Nitrogen fertilizers

Key points

Nitrogen loss pathways

Several factors affect different nitrogen loss pathways:

• Denitrification - occurs when denitrifying bacteria convert nitrate (NO₃^- ) to nitrogen gas (N₂ ) or nitrous oxide (N₂O) under warm and wet conditions.
• Volatilization - is the gaseous loss of nitrogen through the conversion of ammonium (NH₄⁺ ) to ammonia (NH₃ ). The risk of volatilization losses increase with surface-applied fertilizers and manures, liquid vs. solid fertilizers, high temperatures, moist soil surfaces and high pH soils.
• Leaching - Nitrate is a soluble form of nitrogen that readily moves with water. When nitrate moves out of a crop's rooting zone, not only is it lost to the crop, it can contaminate drinking water, impact aquatic life and contribute to the hypoxic zone in the Gulf of Mexico.
• Harvest - Large amounts of N are removed through harvest, but it's difficult to calculate a nitrogen budget due to transformations and movement.
• Ammonium fixation - occurs when plant-available nitrogen is used by microorganisms and is subsequently unavailable to the crop. Eventually, that nitrogen will be mineralized and become plant-available again.
• Runoff or soil erosion - Typically, N loss through runoff or soil erosion is not a large portion of the overall soil N budget, but it can impact water quality.

Estimates of how much of the applied fertilizer nitrogen might follow a particular loss pathway are included in the following table.

Nitrogen fertilizer sources

• Industrial fixation - The Haber-Bosch process converts atmospheric nitrogen (N₂) to ammonia (NH₃ ). This process requires a great deal of energy and is the basis for synthetic nitrogen fertilizers.
• Anhydrous ammonia (NH₃ ) - N content = 82% (82-0-0). This fertilizer must be handled with the proper safety precautions and equipment. It must be injected at least 4" below the soil surface, since it can be readily lost through volatilization.
• Urea (CO(NH₂)₂) - N content = 46% (46-0-0). Urea is the most widely used fertilizer, due to favorable manufacturing, handling , storage and transportation economics. Volatilization is a significant concern when urea is applied to the soil surface and not incorporated. Placement with the seed can cause significant damage.
• Slow-release urea fertilizers - can be useful when there is high potential for leaching or volatilization and are designed to match N release with greatest N need. These include polymer-coated (PCU) and sulfur-coated (SCU) ureas, urea-formaldehyde and methylated urea. They are more expensive per unit N.
• UAN solutions (various mixtures 28-32%) - 28% N solutions used in Minnesota. Solution is approximately half urea and half ammonium nitrate. Do not fall apply, since the nitrate-N (25%) is susceptible to leaching. UAN is also susceptible to volatilization. Advantages include uniform application, compatibility with many pesticides and can be applied through irrigation systems.
• Other inorganic N fertilizers
  • Ammonium nitrate (33% N) - Use decreasing; this N form is explosive.
  • Ammonium sulfate (21 % N) 21-0-0-24 - Economical where S is needed; most acidifying N fertilizer.
  • Ammonium phosphates (10-20% N) & ammonium thiosulfate (12% N) - are primarily P and S sources. N should be accounted for when calculating fertilizer rates.