Newsletter Articles
Plants That Add Nitrogen to Your Garden Soil
Written by David S.
The plants that add nitrogen to your soil are legumes: peas, beans, fava beans, clover, vetch, and their relatives, which host Rhizobium bacteria in their roots that pull nitrogen straight out of the air. The catch is that most of that nitrogen stays in the plant while it is alive and only enriches the soil once the plant is cut down and allowed to decompose, which is exactly how a winter cover crop works. In Southern California, you sow legume cover crops in fall, let them grow over winter, then turn them under in late winter to feed the soil for spring.
Quick answer: Nitrogen-fixing plants are legumes (peas, beans, fava beans, clover, vetch, lupine) that convert air nitrogen into soil nitrogen using Rhizobium bacteria in their root nodules. They give that nitrogen to the soil mainly when their roots and residue decompose, so grow them as a fall-sown winter cover crop and cut them down at flowering. California lilac (Ceanothus) and alder also fix nitrogen, using Frankia bacteria instead. Buckwheat, mustard, and cereal grains do not fix nitrogen.
| Plant | Type | Best use |
|---|---|---|
| Fava and bell beans | Legume (cool-season) | Winter cover crop and edible bean |
| Austrian winter peas, field peas | Legume (cool-season) | Classic winter cover crop |
| Vetch (common, hairy) | Legume (cool-season) | Winter cover crop, often mixed with a cereal |
| Clover (crimson, red) | Legume | Cover crop and living mulch |
| Garden peas and beans | Legume (edible) | Grow and eat; roots add nitrogen as they decompose |
| California lilac (Ceanothus) | Native shrub (Frankia) | Nitrogen-fixing native, not a legume |
What Plants Add Nitrogen to the Soil?
Almost all of the plants that genuinely add nitrogen to soil are legumes, the pea and bean family (Fabaceae), which includes peas, beans, fava beans, clover, vetch, lentils, and lupines. Legumes partner with Rhizobium bacteria that live in nodules on their roots; inside those nodules the bacteria take nitrogen gas from the air and convert it into a form the plant can use, in exchange for sugars from the plant.[1] This process is called nitrogen fixation, and it is why legumes, not fertilizer, are nature’s way of restocking soil nitrogen. For more on soil nutrients generally, see our guide to soil amendments and what they do.
How Nitrogen Fixation Actually Works
Here is the part most gardeners get wrong: a growing legume keeps most of the nitrogen it fixes for its own leaves and stems, so that nitrogen does not become available to the soil or the next crop until the plant decomposes.[2] That is why the real payoff comes from cutting the cover crop down and leaving the roots and their nitrogen nodules in the ground to break down, which releases the nitrogen into the soil.[3] A living legume does share a little nitrogen with nearby plants through its roots, but most of the benefit shows up the following season as the residue decomposes, not as a real-time handoff to its neighbors.[4] As one measure of the potential, researchers recorded as much as 140 pounds of nitrogen per acre released within days of turning under a hairy vetch cover crop.[2]
Best Winter Cover Crops for Southern California
The most useful nitrogen-fixing plants for a home garden are cool-season legume cover crops, and Southern California’s mild winters are ideal for them. Good choices are fava and bell beans, Austrian winter peas, vetch, and clover, and they are often sown in a mix with a cereal grain like oats or rye for extra biomass.[5] Sow them in fall, roughly late September through October, so they establish before winter and grow through the cool months.[5] Besides adding nitrogen, a winter cover crop protects bare soil from erosion, crowds out winter weeds, feeds soil life, and builds organic matter and structure.[2] For what to plant alongside them, see our guide to vegetables you can grow in SoCal in the fall and winter.
Turning Cover Crops Into Green Manure
To capture the nitrogen, cut the cover crop down when it starts flowering, when about a quarter to a half of the plants show flowers, which is when they hold the most nitrogen and before they set hard seed or turn woody.[3] Chop up the tops, dig the greens into the soil, and wait about three to six weeks for everything to decompose before you plant your spring crop.[5] One tip for success: if you have not grown that legume in your garden before, coat the seed with the correct Rhizobium inoculant, since the right bacteria have to be present for good nodulation and nitrogen fixation.[1]
Nitrogen-Fixing Shrubs and Natives
Legumes are not the only nitrogen fixers. California lilac (Ceanothus), one of our most beautiful native shrubs, also fixes nitrogen, but it does so with a different partner, a bacterium called Frankia rather than Rhizobium.[6] Alders work the same way; plants that fix nitrogen with Frankia instead of Rhizobium are called actinorhizal, and they are a great way to build soil in a permanent planting.[7] One caution worth knowing: many plants sold as cover crops, including buckwheat, mustard, and cereal grains like rye and oats, do not fix nitrogen at all. Grains instead scavenge and hold the nitrogen already in your soil, which is useful but different.[2] If you are adding natives, our guide to the best native plants for SoCal yards includes Ceanothus and more.
Ready to build better soil? Visit any Green Thumb Nursery location and our team will help you pick the right cover crop seed, inoculant, and timing for your part of Southern California.
Nitrogen-Fixing Plants FAQ
What plants add nitrogen to the soil?
Legumes do: peas, beans, fava beans, clover, vetch, and lupines, which use Rhizobium bacteria in their root nodules to fix nitrogen from the air. California lilac (Ceanothus) and alder also fix nitrogen, using Frankia bacteria.[1]
Do beans and peas add nitrogen to the soil?
Yes, but mostly after they finish. A growing bean or pea keeps most of its nitrogen; the soil gets the benefit when you leave the roots in the ground and let the plant decompose.[2]
What are the best winter cover crops for Southern California?
Fava and bell beans, Austrian winter peas, vetch, and clover, sown in fall (late September through October) and often mixed with a cereal grain.[5]
How do nitrogen-fixing plants work?
Bacteria living in nodules on the plant’s roots convert nitrogen gas from the air into a form plants can use, in trade for sugars from the plant. This is called nitrogen fixation.[1]
Do you have to till in a cover crop?
You get the most nitrogen by cutting the crop at flowering and digging the greens in, then waiting a few weeks before planting. Leaving the roots to decompose in place is what releases the nitrogen.[3]
Sources
- [1] Clemson Cooperative Extension, Legumes, Inoculation and Nitrogen Fixation FAQ: blogs.clemson.edu
- [2] SARE, Building Soil Fertility and Tilth with Cover Crops: sare.org
- [3] UC ANR Master Gardeners, When to Cut Cover Crops: ucanr.edu
- [4] SARE, Intercropping Legumes with Nonlegumes: sare.org
- [5] UC ANR, Cover Cropping in Home Vegetable Gardens: ucanr.edu
- [6] UC ANR Master Gardeners, Ceanothus, a Beautiful and Hardy Native: ucanr.edu
- [7] USDA Forest Service, Ecological Significance of Nitrogen Fixation by Actinorhizal Plants (GTR-178): fs.usda.gov
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