(This first appeared in the Wyoming Livestock Roundup in April, 2015)
Ah springtime! Birds are singing, days are longer, even in Laramie it’s warming up, and our thoughts turn to…. fertilizer. Well, if it’s not the first thing on your mind, it’s probably close. That’s because if you’re an ag producer you have to lay out a lot of money for plant food. Here’s some food for thought: half or more of the nitrogen you apply can escape from the soil as ammonia gas; a process called volatilization. Nitrogen has many forms during its journey through the nitrogen cycle, including organic forms in plants and microbes, soluble mineral forms (nitrate and ammonium) used by plants, and gases like di-nitrogen (most of our air) and ammonia – many opportunities for loss. Cycling of our other big fertilizer investment – phosphorus – is less complicated. With no gaseous form, it transforms among organic, fixed, and plant-available forms. Phosphorus fixation in calcareous soils is another source of heartburn for growers, but that’s a different story. This article describes factors that cause ammonia volatilization of nitrogen fertilizer, and how to minimize those losses.
Nitrogen in any type of fertilizer can be lost to volatilization, but the problem is most prominent, and most studied, in urea and urea-containing fertilizers (like UAN 28 and 32 solution). Each transformation in the nitrogen cycle is mediated by soil microbes, so conditions that favor microbial activity – the same optimum temperature and moisture as for plant growth – accelerate nitrogen cycling, including ammonia volatilization. When urea hits the soil it must go through a process called “hydrolysis” to become available to plants as ammonium . This process is catalyzed by a natural enzyme from soil microbes called urease. Hydrolysis raises pH in the immediate microsite around the urea, which favors ammonia gas over ammonium. Thus volatilization is a bigger problem for alkaline soils (like yours!) than for acidic soils in higher rainfall regions.
If warm temperatures speed volatilization, then putting down urea on cool days in fall or early spring, or on the snow, should be a safe practice, right? Wrong. For many years that was assumed to be so, but recent research in Montana shows that as much as 44 percent of nitrogen applied as urea between October and April was lost to volatilization, and the average loss across 23 study sites was 16 percent. This is enough to really hurt yields and represents money drifting up off your field and out of site in the Wyoming wind. Even though the cool temperatures slow down volatilization, all the other transformations, including plant and microbial uptake, are slowed down too, so ammonium is left exposed to the volatilization pathway for a long period. Surface crop residues also increase potential ammonia volatilization because they keep the soil surface moist, reduce the amount of urea diffusing into the soil, and have high urease activity.
Minimizing nitrogen volatilization
Choosing a nitrogen source with lower risk of volatilization is one way to minimize losses, but of the two with low risk factors, one (liquid anhydrous ammonia) is not widely used in Wyoming, and the other (ammonium nitrate) has limited availability because of explosive properties. Other alternatives to urea, like 28 or 32 solution, ammonium sulfate, and others each have lower volatilization potential, but in alkaline soils under the wrong environmental conditions the potential is still high. The Montana study showed that urease inhibitors like NBPT, the active ingredient in Agrotain®, slowed volatilization, especially under cold dry conditions. Other enhanced-efficiency urea fertilizers can slow volatilization and can work well for no-till or perennial crops under the right conditions, but proper placement and timing of fertilizer is the best route.
Broadcasting without incorporating to at least two inches should be avoided. The best alternative to incorporating fertilizer is to broadcast it just before a single rain or irrigation event of at least ½ inch; enough to dissolve and carry the fertilizer into the soil. Subsurface banding to at least two inches can be effective, but the slit produced by banding needs to be closed tight to avoid loss of ammonia from the concentrated band of urea. In conservation tillage or no-till systems fertilizers should be knifed into the soil below the residue and the slit closed tightly. That also goes for fertilizing perennial pastures or hayfields. Applying liquid nitrogen with a spoke-wheel injector has been effective for these situations.
Montana State University has produced some excellent guides on managing nitrogen fertilizers to minimize volatilization. You can access them by going to http://www.uwyo.edu/soilfert/ and clicking on Soil Links> Montana State University Soil Fertility Program>Ammonia Volatilization.