Nitrogen comes in many forms; which you choose — and what you apply with it — can significantly affect plant and soil health
The fixation of nitrogen from the atmosphere is critical for life on earth. While nitrogen constitutes 78 percent of the air we breathe, that molecular farm is extremely inert, not wanting to interact with other molecules. Biological processes transform this inert nitrogen into reactive nitrogen, which can be assimilated into the bodies of living organisms.
While in a reactive form, a nitrogen ion may be found in any number of nitrogen-containing compounds. Eventually, the nitrogen ion will end up back in the atmosphere in an inert form, thus completing the nitrogen cycle.
Nitrogen is particularly essential for plants — it is part of all amino acids, nucleic acids, enzymes and chlorophyl. Prior to the 20th century, agriculture depended solely on nitrogen-fixing microbes to make atmospheric nitrogen available to living organisms.
The discovery of the Haber-Bosch process marked a radical shift in how we acquire and use nitrogen. This method produces ammonia from purified atmospheric nitrogen. The process is energy intensive, accounting for 1 to 2 percent of global energy consumption. The amount of nitrogen fixed through industrial means is now nearly as great as what is fixed by biology globally.
This has had a major impact on the nitrogen cycle. Nitrogen fertilizer accounts for a major proportion of agricultural pollution. Groundwater contamination, oceanic dead zones, toxic algae blooms, and ozone-destroying gases are all the result of excess nitrogen fertilizer use. Rising energy prices and continued environmental degradation has put pressure on farmers to evaluate and reconsider how they use nitrogen.
By understanding the different forms of nitrogen and how to use nitrogen fertilizers most efficiently, farmers can improve profitability while also improving environmental health.
Excess and Deficiency
Plants deficient in nitrogen will show a yellow or pale-green coloring called chlorosis. They will also appear thin and stunted. Protein content of the plant will be low and sugar content will be high because there is not enough nitrogen available to form carbon compounds into protein. Nitrogen is mobile within the plant, meaning the plant can translocate nitrogen to the parts that needs it most. In nitrogen-deficient plants, old leaves tend to die off because the nitrogen they contain is needed for new growth.
An oversupply of nitrogen results in weak, watery plant growth. Plant cells become turgid and elongated. Susceptibility to insect attack is increased, and the nutrient concentration of the crop is diluted. Excess nitrogen can also lead to elevated levels of nitrates in plant tissue. This poses a health risk to both humans and livestock, and it makes the plant more susceptible to disease and insect pressure.
Farmers can control nitrogen levels in plants by altering their nitrogen source, application rate, timing of application, and the application method.
Nitrogen Sources
Nitrate (NO–3). Nitrate is one of the two main forms of mineral nitrogen absorbed by plants. It is a negatively charged ion, so it is unable to bind to the soil colloid. This makes nitrate prone to leaching. Leached nitrate wastes nutrients, acidifies the soil and pollutes ground water. Leached nitrate also co-leaches calcium, magnesium and potassium.
Due to nitrification, nitrate is the most common form of mineral nitrogen in the soil. While nitrate is absorbed rapidly by the plant, it is metabolically inefficient. For a plant to use nitrate, it must be converted to ammonium after it is absorbed. This conversion process uses water and energy supplied through photosynthesis. A plant absorbing nitrogen as nitrate can use as much as 25 percent more energy than it would have if the nitrogen was in the form of ammonium.
While some of the energy is used to convert nitrate to ammonium, a proportion of the energy expended by the plant is used to regulate the redox potential and pH of the rhizosphere. Nitrate has an oxidizing effect and lowers the pH of the soil. This impacts the availability of other nutrients because they will not be in a plant available form if the redox potential and pH are not within a certain range.
In addition to increased energy expenditure, the nitrate to ammonium conversion requires water. For every one molecule of nitrogen in nitrate, four molecules of water are needed to convert it to the ammonium form. In this way, the source of nitrogen fertilizer can significantly increase a crop’s susceptibility to drought stress. Nitrate-containing fertilizer sources include ammonium nitrate, potassium nitrate, and calcium nitrate. Excessive applications of manure can also contain appreciable amounts of nitrate.
Ammonia (NH3). All synthetic fertilizers begin as ammonia. Through industrial processes, atmospheric nitrogen is converted into ammonia gas. It is then made into anhydrous ammonia.
Anhydrous ammonia is highly concentrated and toxic. From a soil health perspective, it is one of the worst nitrogen fertilizer sources. Before it can be used by plants, bacteria must convert ammonia into ammonium. In the process, some of the ammonia is lost through volatilization, especially in sandy soil.
Anhydrous ammonia kills soil life, solubilizes humus, dries the soil and alters pH near the injection site. The deleterious effects of anhydrous ammonia make it a poor choice for regenerative farmers.
Ammonium (NH+4). Ammonium is the second of the primary forms of mineral nitrogen absorbed by plants. Unlike nitrate, ammonium ions are positively charged. Ammonium is less prone to leaching because it will bind to the soil colloid; however, much of the ammonium in the soil will be converted into nitrate through nitrification. Another portion of ammonium will be lost as ammonia.
Chemicals that inhibit nitrification can be applied, but the use of biology-suppressing chemicals runs counter to the aims of regenerative agriculture. Ammonium can also be fixed in some clay particles such that it becomes unavailable to plants. While excess ammonium applications are still prone to leaching, it is more metabolically efficient than nitrate. This is because the plant does not have to undergo the nitrate to ammonium conversion.
