If you find that your plants are yellowing, slow-growing and struggling to thrive, a lack of nitrogen in the soil may be the problem. By contrast, if you find that your plants are dark green, leafy and slow to develop, an excess of nitrogen in the soil may be responsible. Before getting into the specifics on nitrogen, it is important to remember that soil and plant health rely on a synergistic process.
Research suggests that humic substances found in the decomposed matter or humus of a soil may be involved in regulating the uptake of nitrogen. So, solely adding nitrogen to a soil may not solve a growth problem and may actually have detrimental environmental consequences long-term. Whenever there is advice on a single nutrient, it is best to take it with a grain of salt (or, better yet, a grain of humus) and think holistically.
Why do plants need nitrogen?
Plants need nitrogen for the same reason that humans need nitrogen: to form amino acids. Amino acids are nitrogen-containing compounds that make up proteins. Protein molecules are essential for life and perform numerous functions within a plant. Proteins are responsible for synthesizing DNA, the genetic material or instructions necessary for a plant to live and grow; proteins also provide structural support in cell walls and are integral in nutrient transport and enzymatic reactions.
What exactly is nitrogen?
Nitrogen is an element that is essential to all life on earth. It is usually found in its gaseous state as the compound dinitrogen (i.e., two nitrogen atoms bonded together with an incredibly strong triple bond). In fact, 78 percent of our atmosphere is made up of dinitrogen molecules. So, up in the atmosphere and hovering over our fields is dinitrogen gas. With such an abundance in the atmosphere, it would seem that acquiring usable nitrogen would be an easy task, but unfortunately nitrogen in the form of dinitrogen is unusable or inaccessible to plant life. So, what is a plant to do? How does a plant satisfy its nitrogen requirements?
How do plants acquire nitrogen?
Plants acquire nitrogen through a process called nitrogen fixation. One pretty awesome method of fixing nitrogen (i.e., making nitrogen usable to plants) involves lightening. During a thunderstorm, the heat and pressure of a lightening strike is enough to break the triple bonds of dinitrogen, allowing it to mix with oxygen and rainwater to create nitric acid, a plant-friendly form of nitrogen, which falls to earth in the form of precipitation and is readily used by plants.
So, plant-unfriendly dinitrogen with its unbreakable triple bond is basically Track & Field prior to Usain “the lightning” Bolt. You have dinitrogen with its top three incredibly difficult world records (i.e., bonds) and then Usain Bolt enters the arena… Bonds are going to be broken and the world will be a better place for it.
Although not as seemingly exciting as a Jamaican sprinting superstar, another method of fixing nitrogen is just as amazing. In the soil, certain types of bacteria actually have the ability to capture dinitrogen gas from the air and convert it into a more usable or plant-friendly form of nitrogen.
These bacteria form symbiotic relationships with plants by creating nitrogen rich nodes on plant root systems. So, while these bacteria work to assist plants with their nitrogen requirements, the plants, in turn, provide the bacteria with energy and nutrients.
New research also suggests that as much as 26 percent of the nitrogen in ecosystems may come from the earth’s bedrock and not solely from atmospheric fixation by lightening and bacteria. As bedrock breaks down or weathers, it releases nitrogen into the surrounding soil, making it readily available for plant life.
Plants also form special symbiotic relationships with fungi. By teaming up with fungi, plants can extract more nutrients from the soil than would be possible with their root systems alone. Not only do fungi help with nitrogen uptake, but they also assist with another elemental nutrient that is critical to plant growth (and the subject of a future blog post): phosphorus.
Cobb, A. B. (2007). Chemistry matters! Non-metals (Vol. 7). Scholastic Library Publishing, Connecticut.
Davies, B., Eagle, D., and Finney, B. (2001). Resource management: soil. Farming Press: United Kingdom.
Gardiner, D.T., and Miller, R.W. (2004). Soils in our environment, 10th Ed. Pearson Education, Inc., New Jersey.
Nardi, J.B. (2003). The world beneath our feet. Oxford University Press, New York.
Quaggiotti, S., Ruperti, B., Pizzeghello, D., Francioso, O., Tugnoloi, V., and Nardi, S. (2004). Effect of low molecular size humic substances on nitrate uptake and expression of genes involved in nitrate transport in maize (Zea mays L.)
Schwartz, J. D. (2013). Cows save the planet and other improbable ways of restoring soil to heal the earth. Chelsea Green Publishing, Vermont.