Nitrogen is an essential nutrient for all living things. Where does that nitrogen come from? It comes from the atmosphere, which is composed of 78% nitrogen; but that gas is quite inert, meaning it can’t combine with other elements until it is broken into a simpler form. This process takes a lot of energy to “fix” the nitrogen. There are three processes that can fix nitrogen: atmospheric, Haber Process and biological.
Atmospheric fixation occurs when the high temperature of lightning splits the nitrogen gas so it bonds with oxygen and moisture in the air to form nitrates that fall to the earth with rain. This natural fertilization benefits plants. Some people have asked, Is it my imagination that my grass looks greener after a thunderstorm? Maybe not; it could be due to the nitrogen as much as to the water.
The Haber Process is a man-made method that uses natural gas to fix nitrogen for making chemical fertilizers used in industrial agriculture. These water soluble nitrogen fertilizers stimulate plant growth, but less than 50% of the nitrogen is used by the plants before washing downstream, where it causes nitrate pollution in our groundwater and streams and causes dead zones in gulf areas of the ocean. Chemical fertilizers also are a significant expense.
The third method of nitrogen fixation is biological, caused by special bacteria, mostly Rhizobium and Actinobacteria. It is remarkable that these bacteria can generate the necessary energy to fix nitrogen. Rhizobium bacteria are symbiotic and “infect” or attach nodules to the roots of certain plants, such as legumes (like beans and peas) to make plant-usable nitrogen. Actinobacteria like Frankia are free-living but also fix nitrogen, often in association with woody plants like Alder and Russian Olive. Nitrogen fixing plants provide sugars to the bacteria and the bacteria provide nitrogen to the plants. This relationship gives these plants an extra advantage to adapt to adverse conditions.
This is a completely natural process that Nature has designed for the well-being of the whole ecosystem and the soil food web. In the case of legumes, we gardeners will gain more benefit when we inoculate our peas etc with the right bacteria. If the seed is not pre-inoculated, we can moisten the seed in a container with a little water or milk and add a spoonful of inoculant, shake it and plant as soon as possible, keeping the seed out of direct sunlight.
It is good for us gardeners to understand how to make use of the nitrogen produced by nitrogen fixing plants. There are three main vehicles: composting, annual cover crops, and perennials, shrubs and trees that fix nitrogen. The more nitrogen a plant absorbs from biological fixation or from added fertilizer, the higher the nitrogen content in the leaves and stems. When we compost and incorporate these materials into our soil, we are feeding our soils and plants with some nitrogen. When we dig in raw organic matter, especially high carbon materials like wood chips, the microorganisms that break it down must take nitrogen from the soil to do that, often causing a nitrogen deficiency in the plants nearby. Then when the raw materials are broken down, nitrogen is returned to the soil.
Annual legume cover crops also add nitrogen to the soil and nearby plants. The amount of nitrogen available varies greatly depending on the type of legume, the length of growing season, and how the crop is managed. Very little nitrogen is released into the soil from the legume roots, as is commonly believed. Most of the nitrogen is dispersed to the soil only when the plant dies and the nodules decompose. Maximum benefit is gained if the crop is cut or crimped when the cover crop begins to flower or before seeds form.
It is less clear how nitrogen fixing shrubs and trees transfer nitrogen to the surrounding soil and plants, since it doesn’t make sense to kill them like a cover crop. Some of these woody plants are legumes and some are not. Permaculture gardeners suggest that nitrogen becomes available when roots die naturally, when leaves fall and from the practice called “chop and drop.” “Chop and drop” is a permaculture technique of pruning, or coppicing, that is cutting to near ground level; and letting these branches lay on the ground as a mulch that supports the soil food web. This sheet mulching method not only feeds the soil over time, it insulates and holds moisture which supports all forms of microorganisms and worms. Nitrogen is distributed by worms and by the symbiotic mycorrhizae network of mycelium that connects the roots of a plant community. In addition, mycorrhizae bring phosphorus to the plants and nitrogen fixing bacteria which both need phosphorus.
Since nitrogen fixing plants fit into the soil food web system, everything that feeds the microbes and worms, like rock dust and seaweed, compost, compost tea, humate and raw organic matter, help to support nitrogen fixation. Products like pesticides, fungicides and herbicides and strong chemical fertilizers which kill microorganisms, of course, reduce nitrogen fixing bacterial activity.
More and more research supports the idea of keeping living plants in the soil at all times. Not only do mycorrhizae die out when there are no living plants to continue making sugars for them, nitrogen that is not captured (harvested) into the tissues of living plants is not retained in the soil food web to benefit a continuing fertility.
We still have a lot to learn about how to partner with Nature’s systems of producing fertility. An article in Annals of Biology discussed Azobacteria in current use as a biofertilizer and the possibility that cultivating these free living nitrogen fixers could reduce or replace synthetic nitrogen fertilizers in the future. There are also people experimenting with various plant combinations, somewhat like the Three Sisters (corn, beans and squash) that produce more value than the same plants grown individually as monocrops. And more farmers are learning that using cover crops can reduce their fertilizer costs, support beneficial insects and bees, build healthy soil and improve their income.
Thanks to Katie Jones from the National Ecological Observatory Network, Eric Toensmeier, Wojciech Majda, The Bio-Integral Resource Center, and the Permaculture Research Institute
High nitrogen-fixing woodies and perennials: alfalfa, Trifolium repens, Astragalus cicer, Dalea candida
Medium nitrogen-fixing woodies and perennials: Amorpha, Caragana, Lotus corniculatus, Shepherdia, Alnus incana tenuifolia, Sea Buckthorn