Spring fertilizer decisions: Should you use nitrogen inhibitors and other enhanced efficiency fertilizers?
Enhanced efficiency fertilizers, such as nitrogen inhibitors and slow- and controlled-release products, are fertilizer that has been modified to reduce losses to the environment and increase nutrient availability. These products can help farmers increase yield, save on fertilizer costs, and protect the environment, but there are some things you should keep in mind.
Inhibitors are sometimes referred to as stabilizers, but I believe this word can be misleading. In the soil, there is nothing stable about urea or ammonium. They will eventually transform to a form of nitrogen that can be lost. The most important thing to remember about inhibitors is that they delay nitrogen transformation.
Nitrogen inhibitors are classified into two main groups: nitrification inhibitors and urease inhibitors. Nitrification inhibitors such as nitrapyrin, DCD and pronitridine have been developed to delay the conversion of nitrogen fertilizers into nitrate. The reason we want to delay transformation to nitrate is that nitrate can leach below the root zone or be denitrified if there is excessive precipitation.
Urease inhibitors such as NBPT and NPPT delay the conversion of urea into ammonia. We want to prevent nitrogen transformation to ammonia as this too will end up in the environment and not available to the crop. In this case, though, the loss happens as ammonia goes off as a gas to the atmosphere.
Slow- and controlled-release products
Slow-release products are fertilizers such as methylene urea, urea triazone, and urea formaldehyde that release nitrogen through biological and chemical processes that break the molecule down to urea before it can be transformed to ammonium and then nitrate.
In contrast, controlled-release fertilizers, such as polymer-coated urea, release nitrogen by physical processes. Water moves through the coating, dissolving the nitrogen inside the coating. The nitrogen solution then diffuses through the coating out into the soil, where eventually it gets transformed to ammonium and then nitrate.
Should I use enhanced efficiency fertilizers this spring?
Regardless of which enhanced efficiency fertilizer product you decide to use, there are a couple important points to keep in mind.
First, these technologies are not bulletproof. While they delay nitrogen cycle processes that can result in nitrogen loss, their use is not a safeguard against poor management. These products degrade over time and, in general, the decomposition rate increases with warm temperatures. In the case of polymer-coated urea, while it does protect nitrogen from volatilization, it is best to incorporate it into the soil. If left on the surface, a large amount of rain can lead to runoff of the granules that float with water.
Second, just like a screwdriver is not the right tool to drive a nail into wood, enhanced efficiency fertilizers have specific functions. Using a nitrification inhibitor will not prevent ammonia volatilization any more than using a urease inhibitor will delay nitrification.
Where do enhanced efficiency fertilizers make the most sense?
The answer is wherever there is the greatest potential to protect nitrogen from loss. These products add cost to your nitrogen investment, so it is important to prioritize using them where it will produce the most benefit.
For example, there is absolutely no need to use a urease inhibitor with a broadcast application of urea that is immediately incorporated by tillage at least two inches in the soil. If you were concerned about nitrogen loss from a pre-plant application, I would not use urea-ammonium nitrate (UAN) and include a nitrification inhibitor to protect the investment. While the nitrification inhibitor will provide protection, 25% of the application is already nitrate in this fertilizer source and the inhibitor will do nothing to protect nitrate from leaching or denitrification loss. It would be a much better idea to use the inhibitor with a urea application.
Alternatively, one could consider using a polymer-coated urea or anhydrous ammonia without an inhibitor since anhydrous ammonia itself provides some level of nitrification inhibition as it kills nitrifying bacteria within the ammonia retention zone of the soil. This does not mean that a pre-plant application of anhydrous ammonia will not benefit from a nitrification inhibitor. It only means that the chance of seeing a benefit with a nitrification inhibitor is lower.
Similarly, using enhanced efficiency fertilizers when there is low potential for nitrogen loss does not mean that the product does not work as well, it simply means that the additional cost is probably not justifiable. In other words, you applied something that you did not need. On the other hand, using a urease inhibitor when urea is surface-applied in a high crop-residue field, or a nitrification inhibitor or polymer-coated urea in fields or portions of fields that tend to be wet, is most likely to increase the chance that you see a benefit from your investment.