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Nutrient Navigator: A Moving Target

January 7, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
p20 A Moving Target 1
Efficient nitrogen use requires weighing the effects of the previous crop, soil temperature, soil pH, tillage and other factors.  

Preventing corn from going hungry requires balancing nitrogen and other factors, from year to year and field to field

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 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," Ferrie says. "Inorganic nitrogen is the usable form of soil nitrogen. It comes from applications of nitrogen fertilizer, or from the 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 UAN (urea-ammonium nitrate) solutions, are a mixture of both. The popular 28% and 32% UAN solutions are 50% urea, 25% ammonium and 25% nitrate.

"Of the inorganic sources of nitrogen, nitrate is subject to loss through denitrification and leaching," 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 from urea 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."

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FEATURED IN: Farm Journal - January 2013



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