Nitrogen-Fixing Plants: How to Grow Your Own Fertilizer
Nitrogen is essential for plant growth, but as organic gardeners, we don’t just go get some chemical with the right N-K-P numbers, right? We could go buy some compost or steer manure, and while these can be valuable components to get our garden started, there is a more sustainable way to maintain soil fertility. Nitrogen-fixing plants can do the work for us without cost while reducing our carbon footprint.
Many plants have the ability to take nitrogen, the most abundant element in the atmosphere, and fix it into the soil. These are often the plants that thrive in the worst soils: alder, indigo, clover, vetch, and Russian olive. Consequently, many are considered weeds and, in fact, can be invasive. However, by choosing varieties that are not invasive in your area and mixing them generously in your garden, over time you will create rich, nourishing soil.
How it Works
Nitrogen fixing occurs when symbiotic bacteria adhere to the roots of compatible plants and form nodules containing enzymes for converting inert N2 (nitrogen gas) into usable NH3 (ammonia). Two components are needed to make this happen. First, you need a nitrogen-fixing plant; second, you need its compatible bacteria.
There are numerous plants that fix nitrogen, some of which you may already be growing. These include legumes (beans, peas, indigo, lupine, peashrub, vetch, alfafla, clover), alder, eleagnus (autumn olive, Russian olive, goumi), and locust. It is easy to incorporate some of these into your annual and perennial gardens.
Finding the compatible bacteria can be somewhat trickier. If you have grown beans before, it is likely you already have that bacteria present. Otherwise, you can acquire the appropriate bacterial inoculant online for most legumes and clover. It is virtually impossible to find inoculants for other nitrogen fixers. However, nitrogen fixing plants will often come already inoculated. Also, if you know someone who is successfully growing the plant you want, see if you can get a handful of dirt from near their plant and drop it in the hole when you plant your own. If you have a good mycorrhizal network in your soil (sheet mulching will help this enormously), that should help bacteria get where they need to go.
Ways to Incorporate Nitrogen Fixers
I aim to plant at least one nitrogen-fixing plant for every two other plants. It’s a difficult goal to achieve, because often the nitrogen-fixing plants do not bear the fruit we would like or look particularly attractive. However, in the long run this will help the more desirable plants to grow more of what you really want with less effort from you. Following are some ideas for making the most of your nitrogen-fixers.
- Blue wild indigo is a beautiful flowering perennial that is right at home in any ornamental garden, but will also nourish edibles and attract pollinators. Lupine is another beautiful nitrogen-fixing perennial which comes in many colors and seeds prolifically, though the seeds may grow flowers of a different color. It is a great source of biomass for soil-building.
- Alder trees are known as weed trees because their nitrogen-fixing abilities allow them to grow where few other things can. You can plant them on the north side of your forest garden where they will not block light or your can plant them more centrally and limb it way up so that it doesn’t block very much light. One alder tree can spread and nourish dozens of feet in every direction.
- Siberian Peashrub is a lovely ornamental tree with yellow blossoms, which also produces edible pods that make great animal forage. Use this for a large hedge or windbreak or use it for a small specimen tree.
- Goumi berry, a member of the genus eleagnus (which contains many nitrogen-fixing species), produces an edible berry. The advantages of this plant over others of its genus is that it is not considered invasive and the berries are bigger and tastier. Some have had success planting goumi berry plants (or other nitrogen-fixing plants) right in the same hole as a fruit-bearing tree.
- Clover is a lovely ground cover and can replace a lawn, nourishing itself and surrounding plants. Its deep taproots make it much more drought tolerant than normal sod.
- Desert indigo can be used in the middle of plantings where edibles would be difficult to harvest. Other nitrogen-fixing shrubs such as Siberian peashrub can be used the same way.
- Black Locust, planted 6"-18" apart in a zigzag rapidly forms a large, impenetrable windbreak in addition to fixing nitrogen in the soil.
- Sea Buckthorn or Sea Berry is a superfood that is coming into vogue. For versatility, it boasts hardiness in zones 2-7 and even tolerates salt spray. However, the one-inch spines may be enough to deter most of us from growing (or harvesting) these bright orange berries. Requires male and female for fruit.
- Alfalfa is a nitrogen-fixing perennial that is also a terrific source of biomass. It can be mowed up to six times a year for a bounty of nitrogen-rich material. Also called lucerne, this vegetable is also edible.
With planning, nitrogen-fixers can serve more than one purpose and be a significant contributor to your garden or food forest.
Nitrogen-fixing plants are a boon to the organic gardener, eliminating the need for importing nitrogen-rich fertilizers and reducing your carbon footprint. Try to incorporate them into your food forest by alternating edibles with nitrogen-fixers. Select nitrogen-fixers with edible or useful parts such as peashrub or Goumi berry for more utility. So often, these nourishing plants are omitted from the forest garden in the hope of growing more food, but the reverse is likely the result: the garden is less vital or requires more input from the gardener than if it had contained nitrogen-fixing plants. Plant some of these useful and beautiful plants and reap the rewards for years.