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The Three Biggies: Urea, Anhydrous Ammonia, and UAN

Fabián G. Fernández

Nitrogen (N) fertilizer is extremely important for crop production. There are many sources available in the marketplace, but the three most important in order of tonnage sales for Minnesota are urea, anhydrous ammonia, and urea ammonium nitrate (UAN). Since many are very busy applying N and doing other field operations at this time, my purpose is not to go into a lengthy discussion on N sources but I thought it would be good to review a few important points.   

Urea [CO(NH2)2]

This source is 46% N (46-0-0) all in urea form. This source is easy to use and transport and has become in recent years the number one N source for Minnesota. Urea is very soluble and moves in the soil in any direction water moves.  There have been years, like 2014, when within a couple of days after application several inches of rain have fallen. In this situation, it is possible to get N loss similar to nitrate loss as urea moves with the water. Luckily, the chance for several inches of rain to fall soon after urea application is not very high and urea transforms quickly to ammonium (NH4+) after application, which will not leach.

However, we need to remember that volatilization losses can be important for this N source. After application, NH2 changes to NH3 either chemically or by the enzyme urease, and then to NH4+. The speed of this conversion depends largely on temperature and the amount of urease, which is present in large amounts in crop residue. Conversion is slow at low temperatures but with current temperatures, the conversion will occur within a few days after application.

If the conversion of urea to ammonium occurs on the soil surface, the surface of crop residue or leaves, or even when incorporated at a shallow depth (about 1 inch) some of the resulting ammonia will be lost to the atmosphere as a gas. Generally, when urea is on or near the soil surface the potential for loss increases in warm temperatures like the ones we are having, in soils with low cation-exchange capacity or neutral to alkaline pH, and in moist soil surfaces that are quickly drying with a warm breeze.

The best way to keep urea from volatilization is to incorporate it at least 3 to 4 inches soon (within a couple of days) after application either by tillage or with rain. Normally, to move urea down 3 inches a ¼ to ½ inch of rain is needed. If urea can’t be incorporated by tillage, it is a very good idea to use a urease inhibitor to “buy” more time for the application to be incorporated by rain. However, the use of a urease inhibitor will not protect urea from volatilization for an indefinite period of time because the inhibitor will deteriorate. Urease inhibitors like NBPT will help minimize volatilization losses for about 10 days.   

Occasionally, I get questions about toxicity issues with urea. This was a bigger problem years ago when during manufacturing a toxic byproduct, biuret, was generated. At present, manufacturing techniques have reduced considerably the generation of this byproduct, so toxicity is not a major issue anymore.

Anhydrous ammonia [NH3]

This source is often the least expensive N source and the one with the highest percent N by weight (82%) of all forms of N. Anhydrous mean “without water.” Anhydrous ammonia is a liquid when kept under pressure, but turns into gas when not contained under pressure. One of the drawbacks of anhydrous ammonia is the danger it poses to operators and living organisms in the event it escapes into the air. It requires equipment than can handle high pressure (approximately 200 PSI), and its safe transport and handling represent real challenges.

In liquid form, the weight of this fertilizer is 5.9 lbs per gallon. For most N fertilizer sources we need to account for the amount (percent) N. For example to apply 100 lb of N per acre with urea (46% N) the application needs to be 217 lb of urea per acre. While the same is true for anhydrous ammonia, most anhydrous ammonia controllers are already programmed with this calculation factored in so the prescription should be the pounds of N per acre, not the pounds of product (anhydrous ammonia) per acre. Using the previous example, if the plan is to apply 100 lb of N per acre, the prescription should be “100” not “122.” A simple mistake like this can represent 18% more N than needed. Of course, there are many different models and manufactures, so I would strongly suggest you check the operator’s manual of the unit you use to ensure you are applying the correct amount of N.

Because ammonia under pressure is a mixture of liquid and vapor, it is more difficult to ensure uniformity of application across a tool bar. Again, there are many systems available and some important advances have been made in anhydrous ammonia systems in the last several years. Whatever system you use, make sure the system is optimized to apply a uniform and accurate rate across the toolbar.

Above all, during this busy time of the year, take all the time you need to be safe when handling anhydrous ammonia. 

Urea-ammonium nitrate (UAN)

Urea-ammonium nitrate has 28 to 32% N. These materials have 50% urea, 25% ammonium, and 25% nitrate. The weight of solution per gallon is 10.70 for the 28% and 11.05 lb for the 32% solution; so one gallon of 28% has 3 lb N and one gallon of 32% has 3.5 lb N.  UAN is often dribbled or sprayed on the soil surface, but it can also be injected below the soil surface. The urea portion of UAN will undergo the same reactions as described earlier for urea. Since urea is only half of the total material, the potential for volatilization loss is less than that of urea fertilizer, but injection below the soil surface is the most effective way to minimize volatilization losses.

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  1. is it not true that urea needs both high temps AND available moisture to volatilize?
    In other words, if it is warmor hot but the granules are lying on a dry soil surface, it wont volatilize?

    1. Thank you for the question. You are correct, both temperature and moisture are important. When we talk about dry soil though, it has to be a very dry surface, which is not likely with the conditions I have seen in recent weeks.

    2. we are in central SOuth Dakota. how would I quantify dry enough soil surface to not have significant volatility?

  2. The greatest effect on urea hydrolysis occurs when soils are moist and become dry. When we talk about dry soil surface, we mean really dry. Studies have shown that at permanent wilting point the hydrolysis rate slows greatly and essentially stops in a soil that is air dry. This would be the equivalent of a soil in the middle of the summer in a drought. That's why I said that for most conditions in the spring, this would not be applicable.

  3. Can you suggest that using UAN is far most costlier than using Urea when comparing the purchase prices of both? That means, we need to buy lot of UAN liters to fulfil the N requirement of soil while we need less lbs of the Urea for the same requirement.

    1. As you correctly indicated, the same amount of N with UAN would represent a lot more material than urea, which has a higher nitrogen concentration. Fertilizer prices vary from place to place, so it is difficult to make generalized statements about the cost. That said, normally per pound of N, anhydrous ammonia tends to be the least expensive and UAN the most expensive, but again buyers should always cheque the local price and calculate the cost per unit of nitrogen and not per unit of fertilizer. The reason for this is that different fertilizers contain different amounts of nitrogen and this is the only way to make a fair comparison across different sources. - Fabian Fernandez, Extension nutrient management specialist


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