You don’t have to tell producers: nitrogen is a concern. It’s also necessary.
It’s a beneficial resource to plants and crops, but excess nitrogen leaching into waterways has undeniable negative effects on water supplies. Farmers and applicators alike are always looking for ways to mitigate such losses while ensuring nutrients stay where they’re supposed to: with the plants they’re feeding.
Although there’s more research to be done, and new products may also be introduced in the future, there’s solid advice available at this point about how today’s nitrification inhibitors are best applied with manure in order to get the most out of the nitrogen it contains.
Manure is, of course, a valuable resource, and if products can be co-applied to maximize its ability to boost yield, then all the better – but as with any fertilizer product, application of nitrification inhibitors must be done with care to make it worthwhile.
Nitrification: A primer
Nitrification is a natural process in the soil where certain bacteria (mainly in the genera Nitrosomonas and Nitrobacter) convert nitrogen to a form that’s more available for root uptake. This conversion of ammonium (NH4+) to nitrate
(NO3-) can take a few days to weeks, depending mainly on soil temperature and water content, but also whether or not you apply a product to slow it down. That is, nitrification inhibitors do not completely inhibit nitrification, but slow down bacterial activity, providing a more gradual release of nitrate to plants.
But besides the benefits to crop performance, the amount of N that is present in nitrate form also matters to the environment.
“Because the nitrate molecule and microscopic soil particles are both negatively charged, they are not held together, so when it rains, especially excessively, the nitrate can leach down to lower levels in the soil beyond the root zone,” explains Dan Kaiser, UMN extension nutrient management specialist.
The nitrate can also leach away into nearby bodies of water. Substantial leaching can occur in the early spring (when snow is melting and spring rains arrive) from nitrate-containing fertilizers or manure applied to fields at that point. (Also note that besides leaching, in some conditions nitrate can be lost via the process of denitrification.)
Returning to our nitrification inhibitors, good results using these products with manure can be achieved if application is precise in method and timing, explains Jeff Vetsch, soil scientist at University of Minnesota (UMN). There are a few factors involved with that, mainly how long these products themselves last in the soil, so the application needs to count.
Fall nitrogen application in Minnesota.
Credit: University of Minnesota
Range of products
Before we look at best application practices, it’s important to distinguish the two types of inhibitors currently on the market that are suitable to apply with manure. (That is, other inhibitors will only truly work well with fertilizers. Vetsch says Instinct NXTGEN works well with urea, for example, and N-Serve is used strictly with anhydrous ammonia fertilizer.)
One type acts directly on bacteria, and because it does so, it’s classified as a general use pesticide in the U.S. Commercial applicators therefor need to check with their state to determine whether a commercial pesticide applicator’s license is necessary before applying it. However, in Canada, both nitrification and urease inhibitors are regulated* as soil supplements.
The well-known active ingredient in many of these products is a chemical called nitrapyrin. It has been around for decades, but new forms have emerged in recent years. Instinct and Instinct 2 both contain micro-encapsulated nitrapyrin, for example, which greatly reduces volatization. Another product with direct action is CENTURO, which targets the ammonia monooxygenase enzyme of Nitrosomonas bacteria to disrupt its N conversion in two ways.
The other type of inhibitor does not act directly on bacteria (mechanism uncertain) and the chemical here is dicyandiamide (DCD). It’s offered in various products such as SUPERU. Vetsch notes that some retailers prefer DCD products as they are not as corrosive on equipment as products with nitrapyrin can be. The makers of CENTURO, who also make SUPERU, also claim CENTURO is less corrosive on equipment than nitrapyrin.
Melissa Wilson, UMN associate professor and extension specialist, reports that the operators she’s observed using nitrapyrin “have a separate tank on top of their tank where a stream of nitrapyrin is injected into the manure stream as it’s exiting, rather than having it mixed in.”
Returning to DCD, Kaiser explains that “it was first developed as a fertilizer. The molecule has an ‘amide’ in it, which is ammonia. The concentration of N in straight-up DCD is higher than it is with urea, so on paper it would seem to be a very good fertilizer by itself, but when it was tested, there were a lot of issues with plant toxicity. But researchers also found that if you mix DCD as a certain percentage of your total N, you get some delay of nitrification.”
Comparing other factors, while DCD is more mobile in the soil than nitrapyrin, Vetsch says this is not a huge problem in medium-textured and fine-textured soils. That is, DCD products mostly stay in the root zone. “From a research standpoint, my studies have used medium to fine-textured soils and that dominates the other research as well,” he says. “There is some research in coarse-textured soils that shows better retention with nitrapyrin.”
