NITROGEN FIXERS: How We Can Use Them

By Mikl Brawner:

Nitrogen is an essential nutrient for all living things. Earth’s atmosphere is 78% nitrogen but it is inert, meaning it can’t combine with other elements until it is broken into a simpler form. The process takes a lot of energy to “fix” the nitrogen. Three processes can fix nitrogen: atmospheric (lightning), Haber Process, and biological.

When the high temperature of lightning splits the nitrogen gas 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 say their grass looks greener after a thunderstorm, which could be due as much to the nitrogen as to the water.

The man-made Haber Process uses natural gas to fix nitrogen for making chemical fertilizers. These water soluble nitrogen fertilizers stimulate plant growth (and sparked the “Green Revolution” of industrial agriculture), but less than 50% of the nitrogen is used by the plants before washing downstream, causing nitrate pollution in our groundwater and streams, and dead zones in gulf areas of the ocean.

The third method 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; they “infect” or attach nodules to the roots of certain plants, such as legumes (beans and peas) to make plant-usable nitrogen. Actinobacteria 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.

Nature has designed this completely natural process to promote the well-being of the whole ecosystem and the soil food web. In the case of legumes, gardeners benefit when we inoculate our peas, etc., with the right bacteria. If not pre-inoculated, we can moisten the seed in a container with a little water or milk, add a spoonful of inoculant, shake, and plant as soon as possible keeping the seed out of direct sunlight.

It is good for gardeners to understand how to make use of the nitrogen produced by nitrogen fixing plants. Composting, annual cover crops, and perennials, shrubs and trees that fix nitrogen are the three main vehicles. 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 feed our soils and plants with some nitrogen. When we dig in raw organic matter, especially high carbon materials like wood chips, the microorganisms must take nitrogen from the soil to break it down, often causing a temporary nitrogen deficiency in plants nearby. Once the raw materials break down, nitrogen returns to the soil.

Annual legume cover crops also add nitrogen to the soil and nearby plants. The amount varies greatly depending on the type of legume, length of growing season, and how the crop is managed. Very little nitrogen is released into the soil from legume roots, as is commonly believed. Most 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 surrounding soil and plants. 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” - a permaculture technique of pruning, or coppicing. It involves cutting branches to near ground level and letting them lay on the ground as 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 microorganisms and worms. Nitrogen is distributed by worms and by the symbiotic mycorrhizal network of mycelium that connects the roots of a plant community. In addition, mycorrhizae bring phosphorus to plants and the nitrogen fixing bacteria; both need it.

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, helps to support nitrogen fixation. Products like pesticides, fungicides, herbicides, and strong chemical fertilizers that kill microorganisms, 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 discusses Azobacteria use as a biofertilizer and the possibility that cultivating these free living nitrogen fixers could reduce or replace synthetic nitrogen fertilizers in the future. Some are experimenting with plant combinations that produce more value than the same plants grown individually as monocrops (somewhat like the Three Sisters corn, beans, and squash). 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.

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

Thanks to Katie Jones from National Ecological Observatory Network, Eric

Toensmeier & Wojciech Majda, The Bio-Integral Resource Center, and Permaculture Research Institute.

Mikl Brawner and his wife Eve co-own Harlequins Gardens in Boulder, specializing in organic veggie starts and herbs, natives, sustainable roses, xeriscape, unusual perennials, and products to build healthy soil.