Nitrogen is one of the most abundant elements on Earth, yet it frequently limits plant growth globally. It is an indispensable component of biological molecules, including amino acids (the building blocks of proteins) and nucleic acids like DNA. Nitrogen is also a major structural part of chlorophyll, the green pigment necessary for capturing light energy during photosynthesis. While the atmosphere consists of approximately 78% nitrogen gas (\(\text{N}_2\)), this form is inert and cannot be directly absorbed or utilized by most plant life. Specialized plants have developed a mechanism to convert atmospheric nitrogen into usable compounds, and understanding which species participate in this conversion is important for maintaining soil health.
The Biological Process of Nitrogen Fixation
The conversion of atmospheric nitrogen into a plant-usable form is achieved through a biological process called nitrogen fixation. This chemical transformation is performed exclusively by certain types of microorganisms, primarily bacteria, that possess the enzyme nitrogenase. These bacteria enter into a symbiotic relationship with specific host plants. The plant supplies the bacteria with carbohydrates (energy compounds created during photosynthesis), while the bacteria provide the plant with fixed nitrogen.
The bacteria, often belonging to the genus Rhizobium or Bradyrhizobium, inhabit specialized structures on the host plant roots known as nodules. Root nodules form after the plant releases chemical signals, such as flavonoids, which attract the bacteria. Once the bacteria enter the root, the plant’s cells divide rapidly, encapsulating the microbes and forming the nodule structure.
Inside the protective environment of the nodule, the bacterial enzyme nitrogenase catalyzes the reduction of atmospheric nitrogen (\(\text{N}_2\)) into ammonia (\(\text{NH}_3\)). Ammonia is immediately converted into ammonium (\(\text{NH}_4^+\)), a form the host plant can readily absorb and integrate. Because the nitrogenase enzyme is sensitive to oxygen, the nodules produce a protein called leghemoglobin, which binds oxygen to maintain the necessary low-oxygen conditions. This intricate partnership ensures a steady supply of nitrogen for the host.
Key Plant Categories That Enhance Soil Nitrogen
The majority of plants known for their ability to enhance soil nitrogen belong to the family Fabaceae, commonly known as legumes. This category includes well-known species such as clover, alfalfa, peas, beans, and peanuts, all of which form nodules with Rhizobia bacteria. Forage legumes, like alfalfa and various clovers, are particularly effective fixers. Grain legumes, including soybeans and fava beans, are also proficient fixers.
The nitrogen fixed by these plants is not immediately available to surrounding plants while the host is actively growing. Most fixed nitrogen is channeled into the host plant’s own growth, incorporated into its leaves, stems, and seeds. The soil benefits primarily when the host plant dies or when its biomass is intentionally cut down and incorporated into the soil. As the plant residue decomposes, the fixed nitrogen is mineralized and released into the soil as plant-available compounds.
Beyond legumes, a smaller but significant group of non-leguminous plants also participates in nitrogen fixation. These plants, often referred to as actinorhizal plants, associate with a different genus of bacteria called Frankia. Examples include alder trees, bayberry shrubs, and Russian olive. This interaction still involves the formation of root nodules where the bacteria convert atmospheric nitrogen into usable forms.
Utilizing Nitrogen-Fixing Plants in Practice
Gardeners and farmers employ several practical methods to integrate nitrogen-fixing plants into their systems to improve soil fertility.
Cover Cropping and Green Manure
One common method is using them as a cover crop, grown specifically to protect and improve the soil during the off-season rather than for harvest. The entire plant, including the roots and nitrogen-rich nodules, is then tilled directly back into the soil before the next cash crop is planted. This technique is often referred to as “green manure” because the decomposing material acts as a slow-release natural fertilizer.
Timing of Incorporation
The timing of incorporation is important to maximize the benefit for subsequent crops. To ensure the nitrogen is available when needed, the green manure should be turned into the soil when the plant is still young and succulent, often during or slightly before flowering. If the plant matures fully, its carbon-to-nitrogen ratio increases. This causes the nitrogen to be locked up temporarily by soil microbes during the slower decomposition process. Following a nitrogen-fixing cover crop with a heavy-feeding plant, such as corn or leafy greens, helps the subsequent crop take advantage of the newly released nutrients.
Crop Rotation and Intercropping
Crop rotation involves sequentially planting different types of crops on the same land over several seasons. A typical rotation cycle may involve a nitrogen-fixing legume, followed by a nitrogen-demanding crop, and then perhaps a root crop, to prevent nutrient depletion and break disease cycles. Intercropping, or companion planting, involves growing nitrogen-fixers simultaneously alongside a non-fixing crop. For example, planting beans with corn allows the beans to fix nitrogen that can be shared with the corn, resulting in a more efficient use of field space and resources.