Remember the 4Rs of nitrogen fertilizing in the fall

Broadcast Manitoba Sask.


I am sure I will be getting a number of calls again this year about the fall application of fertilizers, so I thought it would be a good time to look at this through the four lenses that make up the 4Rs of nutrient management.

Right time

Is the fall the right time? The answer is that it depends. On the Canadian Prairies and Northern Great Plains, the fall application of fertilizers has generally performed well, provided the right source and right place are looked after.

Areas that should be avoided for fall applications include zones of a field that may encounter an extended period of saturation in the spring, as well as light textured soils such as sandy loam. The fall banding of fertilizers can be highly effective in drier situations.

In most cases, the later you can apply the fertilizer, the less time there is for nitrogen product conversions. Biological processes are greatly reduced once the temperatures approach 10 C, so fertilizer applications are usually initiated once this temperature has been reached. Applications can be continued until soil freeze-up.

Right place

Banding can be highly effective. In fact, it should be considered the gold standard for fertilizer applications on the Prairies and Great Plains. All other treatments should be compared to this placement and at best are usually only as good as banding. When we say banding, it is implied that we are placing the fertilizer in a narrow, three-quarter inch band a minimum of two inches below the soil surface. In fact, work done by Westco Fertilizer 25 to 30 years ago showed a distinct advantage to fall banding in drier years.

This is thought to be due to two factors:

  • The fertilizer is below the seed and as the growing plant’s roots moves deeper into the soil to access moisture, they encounter the fertilizer band.
  • There is an opportunity for winter snow and rain to counter any drying out of the seed bed that may occur. There is less of an opportunity if fertilizer is spring banded.

Right source of product

A wide variety of products are available in today’s fertilizer world. Here are some treatments to consider for fall applications:

  • Urea (46-0-0) has become the dominant nitrogen fertilizer source since the mid 1980s. It performs well if applied as a band in the fall. Broadcast applications expose the product to losses through volatilization and immobilization unless rainfall occurs very closely after applications.
  • Anhydrous ammonia (82-0-0 or NH3) is also an excellent source for nitrogen in the fall. Following the same parameters around applications outlined in right time and right place will ensure optimum performance of anhydrous. Application of NH3 under dry conditions where soil is lumpy may cause significant gassing-off losses of the fertilizer.
  • Urea-ammonium nitrate (28-0-0, 32-0-0 or UAN) is not a recommended product for fall application. A significant portion of this product is in the nitrate form, and losses from nitrification can be expected in the spring. As well, spraying the product on the ground leaves the urea portion exposed to volatilization losses.
  • Ammonium sulfate (20-0-0, 21-0-0 or AmSul) can be an effective product for the application of both nitrogen and sulfur. Ammonium sulfate fines can be attractively priced and effective. Apply no more sulfur than the requirements for the next three years of cropping.
  • SuperU (46-0-0) is a product that contains both a urease inhibitor and a nitrification inhibitor. These additions delay the conversion of urea first to ammonium and then from ammonium to nitrate. This will help reduce losses to volatilization and denitrification. These products are applied at the time of granulation of the urea. It can be effectively applied as a broadcast. However, it should not be applied on wet, frozen ground.
  • ESN (44-0-0) is a urea product encased in a polymer coating. Fall band application worked very well in research we conducted at the University of Minnesota. You can blend your phosphorus, potassium and ESN and apply your entire fertility program in a strip-till operation in the fall.

Another option is to apply most of the nitrogen as ESN in the fall but hold back some portion for an in-season side- or top-dress. This option requires an additional application but gives more flexibility to adjust for changing conditions and yield potential. For this option it is suggested that about 70 to 80 percent of nitrogen be ESN in the strip with the balance applied in-season according to need.

ESN is not a recommended fall application in sandy soils because it is prone to winter or early spring leaching. It is also not recommended in soil that does not stay frozen through the winter or for fall broadcasting.

  • N-Serve is a nitrification inhibitor that can be added to anhydrous ammonia. This delay will help reduce spring nitrogen losses due to denitrification and leaching. Special application equipment is required by suppliers to apply N-Serve.

Right rate

Several pieces of information are required to determine the correct nitrogen rate:

  • What is the crop to be grown? Different crops have different nitrogen requirements. However, you may not want to apply excessive rates for a crop on all your acres if you may wish to change your cropping plans. Moderate rates can be topped up in the spring.
  • What are your yield goals? Of course, higher yield goals require higher fertilizer rates. Realistic goals will ensure optimum nitrogen fertilizer utilization. An accurate yield goal can be set by using such things as available soil moisture, soil texture and long-term weather forecasts. However, we all know Mother Nature has the last say when it comes to yields.
  • Do you have nitrogen credits? These credits are applied after the production of a legume crop such as peas, lentils or soybeans. Those crops have the ability to fix nitrogen from the atmosphere and return more nitrogen to the soil when harvested. These credits will vary depending on crop type, tillage and yield. Forage crops such as alfalfa will also give a nitrogen credit, not only for the year following termination but for two to three years.
  • How much nitrogen is in the soil? An analysis of soil samples will give an accurate prediction of how much nitrate nitrogen remains. A more accurate picture comes from using production zones to guide testing. This takes into consideration the difference in productivity in your fields.

Another novel approach to taking a soil test is to use what is called a virtual soil test. This system uses an algorithm to predict nitrogen levels in a zone by using previous soil test results, physical and chemical soil characteristics, previous fertility applications and crop removal. There is a very high correlation between these results and actual soil test results, provided accurate local weather data is used.

An accurate nitrogen rate can be determined using the four steps above — hopefully not too little, not too much but just right.

Next week we will talk about the other nutrients and how they fit into a fall fertility program.

Thom Weir PAg is an agrologist with Farmer’s Edge. He can be reached by emailing thom.weir@farmersedge.ca.

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