How Plants Use Nitrogen

Learn how complex nitrogen molecules are transformed into a substance that plants’ roots can access and use.

| Spring 2017

Farmers, gardeners, and students of science all know that plants need an environmental supply of nitrogen to survive, much less thrive. Unlike carbon dioxide, which green plants can harvest from the gaseous atmosphere through photosynthesis, nitrogen gas (N2) isn’t available for plants’ aboveground parts to use. Instead, we growers apply nitrogen fertilizers to garden beds or manage soils with cover crops, green manures, and even animal manures to supply nitrogen in a form that plant roots can readily take up. To understand how nitrogen from the atmosphere can actually be used by plants, it’s helpful to expand our knowledge of nitrogen chemistry.

Nitrogen’s Properties

Symbol: N
Atomic number: 7
Atomic mass: 14.0067 u ± 0.0001 u
Electron configuration: 1s22s22p3
Electrons per shell: 2,5
Melting point: -346 degrees Fahrenheit
Boiling point: -320.4 degrees Fahrenheit

How Nitrogen Works in the Soil

Nitrogen atoms are energetically predisposed to exist as pairs of individual nitrogen atoms bonded tightly to one another. This is why we use the symbol N2 to represent the relatively nonreactive nitrogen gas (see infographic, below). It takes a lot of energy to split the nitrogen atoms apart. This is problematic for plant roots, which prefer to take up nitrogen in the form of the soluble nitrate ion (NO3¯), which is a negatively charged molecule consisting of a single nitrogen atom bonded to a trio of oxygen atoms (O). Other forms of nitrogen that you might find in your garden’s soil include the soluble, negatively charged nitrite ion (NO2¯) and the soluble, positively charged ammonium ion, which consists of a nitrogen atom bonded with four hydrogen atoms (NH4).

In simplified terms, the principal forms of nitrogen that plant roots can utilize are nitrates and ammonium. A group of fungi and bacteria called decomposers convert animal and plant waste in the soil into ammonium, and in exchange they extract energy to support their metabolism. Nitrogen-fixing soil bacteria — such as those found in the root nodules of legumes — also help create ammonium, and nitrifying bacteria convert ammonium ions into nitrite ions, which they in turn transform into nitrate ions that are finally available for uptake by plant roots. Denitrifying bacteria can convert nitrates to N2, which is then released into the atmosphere and is no longer available for plant root uptake (unless it re-enters the soil and begins the whole cycle over again).

In this simplistic version of the nitrogen cycle as it relates specifically to photosynthetic plants, you can see that it takes a while to pull nitrogen from complex forms, like proteins and nucleic acids, and that the process relies on different groups of soil organisms to make it happen. As you may imagine, the processes are not perfectly efficient and, therefore, some ammonium will be lost to the system.

At this point, it’s clear how important it is to compost organic matter in or on the soil to keep as much nitrogen in a place — and in a form — that can become available to plants. Nitrogen-fixing soil bacteria, such as Azotobacter, Bacillus, Clostridium, and Klebsiella certainly play an important role in getting atmospheric nitrogen back into the system, but in some plants there’s an even better way.

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