Leaching deep into the ground, gassing off into the atmosphere, soil denitrification from microbial activity — all ways you can lose your valuable nitrogen fertilizer.
Finding ways to reduce these losses and help producers get more value from their N fertilizer is crucial, and that begins with understanding how these losses occur, said researcher Fabian Fernández at the recent Manitoba Agronomists Conference in Winnipeg.
Ammonium volatilization usually occurs with urea but can occur with any N source that will transform to ammonia, Fernández, a professor with the department of soil, water and climate at the University of Minnesota, told the conference.
When urea comes into contact with soil water it produces carbon dioxide and then ammonia. If that transformation occurs on the soil surface a lot of the N is lost as ammonia gas.
Fernández shared some research from 2014 which showed that urea applied at the surface lost 50 per cent of its N into the air after 25 days, and when applied at one inch below the surface there were still significant losses. “This study would suggest that producers need to be looking at incorporating the urea into the soil to a depth of at least two inches to protect that investment from getting lost into the atmosphere,” said Fernández.
Volatilization losses are often highest in moist soils that are drying quickly in the spring. Dry soils are not usually as prone to volatilization losses, but high soil pH can also enhance the process and lead to greater losses.
The biggest potential for volatilization is where producers have a lot of residue on their fields. The more urease enzymes that are in the soil, the greater is the potential for the breakdown of urea and volatilization of ammonium. Crop residue typically has 20 to 30 times more urease concentration than the soil underneath. “That is one of the reasons why we incorporate urea into the soil, not only to help ammonia be retained in the soil, but also to pull the urea away from the high concentration of urease that you typically have on crop residues,” said Fernández.
Urease is an enzyme that is present in the soil, especially in crop residue, and it’s extremely persistent because it’s hard to break down. Soil temperature has a big impact on that process. At high temperatures — around 29 C — urease breaks down at double the rate it will at 1 C, but even at lower temperatures there can still be losses due to volatilization.
“A fall application in cooler temperatures might be better to reduce losses, but it is still better to incorporate the urea into the soil to protect it from volatilization,” said Fernández.
Urease inhibitors — such as the trade names Agrotain and SUPERU, from Koch Agronomic Services — contain thiophosphoric triamide (NBPT) which blocks the reaction that turns urea into ammonium carbonate, reducing the amount that converts to ammonia and escapes into the atmosphere.
The N form will have an impact upon how effective urease inhibitors and nitrogen stabilizers are.
“If I had to make a decision about where to put my money, urea would be No. 1 and UAN No. 2 because with UAN you have less potential for volatilization losses than you have with urea,” Fernández said. “One hundred per cent of urea is subject to volatilization, whereas only a portion of the UAN is in urea form.”
Studies in Illinois have looked at different broadcast methods with different N sources and inhibitor or stabilizer products, and found that when applying N on the soil surface, a product with NBPT will help protect it from volatilization, but dribble banding the N is better and injecting it below the surface is best for preventing volatilization losses. That said, if producers are able to inject the urea into soils they may not need to add the inhibitor product as they are already better protecting the N source, said Fernández.
An inhibitor is definitely a good investment when broadcasting urea, in no-till situations where there is a lot of residue, in sandy soils, under high pH conditions and in moist spring soils that dry out quickly.
Nitrification is the conversion of ammonium to nitrate by bacteria in the soil making it vulnerable to leaching and denitrification.
Nitrogen stabilizers such as N-Serve and eNtrench from Dow Agro Sciences Canada contain nitapyrin, which delays the process of conversion. N-Serve is for use with anhydrous ammonia, and the eNtrench, which is encapsulated, can be used with urea, UAN and also manure.
N stabilizers degrade slower and so provide better protection for the N source in high organic matter (OM) soils than in low OM soils. It’s best to apply these products at lower temperatures, because they break down more slowly and bacterial activity that increases nitrate formation is also lower. Data from Illinois studies show that using an N stabilizer increased the ammonium recovered at around 16 C to 20 C, whereas in a test strip where none was applied only half the ammonium remained — the rest had converted to nitrate.
“As farms get larger it is a challenge to find a window of opportunity to apply N in the fall,” Fernández said. “Temperatures vary day to day and year to year, but trying to ensure that temperatures are lower than 10 C, and likely to keeping getting cooler when N is applied, is important because at this temperature the relative amount of nitrate accumulation goes down quickly, but will increase if it gets warmer.”
Nitapyrin doesn’t move far in the soil — typically no more than three inches from the injection site.
“It stays where you put it in the soil,” Fernández said. “In studies, a majority of the nitapyrin was found within one inch of the application point, and in drier soils it will protect a smaller amount of N than when it is applied under adequate moisture conditions. It’s important to know how these products work because how much of the N you are applying will be protected by that product is nothing to do with the chemistry or the atmosphere changing, it has everything to do with the soil conditions.”
ESN polymer-coated N is a slow-release, granular N product that also has demonstrated a good ability to protect against N losses from leaching, volatilization and denitrification.
Whether producers choose to use a fertilizer enhancer not, in general, a spring fertilizer application is more beneficial for keeping N in the soil for the crop than a fall application because there is a shorter window of time for potential N loss, said Fernández. When they do use these products they shouldn’t be lulled into a false sense of security.
“Ammonium is not stable. When we talk about stabilizing N it’s not stable for good, and we still need to use good agronomy with all of these technologies,” he said.