Beans, peas and other legumes only fix nitrogen with the right soil bacteria
Beans, peas and other legumes only fix nitrogen with the right soil bacteria. Ground that hasn't grown their kind recently may have too few, so they fix less.
As recorded: A legume fixes atmospheric nitrogen only in symbiosis with rhizobia specific to its cross-inoculation group; ground with no recent history of that group may carry too few of them to nodulate well.
Promising A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Nitrogen fixation happens in root nodules formed by Rhizobium or Bradyrhizobium bacteria specific to each legume group. On ground that has not grown that legume group before (a new bed, a soil-less mix, a long gap in the rotation, a fumigated or strongly acid soil), the matching strain can be absent or sparse, so nodulation and fixation are reduced until a population builds.
What to do about it
On first-time legume ground, coat the seed at sowing with the inoculant matched to the crop's group (garden peas and beans, soybean, and clover each take a different one). Ground that has grown that legume recently usually needs none. It is a cheap one-time step, advised where the history is absent and never required.
How much it matters: No yield figure is stated or estimated. The benefit is largest where the specific rhizobia are absent (new or long-rested ground) and negligible where the crop has grown before; because soils carry these bacteria unevenly, the rule only advises.
The evidence
Evidence: checked against 1 source in the research-based literature.
The source
Rhizobia bacteria are fairly specific as to which legumes they will infect, form nodules on the roots of, and for which they will fix nitrogen
Plants it applies to
- Chickpea
- Common bean
- Cowpea
- Crimson clover
- Edamame
- False indigo bush
- Fava bean
- Hairy vetch
- Lentil
- Lima bean
- Pea
- Runner bean
- White clover
- Wild lupine
Read next
- Cover crops and green manure for the home gardenA cover crop is a plant you grow to feed the soil. Here's what they do, which kind fixes nitrogen and which just builds organic matter, and how to fit one into a small garden's off-season.
- Growing beans: pole or bush, and what the roots leave behindThe same bean is a knee-high bush or an all-season climbing vine, and the packet says which. How to grow both, and the truth about the nitrogen everyone repeats.
- Growing fava beans, a cool-season bean you grow like a peaFava beans (Vicia faba) are a legume you grow like peas. Sow them early into cold ground and they finish before real summer heat arrives.
How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Wording made clearer
- Wording made clearer
- A person read the sources and confirmed it
- Added to the rules
Cite this page
Milpa Gardens. "Beans, peas and other legumes only fix nitrogen with the right soil bacteria. Ground that hasn't grown their kind recently may have too few, so they fix less." Evidence grade: Promising. https://milpa.garden/rules/legume-fixes-atmospheric-nitrogen-only-symbiosis-rhizobia/
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How we grade evidence
- Well established
- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
- Promising
- The mechanism is named and the literature backs it. A few questions are still open.
- A good hunch
- Plausible by its mechanism, but field studies are thin, so treat it as a lean.
- Contested
- The evidence points both ways, or no study has tested it, so the planner does not enforce it.
- Refuted
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