Nitrogen management needs to be tailored to your specific crop rotation, types of tillage, or lack of tillage, and the needs of your crop.
Stabilizers and controlled-release products help keep the Houdini of nutrients where your crop needs it
Applying nitrogen—corn’s most important and trickiest nutrient—doesn’t do a lick of good if it escapes into the air or, even worse, into water sources before plants can use it.
Products that slow the loss of nitrogen can help you hold it in place, to feed hungry corn and boost yield—but only if you understand them well enough to choose the right one, says Farm Journal Field Agronomist Ken Ferrie.
To select the right product, consult the table on page 32. But first, decide what type of nitrogen loss—denitrification, leaching or volatilization—you need to prevent.
Denitrification and leaching occur only with the portion of your soil nitrogen that is in the nitrate form. Denitrification occurs when soil is saturated with water, especially when standing water is combined with warm temperatures and a carbon source. In those con- ditions, denitrifying bacteria increase. They strip oxygen away from nitrate-nitrogen (NO3-N) molecules, releasing nitrogen as a gas.
Sandy and well drained soils, where water percolates through, can leach NO3-N. "Stabilizers that reduce denitrification and leaching do it by slowing the nitrification process, in which ammonium is converted to nitrate," Ferrie says. "They either kill some of the nitrifying bacteria or slow down their activity."
Since you can’t protect nitrate, you have to keep it in the stable ammonium form, by preventing nitrification, until the crop is ready to use it, Ferrie summarizes. Soils that stay wet and sandy soils that are subject to leaching are situations where nitrification inhi-bitors can help.
Volatilization affects surface-applied urea, which must break down, or hydrolyze, into ammonium. The ammonium then goes through the nitrification process to produce nitrate. Both forms can be used by corn plants, but until it is either tilled into the soil or washed in by rainfall or irrigation,
ammonia can be created during the hydrolysis process and be lost as a gas.
The urease enzyme. Bio-inhibitor products reduce the risk of volatilization of urea prior to tillage or rainfall by blocking the urease enzyme. That enzyme, found in crop residue and in the upper few inches of soil, triggers the breakdown process that creates the risk of volatilization.
Managing urease enzymes is very important if urea is applied on the surface and not incorporated—which is becoming more common as farmers adopt no-till, strip-till and pre-emergence weed and feed programs. That’s because urea hydrolysis causes soil pH to go up around the nitrogen molecule; high pH speeds the conversion of stable ammonium molecules to ammonia gas, which can be lost by volatilization.
The same concern arises in soils with a naturally high pH and where lime is applied without incorporation, also a no-till/strip-till situation.
Another approach to nitrogen stabilization involves controlled-release products, which slow the breakdown of urea by encapsulating the particles. They can reduce both volatilization and denitrification.
So far, so good, but it gets more complicated in the field because you have to understand your nitrogen source. "For example, 28% and 32% UAN [urea-ammonium nitrate] solutions contain 50% urea, 25% ammonium and 25% nitrate," Ferrie says. "So you may need two products: one to help prevent the urea from volatilizing—unless you incorporate immediately after application—and one to slow the conversion of stable ammonium into nitrate, which is easily lost."
Picking the correct stabilizer depends on various factors, including your nitrogen product and whether you plan to incorporate. "Say you’re in no-till or strip-till and you apply 28% UAN solution on the soil surface and you added a stabilizer that protects only against denitrification and leaching," Ferrie says. "Late in the growing season, you’ll be wondering why your crop ran out of nitrogen.
"The reason is that part or all of your urea nitrogen volatilized and was lost as a gas before you got rain to wash it into the soil," Ferrie adds. "You should have used a stabilizer to protect against volatility. It would have slowed or stopped the urease enzyme from triggering the conversion process.
"The nitrification inhibitor that you applied did its job later, by slowing the rate at which ammonium converted to nitrate in the soil," he continues. "But some of your nitrogen never got into the soil because it was lost as a gas."
Season-long nitrogen. "The purpose of stabilizers, like everything else we do to manage nitrogen, is to keep nitrogen available to the crop all season long," Ferrie emphasizes. "Nitrogen applied in March is of no value if it’s gone by July."
