate form and the charge on the molecule does not matter to the plant.  However, since soil particles have a negative charge the nitrification process has a big impact on potential nitrogen loss.  Once nitrogen is in the nitrate form, it is not held by soil particles and is very susceptible to leaching losses and moving down through the soil profile and out of reach of plant roots with each rainfall event.  Ideally nitrate nitrogen would quickly be used by the plant.  The saying with nitrate nitrogen is; Use it or lose it.  So, wouldn’t it be great if nitrogen could just stay in the positively charge ammonium form and be held by soil particles?  Yes it would, but the soil bacteria are active, especially as soil temperatures warm up and they can convert ammonium nitrogen to nitrate nitrogen pretty quickly.

This is where nitrification inhibitors enter the picture because they function by killing off and inhibiting some of the bacteria species that are required to convert ammonium nitrogen to nitrate nitrogen.  Depending upon soil conditions, these products might slow the process by 2 or maybe up to 3 weeks with a spring application.  Ideally a nitrification inhibitor would be timed with a nitrogen application and crop growth so that as the conversion to nitrate nitrogen ramps back up the corn plant is at a growth stage (V-10) where it needs lots of nitrogen and will rapidly take up nitrogen before it could be leached. 

A University of Kentucky Extension publication on the topic of nitrogen inhibitors written by G.J. Schwab and L.W. Murdock from the Department of Plant and Soil Sciences said that: “In order for these new products to be agronomically useful, the producer must reduce the rate of applied nitrogen by the amount expected to be saved as a result of using the additive.

To be economical, the cost of the saved N must exceed the price of the additive.”  In other words, nitrification inhibitors are less likely to show an economic benefit when high nitrogen rates are used.

Denitrification occurs when nitrate nitrogen is converted to a gas and escapes into the atmosphere.  This conversion generally happens when soils are water logged and lacking in oxygen.  The other avenue of nitrogen loss into the atmosphere is through ammonia volatilization.  This volatilization process can occur when urea fertilizers are applied to the soil surface or it anhydrous ammonia is not properly injected into the soil.  Soils contain an enzyme called urease which converts the nitrogen to ammonia gas.  If that conversion happens below the soil surface the ammonia gas can be held by soil particles, but if the conversion happens at the soil surface, the ammonia gas can be lost into the atmosphere.  Soil temperature, soil moisture, surface residue, soil pH and the time between surface application and a rainfall or irrigation event determines the extent of the loss.

Urease inhibitors work by inhibiting the urease enzyme for a limited period of time, somewhere around 14 days.  If a rainfall event occurs during that period and moves the urea down into the soil volatilization losses can be avoided or minimized.  Urease inhibitors are most likely to produce an economic benefit when urea fertilizers are surface applied under warm conditions and/or in high residue situations.  Dribble applications of liquid UAN in high residue fields are another situation where adding a urease inhibitor could be beneficial.

For more information about nitrogen inhibitors, contact the Wayne County Extension office at 330-264-8722.