Nitrogen – I’M LOSING IT!

Fertilizer is expensive. For many farm operations, fertilizer is the chief expense and nitrogen typically accounts for the largest portion of that expense. Needless to say, nitrogen fertilizer is very important, and making sure you’re using the correct amount, in the right form, at the right time, and with precise placement are all equally important so you don’t lose it. Nitrogen can be added to crops as a synthetic fertilizer, as animal manure, and through nitrogen fixation by a legume.

Plants take up nitrogen in two forms: ammonium and nitrate.  This is important because if the nitrogen that has been added is not in either of these two forms it must undergo a chemical reaction before it is plant available.

In Western Canada, synthetic fertilizer comes in three commonly available forms: urea, urea ammonium nitrate (UAN), and anhydrous ammonia.  All forms of fertilizer are at risk of being lost in certain situations and conditions which include: leaching volatilization, and denitrification.

Urea must be converted to ammonium in the soil by the urease enzyme. It will then convert from ammonium to ammonia (NH3), which can gas off under high pH. With low pH it can convert from ammonium to nitrate in a reaction driven by Nitrosomonas and Nitrobacter bacteria.

Urea ammonium nitrate (UAN) is half urea and half ammonium nitrate. Nitrate (25% of total N) is immediately available for plant use. Ammonium (25% of total N) is rapidly oxidized by the soil bacteria, forming nitrate. The urea portion is converted to ammonium from the urease enzyme. This will then convert from ammonium to nitrate under ideal soil conditions.

When anhydrous ammonia (NH3) is applied to the soil it reacts with hydrogen ions as well as water molecules to make ammonia.  This creates a cylindrical zone about 2-5” in diameter.  This zone means that placing this product deep (at least 2.5-3 inches) is important so that the NH3 in this zone does not gas off.  Anhydrous ammonia turns to a gas at atmospheric pressure; therefore, it is important that good ground closure is achieved to keep it from being lost into the atmosphere. NH3 retention depends also on soil texture, row spacing, and soil moisture.  Sandier soil tends to have less soil NH3 retention as compared to clay soils.  Wide row spacing has less retention as compared to narrow row spacing.  Fifteen percent soil moisture has more NH3 retention than soil that is too wet or too dry.  Under low soil pH, the anhydrous ammonia has a higher likelihood to convert to ammonia, and from that point a portion of the ammonia will be converted to nitrates from the same reaction that happens in urea.  (Soil Fertility and Fertilizers: An introduction to Nutrient Management. Page 147-148).

Why does the right amount matter?  It matters because nitrogen is such a large percentage of your variable expenses. Using the correct amount will help you achieve your desired yields at the same time not over or under applying N.  This can be solved with soil testing before applying fertilizer.

Why does the correct time matter?  It matters because if you’re applying your fertilizer at a time where the soil is at risk of becoming saturated you risk nitrogen denitrifying and being lost to the atmosphere.  A nitrification inhibitor will lower your losses to denitrification.

Why does the form of nitrogen matter?  It matters because ammonia will gas off, nitrates will leach and they will also undergo denitrification.  All nitrogen that undergoes a transformation through the ammonia stage is subject to volatilization, and unfortunately, that accounts for all three forms of nitrogen.  UAN is 50% urea, and 50% ammonium nitrate; this form is nice because only half (urea) is very susceptible to volatilization, and the other half (ammonium nitrate) is plant available right away.  For this reason many people have decided that UAN is the right form for their farm.

Why does placement matter?  The fertilizer we use today should not be broadcasted or placed near the surface without applying a nitrogen stabilizer because it is at risk of volatilization.  A urease inhibitor will keep your urea from changing to the ammonia and ammonium form, and allow moisture to wash it deep enough in the soil profile so that it is not lost to volatilization.

Fertilizer is expensive.  Nitrogen is important.  LET’S NOT LOSE IT.


Written By: Duane Horvey