Kaiser also notes that there’s better retention of nitrapyrin in soils with higher organic matter level and clay content. “Higher levels of these components result in better adsorption, so nitrapyrin levels stay steadier,” he says.
Nitrification inhibitors are also affected by temperature, with warmer soils having bacteria that are more active in degrading these chemicals. With nitrapyrin, there’s also evidence of interaction between soil type and temperature. Kaiser points to studies where the time it takes to get 50 percent degradation of nitrapyrin on a silty clay loam was investigated. “At temperatures 55°F versus 70°F, there’s 50 percent degradation in about three weeks,” he says. “At roughly about 50 percent degradation, we see really poor control of nitrification. But with a silt loam, and about 65°F or 70°F, you go from about three weeks to one week to see 50 percent degradation.”
A closer look at soil temperature
Because nitrification slows down in cooler weather, this means ideally, one would apply manure (or ammonium fertilizers for that matter) to fields later in the fall. As winter comes, nitrification greatly slows and then picks up again in the spring – at a slow pace if the spring temperatures rise steadily and smoothly, but very quickly if warm weather suddenly arrives. (And recall that with snow melt and spring rain, if a lot of nitrate is produced in a short time, it can get leached away to lower soil depths or lost through runoff).
In the fall however, as Vetsch explains, there’s typically not enough time and equipment availability to apply manure after the soil temperature has dropped down too low – and that’s where nitrification inhibitors are useful. Indeed, Vetsch and his colleagues have found that with application of manure and nitrapyrin to fields earlier in the fall, nitrification inhibition is roughly the same as delaying manure application only later in the fall when soil temperatures are colder.
“The old school rule of thumb is to apply manure in the fall when the temperature at a soil depth of about six inches is 50°F or below, which is usually the end of October in Minnesota,” says Vetsch. “Farther south, this will be later. However, due to equipment availability and time pressures with harvest, some manure is applied in late summer and early fall, and there are benefits of an N inhibitor in some cases. We’ve found that in early October in Minnesota when you apply manure after soybean harvest, Instinct 2 worked well to slow nitrification. Dr. John Sawyer and his grad student Aaron Sassman at Iowa State had similar results in Iowa.”
Other research found that applying manure with nitrapyrin too early, however, may not work well. When soils are still warm from the summer heat, the inhibitor breaks down quickly and nitrification resumes in the soil. There is a sweet spot where inhibitors are most effective – usually early fall when soil temperatures are going down but have not yet reach 50°F or below.
Inhibitors still work in later fall, depending on temperature, says Vetsch, and can provide some effects following spring. Kaiser notes that in the spring, “in some soils and depending on the soil conditions, we can still see some impact of these products even into mid-to-late May.”
Some solid manure with high organic matter or bedding will have two N conversion steps.
Manure application – and manure
Vetsch says Instinct 2 works best when injected. “That way, it’s applied into in a concentrated volume of soil, in a relatively narrow band,” he says. “This is much more effective than broadcast.”
Looking at manure differences, Wilson tackled this on a recent UMN Nutrient Management Podcast.
She explains that one of the complicating factors with manure is that it contains both inorganic and organic nitrogen, and this ratio changes depending on livestock species.
“In manures with a lot of organic matter, anything with a lot of bedding, there are two N conversion steps, into ammonium and then to nitrate,” says Wilson.
“But in swine and liquid dairy manures, they generally have a lot more ammonium to begin with, and there’s only one step then, from ammonium to nitrate. So with those manures, you’ll see fast conversion to nitrate in warm conditions. Timing with those in the fall is therefore particularly important. You want to wait as long in the fall as you can to apply those manures and if you can’t, an inhibitor is worthwhile.”
Looking at a few studies, Wilson also shares that more consistent results have been seen with swine manure.
Poultry manure is more complex. “It has the bedding in it, but it typically has so much N that even if it’s a small proportion that’s nitrifying, it’s still a lot more than some of our other manures,” explains Wilson. “So, we definitely recommend waiting until later to apply poultry manure as well. But remember it also has more N content.”
Wilson also reports that so far, inhibitors are only applied with liquid manures because they mix well. “There may be products launched that we can apply with dry manure in future,” she notes, “but not yet.” •
*See: https://inspection.canada.ca/en/plant-health/fertilizers/notices-industry/2019-06-07/questions-and-answers
