• Different nitrogen fertilizers are used for tilled or surface applications.Plowed field image by Marek Kosmal from Fotolia.com
  
  Nitrogen fertilizers provide a nitrogen source for plants. Fertilizers are broken down by microbes and weathering to provide nutrients. The main types of nitrogen fertilizers are anhydrous ammonia, urea, ammonium sulfate and ammonium nitrate. Each type has its own advantages and disadvantages.
  
  

Anhydrous Ammonia

• Anhydrous ammonia is 82 percent nitrogen. It converts to a nitrate that is usable by plants more slowly than other forms of nitrogen fertilizers. It is less likely to be lost by leaching or denitrification by bacteria (which causes production of gaseous nitrogen, which then evaporates, or volatilizes, from the soil). Anhydrous ammonia must be injected into the soil in liquid or gas form, and it is a hazardous material that requires special handling. It is a gas at normal temperature and atmospheric pressure and must be liquefied by pressurization. Other forms of nitrogen fertilizers are made from anhydrous ammonia.
  
  

Urea

• Urea is about 45 percent nitrogen. It converts to nitrate in less than two weeks. It is quickly denitrified by bacteria in compacted or wet soils, and is easily leached out of coarse or well-drained soils. Urea that is not tilled into the soil can lose nitrogen to the atmosphere by volatilization. Urea is made by combining carbon dioxide with anhydrous ammonia. Urea is commonly combined with ammonium nitrate in fertilizer solutions (UAN).
  
  

Ammonium Sulfate

• Ammonium sulfate is 21 percent nitrogen and is called for where additional sulfate fertilization is also required. Ammonium sulfate is not prone to volatilization, but it acidifies soils and requires the application of lime to neutralize it.
  
  

Ammonium Nitrate

• Ammonium nitrate is 34 percent nitrogen and quickly converts to nitrates usable by plants. It is prone to leaching and denitrification but is good for surface applications. It has an advantage over anhydrous ammonia because it can be applied in a dried broadcast form.