Some products protect against denitrification and leaching as well as volatilization, Ferrie adds. They’re a good buy if you need both kinds of protection but not so good a bargain if you only need one. "You need both if you are putting a large amount of urea nitrogen on the surface," he says.
In 2012, Ferrie conducted a study of a product that protects against denitrification and leaching with fall-applied anhydrous ammonia, which was applied to soil below 50°F. By March 22, only 48% of the stable ammonium where the stabilizer was used had converted to leachable nitrate. In anhydrous ammonia without the stabilizer, 68% of the ammonium had been converted to nitrate.
"Without the stabilizer, almost 70% of the nitrogen had converted to nitrate, before there was any corn out there to utilize it," Ferrie says.
Equally important, where the stabilizer was used, Ferrie could account for 91% of the nitrogen, compared to only 84% where it wasn’t. Losses were of the leachable nitrate, rather than the stable ammonium form.
"The reason we didn’t lose a lot more nitrogen was that it was so dry," Ferrie says. "In a normal year, we could have lost a lot more fall-applied nitrogen if we didn’t use a stabilizer."
Ferrie also studied the breakdown time of unprotected urea versus a controlled-release product that protects against volatilization, denitrification and leaching. "Twenty-four hours after application, the unprotected urea was beginning to break down," Ferrie says. "Our research showed the controlled-release product could delay breakdown by 40 to 60 days."
Stabilizer payoff. Various studies have confirmed that other products are also effective, when used where needed. One of Ferrie’s 2011 studies showed the benefit of using two types of nitrogen stabilizers to guard against two types of nitrogen loss.
The study involved 28% UAN solution, applied on the surface and not incorporated. It was applied to soil that had been either vertically tilled or moldboard plowed. Some of the UAN was left untreated; some was treated with a product that protects against volatilization; and some was treated with a product that protects against volatilization, denitrification and leaching.
Controlling only volatilization increased yield by 14 bu. per acre in vertical tillage and 8 bu. per acre in moldboard plowing. Controlling both volatilization and denitrification increased yield by 38 bu. per acre in vertical tillage and 30 bu. per acre in moldboard plowing.
Here are some examples of the various situations that you might encounter when selecting a nitrogen stabilizer:
- No-till or strip-till, all nitrogen applied pre-emergence as UAN in the spring. "Here, you’re concerned about both volatilization of urea and denitrification and leaching of nitrate," Ferrie says. "Several products are effective on both." (See table on page 32.)
- Conventional tillage, all nitrogen applied as UAN solution on soybean stubble and immediately worked in. "In this case, you solved the volatility issue by incorporating right after application," Ferrie says. "So your only concern is protecting against denitrification and leaching."
- Long-term corn on corn, broadcasting 60 lb. of nitrogen as UAN solution in the spring, incorporating immediately and sidedressing the rest of the nitrogen at the V4 or V5 stage. "In this situation, if you’re not in a highly leachable soil, you don’t need to apply any stabilizer," Ferrie says. "You applied the 60 lb. of nitrogen in the spring to feed the microorganisms that decompose old-crop residue. If you lock the nitrogen up with an inhibitor, you could create a temporary nitrogen deficiency for the corn plants."
The right time. Nitrogen stabilizers are not designed to let you violate the rules of good stewardship and sound economics. "Using a nitrogen stabilizer with anhydrous ammonia is no excuse for applying ammonia in the fall before soil temperature falls below 50°F and stays there," Ferrie cautions.
"But when soil temperature is fluctuating between 40°F and 55°F, a stabilizer will protect ammonia until the soil cools down permanently," he says.
Avoiding fall application entirely, if possible, is one of the best ways to avoid nitrogen loss.
When calculating the cost of a stabilizer, remember that some products treat the soil and others treat the nitrogen. The rate of soil treatments is constant, but the rate of product treatments will increase along with nitrogen rates. If you understand what you need to accomplish, both types of stabilizer can pay off in higher yield and improved stewardship.
Learn and Profit from Nutrient Navigator
The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher. www.FarmJournal.com/nutrient_navigator
You can e-mail Darrell Smith at firstname.lastname@example.org.
- February 2013
, Farm Business
, Nutrient Navigator
, Soil Health