Features Agronomy Tillage
Surface applied UAN losses can be surprisingly low

With ammonium nitrate (AN, 34-0-0) availability extremely limited, farmers have had to turn to urea granular or urea ammonium nitrate (UAN) liquid fertilizer for surface application. While both these products are subject to volatilization losses to the atmosphere when applied to the soil surface, research by the Indian Head Agricultural Research Foundation (IHARF) and Agriculture and Agri-Food Canada has found that volatilization losses from UAN may not be as much of a concern as initially feared.

“While surface applied UAN is more susceptible to volatilization losses than in-soil applications of UAN banded, our results show that except under unfavourable environmental conditions, losses from surface dribble banded UAN are probably quite low,” says IHARF researcher Chris Holzapfel.

Figure 1. Ammonia losses from surface dribble band UAN during the growing season at various locations. Source: Holzapfel, IHARF.

Factors affecting volatilization
Volatilization occurs when surface applied urea (CO[NH2]2) converts to gaseous ammonia (NH3) and is lost to the atmosphere. Volatilization of surface applied UAN depends on several soil and environmental factors.

Soil and air temperatures are two important factors, with warmer temperatures potentially resulting in higher losses. Soil moisture and rainfall are also important, and previous research has found that as little as 2.0mm of rainfall within two hours of a surface application reduced losses by 15 percent, while 12mm of rainfall within two hours can reduce losses by 81 percent. If rain occurred within 24 hours, losses were higher; 2.0mm of rain reduced losses by just six percent, while 12mm of rain reduced losses by 33 percent.

Holzapfel says the dynamics of soil moisture throughout the soil profile is the most important environmental factor. Looking at soil moisture dynamics, he explains that the net direction of water movement in the soil profile is important. For instance, extended periods of evaporation pull water up from deeper soil profiles carrying any NH3 or NH4 in the soil solution towards the soil surface and increase losses. In contrast, when precipitation occurs, soil water tends to move downward, taking any ammoniacal N not adsorbed to soil and organic matter surfaces along with it.

“High air temperatures during the period following the surface application can increase volatile N losses, but this is not so much an effect of the high temperatures themselves,” explains Holzapfel. “Rather, it is probably more a result of the increased evaporation that occurs during high temperatures and prevents the N from entering the soil.”

The effects of temperature are also due to more rapid urea hydrolysis, reducing the length of time available for the fertilizer to be moved into the soil before it is susceptible to being volatilized. In addition, increasing temperature increases the rate of diffusion of NH3 from the soil solution into the free atmosphere. However, the extent to which temperature can affect the absolute quantity of fertilizer N volatilized is probably fairly limited.

Soil pH and soil texture are also important. The risk of volatilization is lower in low pH, fine textured soils than it is for high pH, coarse textured soils. Holzapfel explains that the potential of loss increases substantially as the soil pH approaches 8 and becomes negligible below pH 7. Losses are higher on coarse textured soils because they have relatively low cation exchange capacity (CEC), which limits the amount of ammonium (NH4+) that can be absorbed by the soil.

“The worst case scenario, which farmers should avoid when surface applying UAN, is when the soil is wet and hot, and dry conditions follow,” explains Holzapfel.

Measuring the losses
Since little research has actually measured the losses of a surface dribble band UAN application, Holzapfel conducted a three year trial to try to quantify the losses. Ammonia losses were measured from plots in spring wheat or canola at Indian Head, Swift Current, Brandon and Ottawa. Four separate treatments were compared, including a check with no fertilizer N, a split application with a portion of N applied at seeding and the balance dribble banded UAN early in the growing season, a split application with the dribble band UAN applied later in the growing season, and all N applied at the time of seeding. The actual amount of fertilizer varied by year and site.

Volatilization losses from UAN dribble band were surprisingly low. Photo By Bruce Barker.

Four NH3 chambers were placed in each plot after seeding and before applying post-emergence UAN. The chambers measured cumulative NH3 emissions from the soil, and by comparing losses from the fertilizer treatments to the check, the quantity of fertilizer N lost was measured.

Based on the first two years of results, with the final analysis pending, Holzapfel says losses were surprisingly low. Looking at the trends, he says that the quantity of N lost as NH3 from surface applied UAN rarely exceeded 10 percent and in several instances, was less than five percent. Holzapfel also notes that the measurement methods may have over-estimated the losses, since the chambers were covered and did not allow rainfall to move the fertilizer down into the rooting zone where it would have been better protected from losses. Holzapfel says that except when moisture conditions at seeding prevent banded fertilizer from being firmly sealed beneath the soil surface, no volatilization losses from urea banded in the soil at seeding should be expected.

To put those losses in perspective, if 70 pounds of N was applied in a post-emergence dribble band, a 10 percent loss would be roughly seven pounds per acre.

Holzapfel notes that while surface applied UAN is more susceptible to gaseous loss than soil placed UAN, his results to-date show that except under unfavourable environmental conditions of low rainfall and high temperature after application, NH3 losses from surface dribble band UAN are probably quite low. Additionally, all of the western Canadian sites were conducted under no-till conditions with crop residues left on the soil surface. Residues on the soil surface decrease the efficiency of surface applied N, which may explain why fertilizer placement is often more important in no-till than conventional tillage systems. -end

The Bottom Line
We struggle with placement/application/types of N every season – and although it is comforting to see this story, I think this could be replicated a dozen more times and find different results each time. Environmental conditions, soil type, residue and timing all play such a significant role that no study can ever determine a ‘hard’ result or trend. The environment trumps all. Kenton Possberg, Humboldt, Saskatchewan.

It is comforting to know losses may be less than we always thought, however any loss at today’s fertilizer prices and crop values is too much. The technology is available to achieve zero losses and I believe almost all of western Canada is either there or planning on being there as soon as they can. We also have at our disposal enhanced efficiency fertilizers albeit at an increased cost. John and Lisa Huvenaars, Hays, Alberta.

There has always been concern and debate over nitrogen losses when applied to soil surfaces.
Even this study admits there are still several ‘if ands and buts’ like the exposure to Mother Nature and ‘unfavorable environmental conditions’ around application times. To me it seems unnecessary to surface apply UAN when there are systems available to inject or band nitrogen and eliminate exposure. Ian McPhadden, Milden, Saskatchewan.