Tending to the fertilizer needs of a field means assessing the past, present and future in order to hit the profitable and environmentally sound bull’s-eye.

Understanding how the previous crop, soil temperature, soil pH, tillage, nitrogen products and application timing interact requires a basic knowledge of nitrogen in the soil and the fertilizer that you apply, says Farm Journal Field Agronomist Ken Ferrie. “Organic nitrogen is tied up in organic matter and microbial tissue, and plants can’t use it,” he says. “Inorganic nitrogen is the usable form of soil nitrogen. It comes from applications of nitrogen fertilizer or mineralization of organic nitrogen.”

Nitrogen fertilizers include ammoniacal nitrogen and nitrate nitrogen. Ammoniacal nitrogen includes nitrogen ions bonded to hydrogen ions, such as urea, ammonia and ammonium. Nitrate nitrogen is nitrogen bonded to oxygen ions, such as potassium nitrate and calcium nitrate. Some products, such as ammonium nitrate and urea-ammonium nitrate (UAN) solutions, are a mixture of both. The popular 28% and 32% UAN solutions are 50% urea, 25% ammonium and 25% nitrate.

“With the inorganic sources of nitrogen, nitrate is subject to loss through denitrification and leaching away,” Ferrie says. “Of the ammoniacal sources, ammonia is subject to loss through volatility or gassing off.”

Acid soil. Soil pH is a major factor affecting nitrogen efficiency.

“With low soil pH, the nitrification process slows down,” Ferrie says. “That happens because the microbes in the soil that mineralize organic nitrogen into inorganic nitrogen, as well as the microbes that convert ammonium nitrogen into nitrate nitrogen, are pH-sensitive. Their populations are limited in acid conditions.

“This is evident in very acidic fields,” Ferrie continues. “There, we see slow breakdown of old crop residue, possibly residue from two or three years ago. This indicates that mineralization of nitrogen held within the crop residue is being slowed or halted. This is nitrogen we count on when trying to select the proper rate of application.

“Fields that are acid are what we call nitrogen-thirsty fields. They require a higher rate of inorganic nitrogen fertilizer to achieve yield goals,” he adds.

While acid soils are slow to break down organic nitrogen, they also are slow to convert ammonium nitrogen to nitrate nitrogen. A nitrification inhibitor applied to an acid soil might not give you the benefit that you would expect on a more neutral soil.

Alkaline soil. “In alkaline soils, we must be concerned with volatility loss when we apply ammoniacal nitrogen in the urea form,” Ferrie says. “Urea must go through a process of hydrolysis to be converted into another ammoniacal source, the ammonium form. For this hydrolysis to take place, we need the urease enzyme, which harbors in the soil and crop residue. I call the urease enzyme the pin puller in the grenade; it has to be pulled before the process will begin.”

Urea hydrolysis causes soil pH to shift upward toward 9.0 in the area around the nitrogen molecule. “This shift in pH can cause the stable ammonium molecule to convert to an ammonia molecule, a gas that can volatilize off,” Ferrie says. “This volatility risk is associated only with the urea form of ammoniacal nitrogen applied to the surface of the soil and not incorporated or rained in.

“Weather conditions and soil pH influence how fast volatilization happens,” Ferrie continues. “With normal weather and soil pH conditions, it can take three to five days. But when we apply urea to the surface of high-pH, or alkaline, soils, urea volatility can occur in a matter of hours. In those conditions, the urea form of nitrogen needs to be incorporated in a matter of hours to stop the volatility.”

Unincorporated surface-applied urea has become more common, as more farmers implement no-till, strip-till and pre-emergence weed-and-feed programs. 

“While volatilization is mainly a concern in soils that carry a naturally high pH, it must also be considered in fields where limestone was applied the previous fall and not incorporated, as often happens in no-till and strip-till situations,” Ferrie says. “That creates a temporary high surface pH.” The higher the rate of urea application, the faster and higher the pH shifts—and the greater the risk of volatilization, Ferrie adds.

“Another form of volatility loss—one that sometimes fails to be detected—occurs when we apply ammonia gas to extremely dry soil,” Ferrie continues. “The ammonia ions will quickly attach to hydrogen ions in the soil, thus converting to stable ammonium.

“Most of this hydrogen comes from water. If soil is too dry, the ammonia gas will continue to move in the soil because it failed to find water to cause the conversion. Some of it may eventually gas back off into the atmosphere. When that happens, you can smell ammonia coming out of the soil hours or days after application. There isn’t much you can do to stop this loss, except wait for soil moisture conditions to improve. This can definitely be a problem following dry seasons.”

