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Controlled Release Nitrogen: Another Tool in the Toolbox

With harvest wrapping up this fall our attentions are being drawn to the next growing season already. Now’s the time to choose what seed hybrid to plant, which tillage method to implement, how much fertilizer to apply, and in some cases what source of that fertilizer to use. Controlled release fertilizers (CRF) have been on the market for some time now and have shown good results in certain situations. Before you jump in, consider how a CRFs work and whether it would be a good fit for your operation.

What are the main forms?

CRFs fall under a broader group of fertilizers called enhanced efficiency fertilizers (EEFs). This group includes:
  • Controlled release fertilizers. Physical barrier such as a resin or polymer. Release affected mainly by temperature, but also by thickness of the coating, moisture, handling, and placement. Rate of release is fairly consistent. 
  • Slow release fertilizers. Microbial or chemical barrier effected by temperature, moisture, soil pH, and microbial activity. Release rate hard to predict. E.g. urea-formaldehyde and isobutylene-diurea
  • Stabilized fertilizers. Additives that have no effect on the fertilizer. Keep the fertilizer in the urea or ammonia form longer. E.g. nitrification and urease inhibitors. 
We see many forms and products of CRFs on the market, especially for horticultural and turf use. In commercial agriculture there are a few that are widely used, including ESN® (Environmentally Smart Nitrogen, Agrium, Calgary, AB), which has a polymer coating. 

One thing to remember is that each CRF is made at a manufacturing facility and not at the local fertilizer dealer. Fertilizer dealers need to buy a select amount from the manufacturer and use that as their supply. That means some fertilizer dealers may or may not carry certain CRF products or have enough product on hand.  

How does a controlled release fertilizer work?

Controlled release fertilizer is a water-soluble fertilizer, like urea, covered with some sort of coating, which can include resin, wax, sulfur, or a polymer. These coatings protect the fertilizer inside from external factors, like water in the soil. Depending on the product and several other factors, the coating delays the release of the fertilizer for an extended period. The idea is that fertilizer gets supplied to the crop as it needs it, instead of in one or two shots.

Factors that affect rate of release

Thickness of the coating. This plays a large part in determining when the fertilizer will be released and how fast. Water in the soil needs to move through the coating to dissolve the fertilizer inside, then back through the coating into the soil. Thus, the thicker the coating, the longer it takes for fertilizer to release. 

Temperature and moisture surrounding the CRF. Since we know that the rate of release depends on water moving through the coating, we know that the amount of moisture we have effects that process. When we have dry conditions, it is hard for any nutrients to move through the soil, let alone through a semipermeable coating. Research has also shown that the higher the temperature, the higher the rate of release, regardless if the fertilizer has soil contact or not. One study has shown that with an increase of 20 degrees Fahrenheit, the CRF released nutrients 2 time faster. With a decreased temperature of 20 degrees Fahrenheit, the release rate slowed almost 4 times the amount as the control (Chissoasahi, 2005). Therefore, it is very important when storing CRFs to keep them in a cool dry place. 

Placement in or on top of the soil surface. Since temperature and water affect the rate of release, we recommend having good contact with the soil while applying a CRF. Depending on the time of year, tillage system, and other factors, the soil temp and moisture can be drastically different than the air temperature and moisture. By incorporating a CRF, you can assure good contact with the soil on all sides of the granule giving it a more uniform environment. However, if you’re in a situation where you must top dress, making sure you’re getting the best contact with the soil is important. 

Handling. Be extra cautious when handling CRFs because the coating can easily fracture. Fractured coating makes the product highly soluble again like a regular urea fertilizer. Handle bags with care, keep equipment clean and without rust or cracks, and use a minimal impact application method.

Where and why to use CRFs 

There are several situations where using a CRF has shown promising results, including:
  • A no-till field where you’re unable to incorporate granular fertilizer. A coating can protect your fertilizer from volatilization losses early in the season. 
  • Low organic matter soils which are unable to hold on to nutrients for extended periods of time. A CRF is a good option if you’re unable to side dress later in the season. It can help you apply all your nitrogen at the beginning of the season with a lower risk of losing much of it to nitrate leaching.
  • Or, fields that are saturated for extended periods of time. A CRF in this situation can also help with volatilization losses, by keeping that fertilizer protected. 
Controlled release fertilizers can be a great option when you’re looking for an alternative to a regular fertilizer that is highly soluble. Use them in certain situations where volatilization and nitrate leaching is a concern. If you’re interested in a CRF but are unsure of the extra cost, consider doing a replicated strip trial to see if they could be of any benefit to your operation. 

By: Anne Nelson, Extension Educator

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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).

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