Ammonium is already in a form that the plant can use to make amino acids. This leaves more energy and water available for plant growth. Ammonium also has a reducing effect on the soil. This can be beneficial because cultivated land is often highly oxidized. The synthetic fertilizer market is full of ammonium-based products, including ammonium sulfate, urea ammonium nitrate, ammonium thiosulfate, monoammonium phosphate and diammonium phosphate.
Organic growers have historically lacked concentrated nitrogen fertilizers. New processes have recently been developed to distill liquid ammonium from hog manure for use in certified organic production (see sidebar).
| A new organic-approved liquid ammonium source Azogen 5-0-0 is a concentrated nitrogen fertilizer that has been approved for use on certified organic farms. Azogen is the flagship product of Solugen Global, a Quebec-based company that has developed proprietary technology for processing hog manure to create a final product that is 80 percent ammonium and 20 percent ammonia. Using large distillation tanks, liquid manure is heated and nitrogen gases are captured and then condensed into a liquid form. The distillation process leaves behind any contaminants from the manure. Azogen is water soluble and is free of suspended solids, making it well suited for a diversity of application methods. There are very few mineral nitrogen products available for certified organic growers. Farmers coming from a conventional background may find that products like Azogen help them make the transition to organic because it comes in a form they are already familiar with. Azogen can be combined with trace minerals, carbon and sulfur to buffer the fertilizer and increase nitrogen use efficiency. |
Organic Nitrogen. Historically, it has been thought that plants primarily absorb nitrogen as simple nitrate and ammonium ions. The mineralization of organic nitrogen compounds and the mechanisms used by plants to transport and assimilate mineral nitrogen have been well studied for decades.
New research is suggesting that another mechanism for nitrogen absorption may be preferred by plants. For a plant to assimilate protein from nitrate, the plant converts nitrate to ammonium through a reduction reaction. Next, the plant uses a series of biochemical reactions to synthesize organic molecules such as amino acids. Amino acids are then joined together to form proteins.
It is more efficient for a plant to absorb nitrogen as ammonium because the plant can skip the first step in the process. Efficiency can be increased further by absorbing amino acids and proteins directly. Through rhizophagy, and similar mechanisms on the leaf surface, plants can absorb organic molecules directly.
In addition to increased nitrogen use efficiency, organic nitrogen is much less prone to leaching than mineral nitrogen because it is held in complex carbon molecules. Non-symbiotic nitrogen-fixing bacteria provide appreciable amounts of nitrogen in some natural ecosystems. This is not true of most agricultural soils because organic matter levels are not high enough to support thriving populations of these organisms.
With improved organic matter management, regenerative farmers can begin to use this free and efficient source of nitrogen. Organic (as in a carbon-containing compound) nitrogen fertilizer sources include compost, manure, plant-based fermented amino acid products, fish hydrolysates and corn steep liquor.
Solugen has a proprietary process to transform hog manure into liquid, organic-certified 5-0-0 that is 80 percent ammonium. (Courtesy of Lucas Hubbard)
Using Nitrogen Efficiently
The source of nitrogen chosen by a farmer can greatly impact nitrogen use efficiency; however, there are additional steps farmers can take to prevent nitrogen losses and to improve their crop response.
Ideally, nitrogen applications will be converted into organic nitrogen as quickly as possible. Farmers can support this process by applying a carbon source along with nitrogen and by ensuring that the nutrients bacteria need to form amino acids are present. Both a stable carbon source and soluble carbon source should be included. Stable carbon, such as biochar or humic substances, provide habitat for bacteria and bonds to nitrogen ions. Stable carbon is not readily consumed by bacteria. Soluble carbon in the form of simple sugars is readily consumed by bacteria and is needed as a food source.
Sulfur should be applied to maintain a 10 to 1 nitrogen to sulfur balance. Sulfur is also needed to synthesize some amino acids. Molybdenum should also be added if adequate levels are not present in the soil. Molybdenum is needed for the nitrate reductase enzyme. Without it, plants will be unable to synthesize nitrate into protein. Once nitrogen is assimilated into bacterial cells, it will not be prone to leaching or volatilization. When plants need nitrogen, it will be present for extraction through the rhizophagy cycle or through mineralization.
Farmers can also increase their nitrogen use efficiency by only applying nitrogen when it is needed. Because plant growth is not consistent across species, different crops require nitrogen at different times. Plants require more nitrogen when they exhibit strong vegetative growth. Crops like winter wheat require more nitrogen in the early spring when they experience a flush of growth. Crops like tomatoes or cucumbers, which experience vegetative and reproductive growth simultaneously, do better with regular nitrogen applications throughout the season.
Plants can absorb nitrogen through both the roots and leaves. Farmers can apply nitrogen directly to the soil, through irrigation, or through foliar sprays. Soluble nitrogen sources can easily be fed through irrigation and spray rigs. Certified organic growers may face technical limitations when applying organic nitrogen through irrigation or as foliar sprays because large particle sizes could clog machinery. Certified organic liquid ammonium products could offer a solution to organic farmers looking for increased precision.
For many farmers, nitrogen use is closely tied to farm profitability. A century of excess synthetic nitrogen applications has proven disastrous for many natural ecosystems and has interrupted some of nature’s most important life sustaining processes. By understanding how nitrogen functions, and by supporting microbial life, farmers can begin to eliminate the negative effects of nitrogen fertilizer while increasing farm profitability.
Lucas Hubbard operates a market garden in southern Michigan. Together with his wife, he is working to develop a farm that feeds his local community while benefiting the natural world around him.
