Making Decisions on Fall N Options

Oct 14, 2016

By: Thom Weir, Senior Precision Agronomist for Farmers Edge
First off, I have to make a full disclosure that I worked with and was mentored by the late John Harapiak.  I saw first-hand evidence of the benefits of banding over broadcasting nitrogen fertilizers.

I also know that on large farms, efficiency often trumps agronomy. However, in recent years, the avalanche of acres being broadcast with nitrogen products has increased an anxiety in me that farmers are not totally aware of the risks that are involved in using these products.

Firstly, any nitrogen based product applied in the fall is at risk of losses.  These losses can be put into 3 categories. The losses are leaching, denitrification and volatilization.

  1. Leaching is the movement of nitrate-N through the soil to a depth where it is no longer accessible to crops. It may also move into groundwater where it acts as a pollutant.  Soils with high water infiltration rates and low nutrient retention capacity (CEC), such as sandy soils are most susceptible to leaching losses.
  2. Denitrification is the process where nitrate-N is converted to nitrogen (N) gases that are lost to the atmosphere. Denitrification occurs when soil bacteria use nitrate for their respiration in the place of oxygen in the air. This process occurs most rapidly in warm, wet soils with an abundance of nitrate.
  3. Volatilization losses result from the hydrolysis conversion of urea to ammonia, and if the urea is not incorporated, the ammonia is lost to the air. Conditions favouring high volatilization potential are:
  • high soil temperatures
  • moist conditions, followed by rapid drying
  • windy conditions
  • high soil pH (>pH 7.5)
  • high lime content in surface soil
  • coarse soil texture (sandy)
  • low organic matter content
  • high amounts of surface residue (e.g. Zero tillage)
  • nitrogen source: urea > UAN solution > ammonium nitrate

From the above, you can see some overlap.  Both leaching and volatilization are favoured in coarse textured soils whereas denitrification is favoured by poorly drained soils.  Generally, these soils should be avoided for most fall fertilizer applications. If fall application is necessary, it should be done in late fall when soil temperatures are below 10 degrees C. Early fall nitrogen at soil temperatures above 10 degrees C can lead to significant nitrification and potential loss of nitrate-N by leaching or denitrification later in the fall or spring.

So when it comes to banding, what are the high risk scenarios for losses?  Firstly, using fertilizers that contain nitrates are immediately at higher risk for losses other than volatilization. Generally, UAN is not recommended for fall application.  Secondly, applications to sandy soils or coarse textured soils will increase the risks of losses. Thirdly, banding at a depth of less than 5cm (2 inches) will increase the chances of volatilization losses.

There are several products that are now on the market that delay the conversion of ammonium to nitrates or controlled release products. These products can reduce the risk of denitrification and leaching from fall applications but surface applications can still be susceptible to volatilization and immobilization.  These products may allow for lower risks when banding into areas not recommended above.  In addition, fall-applied surface applications will be subject to runoff during the snowmelt. However, there may be advantages to controlled release products or nitrification inhibitors over band applications of conventional sources at the time of seeding with long season crops such as corn or potatoes; with these crops, controlled release of nitrogen or reduced nitrification over the growing season may reduce the risk of denitrification and leaching, particularly on wet soils or under irrigation.

Immobilization is often referred to as a nitrogen loss mechanism but is really just a removal of the N for a short (1-2 years) period of time.  It occurs when nitrogen is applied to high Carbon to Nitrogen (C:N ) ratio crop residues.  Examples of this would include cereal stubble or corn residue.  In these situations, microbes feed on the nitrogen to aid in the breakdown of the residues.  These conditions can exist when nitrogen fertilizers are broadcasted onto stubble chaff or when nitrogen products are incorporated with various implements including heavy harrows or vertical tillage implements.

In a paper presented to the Manitoba Agronomist Conference, Dr. Cynthia Grant states “In-soil banding is still the gold standard for nutrient use efficiency in Manitoba. While enhanced efficiency products do not close the gap between in-soil banding and surface applications, they can narrow the gap if used correctly.”  She goes on to comment that surface applications will not normally be as good as in-soil bands from the perspective of nutrient use efficiency, but they may provide advantages in other areas of the farming operation, including more rapid seeding, reduced soil disturbance, less draft requirement, lower equipment and fuel costs and increased field trafficability. So back to my initial comments about agronomy and efficiencies.  However, note that I am talking about management efficiencies and not nitrogen efficiencies.

In summary, using an ammonium based fertilizer (urea or NH3), banded at least 5cm (2 inches) deep and to medium or heavy textured soils that are not prone to spring flooding will be at low risk for N losses.  The use of nitrification inhibiting products or controlled release products may reduce the risks of fall banding into less desirable situations. Use of these products for broadcasting will reduce but not eliminate the risks of volatilization losses. Surface applied products are also subject to immobilization.  Before making a decision on fall nitrogen applications, make sure you do a thorough review of the pros and cons of the various applications and make an informed decision.