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The Chemistry Behind N Loss

The Chemistry Behind N Loss
N stabilizers can help by killing soil bacteria that encourage nitrate formation.

To understand how N-Serve prevents nitrogen loss, a short chemistry lesson is in order.

As anhydrous ammonia leaves the nurse tank, its chemical formula is NH3, explains University of Illinois crop nutrition specialist Fabian Fernandez. Once it comes into contact with water in the soil, it converts to NH4+.

Amir Faghih, Dow AgroSciences nitrogen stabilizers product manager, says NH4+, or ammonium, is a stable form of nitrogen as it bonds with negatively-charged soil particles.

"It just stays bonded to the soil and locked into that root zone," Faghih notes. "You don't here reports of the Gulf of Mexico being full of ammonium for this reason. It's very stable."

If that's where the process stopped, there would be no need for N-Serve, or nitrapyrin. Unfortunately, soil bacteria begin attacking the NH4+ molecule, turning it to NO2- (nitrite) and subsequently NO3-, commonly known as nitrate. Once the molecule becomes nitrate, it's highly susceptible to N loss. Since it's negatively charged, the molecule now repels the negatively charged soil particle, Faghih adds.

As nitrate, the most common mode of loss is leaching. The NH3- molecule easily dissolves in water, which leaves the soil via tile drains and natural drainage.

Nitrate is also subject to denitrification, which is more of a problem in water-logged soils. Faghih explains that a typical soil profile should have 25% air space. Without proper air space, soil bacteria will break the bonds of the NH3-molecule, releasing a number of nitrogen-based molecules into the atmosphere, namely N2O, N2 and NO.

How N-Serve works

N-Serve prevents denitrification and leaching via a fairly simple concept. It kills the bacteria that break down the stable NH4+ molecule. Faghih says farmers can expect control to last for 60 to 90 days when soil temperatures are above 50 degrees F.

Without the protection of nitrapyrin, soil bacteria can break down NH4+ and turn it into nitrate in about two weeks when soil temperatures are at 70 degrees F.

Of course, adding N-Serve to the nurse tank isn't a cure all for protecting against N loss. U of I's Fernandez says the number one concern when fall applying anhydrous ammonia should be soil temperature. The big number to remember is 50 degrees F. Once soil temperatures hit 50, and are continuing to fall, the green light for fall application is lit.

Previously, the U of I Agronomy Handbook said applying anhydrous ammonia in 60-degree-F soils was an acceptable practice as long as a stabilizer was used. Many took this recommendation as a sign that a stabilizer cure problems associated with applying N in warm soils. Two years ago, the recommendation was removed. Now, U of I advises to apply only in 50-degree-F soils and encourages the use of a stabilizer.

Fernandez explains the 50 degree mark is the point at which soil bacteria activity begins to decline significantly. As temperatures exceed 50, soil bacteria ramp up reproduction and feeding, creating an environment ripe for nitrogen loss.

Fall app tips

Even if soil temperatures are below 50 degrees F, fall-applied nitrogen can still go wrong. Fernandez says many do not realize that soils can actually be too dry.

As soon as anhydrous ammonia (NH3) is knifed into the soil, it seeks out water to create the NH4+ molecule. If the soil is too dry, the NH3 will travel a greater distance into the soil and can end up escaping into the atmosphere.

Conversely, if the soil is too wet, it becomes tough to close the knife track, frequently resulting in NH3 finding its way to the atmosphere.

"Another potential problem if the soil is too wet is that NH3 will not move very far from the injection point into the soil and will remain as a concentrated NH3 band," Fernandez explains. "When the soil surface starts to dry and crack along the knife track, some of that NH3 will find its way out and be lost from the soil."

To test if the soil is too dry or too wet, grab a clod of soil and roll it into a ball. Once the ball is formed, push it. If the ball crumbles, the moisture content is just right. If it doesn't crumble it's too wet. If you couldn't make the ball in the first place, it's too dry.

Proper sealing is very important to minimize volatilization losses. If the soil is not sealing properly, the applicator is essentially opening the tank's nozzle and letting the anhydrous ammonia escape as a gas.

To ensure proper closure, Fernandez recommends walking the ground after the first pass. If you can smell the ammonia, you've got a closure problem. "You can try going deeper, and see if that helps," Fernandez adds. "If that doesn't work, it may be too dry or too wet." Adjusting the application depth deeper can remedy too-dry conditions.

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