Every living creature on Earth requires some form of the element nitrogen to live, grow and reproduce. This ever-abundant element—nearly 80% of the Earth’s atmosphere is comprised of nitrogen—plays a key role in the production of amino acids. Amino acids are the building blocks for proteins, and proteins are a part of every cell found in both plants and animals.
Nitrogen is also a factor in the development of nucleic acids. Nucleic acids are important in the formation of DNA and RNA that contain the genetic information responsible for the proper reproduction and growth of living cells.
Of the 13 elemental minerals found essential for proper plant growth and production, nitrogen is the most important and is contained within the plant in the highest concentrations. Only three other elements can rival the importance of nitrogen in plant growth: carbon, hydrogen and oxygen.
Nitrogen is also essential in the production of chlorophyll. Proper chlorophyll production will lead to lush green vegetative growth. Signs of deficiency within a plant are always evident in the yellowing of the plant’s leaves. Nitrogen is relatively mobile inside a plant, so yellowing of leaves often occurs in the older growth first, as the plant will try and maintain proper nitrogen levels in the newest growth.
Nitrogen has a hand in the creation of nearly every plant structure. But, in terms of plant useage and uptake, not all forms of nitrogen are created equally. The key to optimizing a plant’s growth potential is understanding the different forms that nitrogen can take, and realizing how each form interacts with both the plant and the growing medium.
Commonly referred to as atmospheric nitrogen, dinitrogen is the most common form of nitrogen on the planet, yet plants are unable to access it. That being said, this abundant source of nitrogen does not go completely untapped.
Through a process called nitrogen fixation, some micro-organisms living in soil have the ability to convert dinitrogen into ammonia with the help of special enzymes. The fixing (combining with other elements) of nitrogen is also done in small amounts through lightning strikes and combustion, like from an internal combustion engine.
The majority of nitrogen fixing micro-organisms live freely throughout the soil, but some are known to form a type of symbiotic relationship with the roots of certain plants—the most common being legumes.
Legumes are often used as a cover crop between plantings of high-nitrogen commercial crops such as corn, mainly because of their ability to encourage colonization and reproduction of nitrogen-fixing micro-organisms. The soil after a corn crop might be somewhat drained of nitrogen. A legume crop tilled under will help replenish the nitrogen supply in the soil and allow the grower to perhaps use less applied nitrogen fertilizer with the next crop.
Organic nitrogen is the nitrogen found within organic matter. It exists in multiple forms including urine, feces and decaying plant and animal proteins. Organic nitrogen is part of a complex organic carbon molecule and cannot be directly accessed by plant roots.
Organic matter must be further broken down by soil microbes and it is through this decomposition that the organic nitrogen is converted into plant-usable, inorganic forms—the primary one being ammonium nitrogen.
Organic matter does not easily travel through soil, so it is up to the microbes to find and consume it on their own. This can take time. The rate at which organic matter breaks down depends on the environmental conditions within the soil. In warm soil that has adequate moisture levels, the rate of decomposition will be higher when compared to soil with characteristics that do not favor microbial activity.
Since the rates of decomposition can vary with each different organic material used as a fertilizer input, it is rather hard to predict exactly when and how much of the organic matter will be converted into a plant-usable form of nitrogen.
The percent of nitrogen appearing on the labels of earthworm castings and poultry litter (as well as other nutrients) is only approximate. The nitrogen and other elements are contained in an organic matrix of sorts, and given time and favorable soil conditions, it is only through decomposition by soil microbes that they will be released and converted into plant-usable forms.
Ammonium Nitrogen (NH4)
The first form of plant accessible nitrogen to emerge from the decomposition of organic matter within the soil is ammonium nitrogen. The process in which organic matter is broken down by specialized soil microbes (fungal) is referred to as mineralization.
Mineralization, also called ammonification, will occur at higher rates during the summer months when soil is warm and moist. When ammonium nitrogen is taken in by a plant, it is used directly in the creation of proteins.
Ammonium nitrogen exists in the soil as a cation—an ion with a positive charge. This explains how ammonium nitrogen acts within the soil. Soil particles have a negative charge and ammonium ions are attracted to them. This attraction causes the soil to hold on to ammonium nitrogen, allowing it to stay put and not be washed away during rainfall or watering.
