Wetter conditions in recent years have led to more questions about polymer-coated urea. Polymer-coated urea is a urea granule that is coated with a polymer that slows the release of urea into the soil. The most widely available polymer-coated urea product on the market for production agriculture is Environmentally Smart Nitrogen (ESN). The most important thing to understand about polymer-coated urea is that it is not an inhibitor like nitrapyrin or dicyandiamide (DCD).
Temperature and moisture
With polymer-coated urea, urea is released by diffusion. Water diffuses through the polymer coating and dissolves the urea inside. Over time, the urea diffuses through the coating into the soil where it converts to plant-available forms of nitrogen (N).
There are a couple of things to note with this process. First, the coating does not itself break down, releasing the urea. You can often still find the empty coatings in the field at the end of the growing season, but they will eventually decompose. Second, while soil moisture is important for the process of diffusion, how quickly the process goes is controlled by temperature. Colder temperatures will slow the release of urea.
It’s a good idea to consider temperature and moisture when deciding if, when and where polymer-coated urea may fit into your nutrient management plan. For example, if you are applying N early in the spring when it is cold, to crops such as winter wheat, polymer-coated urea probably shouldn’t be your sole source of N. Similarly, while temperatures are higher in June when most sidedress N application for corn is done, using 100% ESN might not be the best option since the goal is to have at least some N immediately available. On the other hand, pre-plant applications with ESN can be useful to protect N in areas of the field where N loss is most likely to occur, such as a lower area that tends to be wetter.
Fall versus spring application
One reoccurring question about polymer-coated urea is whether it is a better source of fall urea than urea itself. Our research shows that fall urea application is not recommended in Minnesota, especially if you’re broadcasting and incorporating it with tillage. In our studies, fall urea applications lead to significant yield losses compared to spring application, and fall application requires more N fertilizer to maximize yield than spring application. Even when we sub-surface band urea in the fall (with or without a nitrification inhibitor), the yields are never higher than a spring pre-plant broadcast and incorporated urea application.
In theory, fall application of polymer-coated urea should perform better than uncoated urea; however, evidence is limited. Across many studies in several different regions of Minnesota over the last five years, we saw that fall application of ESN increased corn yield only about 25% of the time compared to fall urea. Still, averaged across the entire study, fall application of ESN produced lower yields compared to a spring pre-plant urea application. Of course, in these studies, the fall applications were done after soil temperature stabilizes at 50ºF or colder. Fall application of urea in any form (uncoated, polymer-coated or with a nitrification inhibitor) is problematic, regardless of placement method.
Minimizing cost, maximizing benefit
Another issue with polymer-coated urea is that it costs more per pound of N than urea. Therefore, it has to provide a benefit in terms of cost savings from reducing your N application rate or a yield increase from lower N loss. For these reasons, it is important to carefully consider where ESN could give you the greatest benefit. In our studies, the most common situation where we see a significant return from application of ESN is when we have an excessively wet spring or soils where the potential for N loss is very high. Another thing to consider is that, when using ESN, you do not need to manage every acre the same way. Applying different blends of ESN and urea based on soil conditions can be a way to get the greatest benefit while reducing the overall cost.
If you are doing split applications, where a portion of your N rate is applied before planting and the rest at sidedress, remember that ESN will give you the greatest benefit at pre-plant, when there is greater potential for N loss. Applying urea at sidedress will ensure that the N is available to the crop when it needs it, and, since urea is less expensive, it will reduce the overall cost of the application. A seven-year study in southwest Minnesota indicated that a split application was not superior to a pre-plant application. However, compared to uncoated urea applications, using ESN in the pre-plant application (whether it was a single application or paired with a sidedress application of urea), there was a consistent increase in corn yields and a reduction in N loss. On irrigated sandy soils, one study found that split-applying urea (our recommended best management practice for these soils) performed better than a spring pre-plant of ESN without a sidedress application. However, a spring pre-plant application of ESN performed better than the same application of urea.
Can I leave ESN on the soil surface?
While ESN will not volatilize like urea if left on the soil surface, it is best to incorporate ESN into the soil, even if only just shallow tillage. The polymer coating makes the urea granule impermeable and it will float in water. If there is water runoff, ESN granules will float and potentially move off of the field or pile up in spots where the water puddles in the field.
Handling polymer-coated urea
Make sure augers and handling or application equipment are not rusty or have abrasive surfaces. If the polymer coating is cracked or worn out, you will basically be purchasing overpriced urea since the coating is no longer able to do its job. One quick way to test the integrity of the coating is to randomly collect 100 granules of your polymer-coated urea and put them in a jar of water at room temperature for 24 hours. Then count the number of empty granules where the urea inside dissolved. If five of the 100 granules dissolved, then roughly 5% of your polymer-coated urea is damaged.
