Understanding the Nitrogen Cycle

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Nitrogen is a key element, making up the majority of the air we breathe. More than three quarters of our atmosphere is made up of nitrogen, making it the largest source of the element. All organisms need it to live and it’s one of the most essential nutrients involved in plant growth.

"Plants depend on the three primary nutrients: nitrogen, phosphorus and potassium," explains Tim Laatsch, Director of Agronomy, North America for Koch Agronomic Services (KAS). "Arguably nitrogen could be called the most important macro nutrient because it impacts so many different metabolic processes in the plant."

An essential component of plant structure, nitrogen is required to build plant tissue and is integral for reproductive success of the plant. Nitrogen is also the central element of the amine groups which are the key functional groups of amino acids. These are the building blocks of proteins and enzymes, which regulate all plant metabolism. Nitrogen is necessary to determine plant genetics and key to its growth and development, stimulating root growth so that it can take up the nutrients it needs to grow.

"Nitrogen is a critical component of energy transfer and really an essential component of the chlorophyll molecule which enables plants to capture sunlight energy and create green tissue," says Laatsch. "The plant simply cannot optimize yield without nitrogen."

A Continual Process

The nitrogen cycle is a series of continual processes by which nitrogen is chemically and biologically transformed in the environment through various organic and inorganic forms. Nitrogen is in its most stable form when in the atmosphere, but to be easily used by plants, it has to be converted into plant-available ammonium and nitrate.

Dr. Emerson Nafziger, professor emeritus of crop sciences at the University of Illinois describes it this way, "In its organic form, plants can't immediately reach out and grab (nitrogen) up. If they did, we wouldn't really need to use fertilizer, but we would also run out. The biological process that takes microbes in the soil to break this down and to release it is what we call mineralization. It turns the organic form of nitrogen into a mineral form that the plants can then take up."

It takes several different processes to achieve that biological reaction, including nitrogen fixation, nitrification, mineralization and denitrification.

Biological nitrogen fixation is the first step in the nitrogen cycle, occurring when atmospheric nitrogen is ‘fixed’ or converted into ammonium by certain crops, such as legumes.
Industrial nitrogen fixation, is the method used by manufacturers, converting atmospheric nitrogen into nitrogen oxides or ammonium ions used by plants and other organisms.
Mineralization encompasses several different processes but is commonly referred to the liberation of ammonium from organic matter by microbes that degrade organic carbon sources.
Conversely, nitrogen immobilization occurs when nitrogen is "tied up" by the microbial pool, rendering the nitrogen unavailable to the crop.
Nitrification occurs when bacteria in the soil then converts the ammonium into nitrate.
Nitrogen assimilation is the process whereas plants can uptake nitrogen in the ammonium and nitrate form. Almost 80 percent of nitrogen taken up by a plant is in the nitrate form.

The Balancing Act

Even with some naturally occurring nitrogen in the soil, growers need to add nitrogen fertilizer.

"We're adding fertilizer nitrogen to bring it up to the level that we need, getting the right amount of nitrogen to the crop, making sure there's not a deficiency but also making sure that we're just not putting on a great excess," says Dr. Nafziger. "It’s a very delicate balancing act and it’s not as easy as people think."

One of the key uncertainties of managing nitrogen is loss. There are three forms of nitrogen loss:

Volatilization is above-ground loss of nitrogen. This happens when the soil’s urease enzymes break the urea molecules into ammonia gas.
Denitrification nitrogen loss occurs below ground when nitrate nitrogen is converted back to gaseous forms. It is found most often in soils that are poorly drained or are waterlogged.
Leaching is when nitrogen is lost below ground as downward movement of water carries negatively charged nitrate below the plant’s root zone.

Applied Fertilizer

Nitrogen fertilizers come in different forms for growers to use on their crops. According to Laatsch, one of the most common nitrogen fertilizers applied in North America is anhydrous ammonia. An economical and effective source of nitrogen, anhydrous ammonia is a gas that is converted into a liquid and then directly injected below the soil surface in order to avoid being lost to the atmosphere.

The most utilized dry form of nitrogen is urea. Urea is a compound that has to be hydrolyzed, which is the process of being broken down by a biochemical transformation with water. It is converted in the soil by the urease enzyme in order to release the ammonia that makes it plant available.

Another common form is liquid nitrogen, also known as urea ammonium nitrate (UAN). Half of the nitrogen in UAN is derived from urea, one fourth from ammonium and one fourth from nitrate.

Protecting Nitrogen

Growers can protect their fertilizer investment with nitrogen stabilizers and also by following the 4R Nutrient Stewardship.

"Following the 4R nitrogen management framework will help growers optimize their yield potential and profitability, while also protecting environmental resources," says Laatsch. "Selecting the right source of fertilizer, applying it at the right rate, at the right time and with the right placement, a grower is implementing the best practices to sustainably manage nitrogen."

To learn more about the 4R Nutrient Stewardship initiative, visit the website at nurtientstewardship.org. And to determine which nitrogen stabilizer solution you need to optimize yield potential, speak with your KAS sales representative to find your local retailer

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