Tillage. Tillage done prior to nitrogen application reduces the risk of volatility in two different ways, Ferrie explains.

First, it incorporates crop residue, which reduces the amount of urease enzyme at the surface. The urease enzyme is 10 times higher in residue than in the soil. Second, by incorporating lime applications, tillage reduces the issue of high surface pH that causes volatility of surface-applied urea. On the other hand, tillage increases the temporary tie-up of nitrogen in the soil because of the carbon penalty.

“As we incorporate residue into the soil, where the microbes can process it, that process creates a temporary nitrogen shortage,” Ferrie says. “The severity of the temporary shortage will be linked to the amount and type of carbon that was chiseled in and the time of year that it was done. That means corn after corn has a higher carbon penalty than corn after soybeans, and spring-chiseled cornstalks have a greater carbon penalty than fall-chiseled stalks.”

Immobilization and mineralization occur at the same time, Ferrie points out. But at certain times there is more of one or the other, resulting in net immobilization or net mineralization.

“The carbon penalty is the same with either fall or spring chiseling; but with spring chiseling, net  immobilization of nitrogen may occur at the same time the young corn plants need nitrogen. That can create a rough start for the corn crop,” he says.

Tillage also affects the amount and timing of mineralization.

“When residue is chiseled in close to the surface, where there is oxygen and high populations of microbes, the carbon penalty is stiffer, but the rate of net mineralization is faster,” Ferrie says. “When we moldboard-plow the residue into an area where there is no oxygen, that’s when microbial activity is somewhat limited.

“Plowing reduces the early season carbon penalty. The process affecting the deeply buried residue is more like fermentation than decomposition; it takes longer to mineralize the nitrogen in the deeply buried residue so that crops can use it, and less is recovered.”

Impact of crop residue. “If you’re in the carbon-penalty area and need to decompose residue faster in the fall, first check your soil nitrate levels and see where the existing values are,” Ferrie says. “In fields with high net mineralization rates, fall nitrogen loads may be high enough to drive all the decomposition you want.

“The same thing may occur following a drought, when corn did not use all the applied nitrogen.”

If nitrate levels are in the single digits, you may want to add a source of nitrogen to stimulate the soil microorganisms.

“If you add nitrogen, choose the ammonium form of nitrogen [DAP, MAP or ammonium sulfate], so you don’t have to worry about it leaching or volatilizing,” Ferrie continues. “If you apply UAN solution, half of the solution is urea, which could be lost through volatilization in three or four days. The other half of urea is one-quarter ammonium and one-quarter nitrate; but microbes won’t use nitrate until the ammonium is gone, so this nitrate could be lost in fall rains.”

Apply the ammonium nitrogen when the soil is warm because that’s when microbes are active. “This goes against traditional wisdom in terms of environmental safety because the ammonium could be converted to nitrate and leached out of the soil,” Ferrie says. “For that to happen, you would have to apply a large amount—more than the microbes can use.

“If you apply only 30 lb. or 40 lb. broadcast per acre of ammonium nitrogen, the microbes will suck it up so fast it won’t get a chance to convert to nitrate. Our studies show that if you apply ammonium nitrogen, soil nitrate levels actually fall seven days later because the microorganism populations have exploded, used up all the ammonium and started using the nitrate.”

This fall-applied nitrogen will mineralize back, and it’s available for the crop to use it next spring, Ferrie notes. So the fall application is not additional nitrogen; rather, it counts as part of your total application rate for the year.

Fall nitrogen considerations. When anhydrous ammonia is applied in the fall, the concentrated subsurface application does little to help decompose surface residue, Ferrie warns.

“This banded application can’t be applied until the soil temperature is below 50°F,” he says. “Because of the high concentration and the location of the band below the crop residue, nitrates will be produced when the ammonia core starts to go through nitrification. This nitrate could be leached away before the crop needs it next spring.”

In the spring, UAN solution is a good choice and fits the system well. The ammonium wakes up the microbes. The urea breaks down into ammonium later, which keeps them going, and there’s nitrate for the crop.

Your customer’s situation is unique. Timing and rate must be linked to their farming practices. Set your sights on keeping corn plants happy all season long, from planting through harvest. 

This article appeared in the November Crop Fertility Quarterly of Ag Pro magazine

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