How strongly the soil holds on to the ammonium nitrogen is determined by the soil’s cation exchange capacity. Soils that have cation exchange capacity have higher levels of clay and decomposed organic matter (humus) as well as the capacity to hold a fair amount of water. A soil that is sandy and loamy will have a low cation exchange capacity.
Other elements also participate in the cation exchange process including calcium and magnesium. The ability to bind to a soil in this fashion means ammonium nitrogen is not likely to be washed away by a mass flow of water through the root zone and end up leaching into ground waters. There can be a down side, however.
Research through the years has shown the ammonium form of nitrogen to have undesired effects when used as a primary nitrogen source. Over time, symptoms of ammonium toxicity, fruit disorders such as blossom-end rot, and the decay of the internal vascular tissues can occur, ultimately restricting the uptake of water.
However, most of the time this won’t be of concern as the ammonium nitrogen may not remain in the soil for long. Through another biological process, ammonium will be converted to a different form of nitrogen—the nitrate form.
Through a process called nitrification, ammonium nitrogen is changed into nitrate nitrogen by specialized soil micro-organisms (bacterial). Like ammonium nitrogen, nitrate nitrogen is a form of the nitrogen element that is readily used by plants. Like mineralization, nitrification is a biological process that takes place at higher rates when soils are warm and moist.
During hot months , nitrification of ammonium nitrogen to nitrate nitrogen can happen in just a few days. Nitrate is the form of nitrogen most often used by plants because of its accessibility when found in the rooting zone and it is used directly in the production of new leaves and stems. Nitrate nitrogen in new leaves will be converted to amino acids by the energy produced through photosynthesis.
Unlike ammonium, the nitrate ion is a negatively charged anion and does not participate in the cation exchange process. It is this subsequent negative charge that can pose potential problems with nitrates in the soil. As stated earlier, soil particles also have a negative charge, so they will effectively repel nitrate ions.
The reason this is a potential problem is that with the next watering or rainfall, the nitrate nitrogen can easily be washed away (leached) through the soil, potentially ending up in lakes, rivers, streams and groundwater. The harms that nitrate runoff can cause are highlighted by the blue algae blooms in the Gulf of Mexico that have devastated entire ocean ecosystems.
Nitrates are a part of the natural biological process in which organic matter is decomposed, but it is the excessive use of nitrates in agriculture that leads to the high amounts of nitrate leaching and runoff.
On the other hand, nitrate nitrogen, with its accompanying negative charge, is suitable in hydroponic growing methods that incorporate the re-circulation of the elemental nutrient solution. It mixes well with, and travels easily in, water and tends to flow freely through a rooting medium or substrate without the risk of excessive build-up.
The different transformations nitrogen undergoes in its journey from the atmospheric nitrogen (dinitrogen) state all the way to the nitrate form are all part of a bigger overall process called the nitrogen cycle.
With the assistance of specialized microbial life and the right soil conditions, the nitrogen in the atmosphere is converted into plant-usable forms just as it has been for possibly millions of years. But to complete the nitrogen cycle, there is still one more transformation to undergo.
Through a process termed denitrification, nitrogen is changed from the nitrate form back into the gaseous dinitrogen form where it can then slowly escape from the soil to rejoin its companions in the atmosphere.
Denitrification is another process involving the skill of specialized microbes, but it takes place under much different soil conditions then those mentioned earlier. This process will only occur under anaerobic conditions when there is little or no oxygen present. During periods when the soil is completely saturated by water, like after a flood, denitrification of nitrates will occur and some plant-accessible nitrogen will be lost from the soil.
By taking a careful look at the nitrogen cycle, it is possible to see the effect it can have on the overall availability of plant-accessible nitrogen. Without adequate nitrogen, plant growth will be slow and weak.
A yellowing of the leaves will begin to replace the beautiful, healthy, green growth we all know and love. And, if allowed to continue down the path of nitrogen deficiency, the size and quality of plant yield will surely suffer.
Understanding the different forms of nitrogen and the ways they behave in the soil will allow a grower to make sound decisions regarding plant health and fertilization. I say this from experience. It only takes one season of improper nitrogen management for a grower to see and realize exactly why this element has been deemed essential.