The scientific reports written about plants who convert and store atmospheric nitrogen in nodules on their roots are so interesting! They are called nitrogen-fixing plants, plants who mine nitrogen, and nitrogen accumulators. We plant beans, peas, and cover crops of various legumes, so I wanted to understand this wonderful plant event.
There is a symbiotic relationship operating between the plant and bacteria in the soil. There are a lot of misleading claims about this subject. My conclusion came in part from my own observation of pulling up plants and soil testing. The below link from a New Mexico State University page also influenced my thinking because it offers such a balanced explanation. As a gardener and not a scientist, I’ve written my own summary points that serve as a basic understanding for this unseen miracle beneath the soil.
The fixed nitrogen does not magically appear: the plant pays a price for the benefits it receives of nitrogen storage. The plant gives photosynthates that are a type of sugar from the photosynthesis process. The other living being in this relationship is the bacteria in the soil around the plant’s roots who needs the nutrients that the plant works hard to produce.
This working relationship continues during the growing season, but is greatly reduced at the time that seeds in the pods of annual legumes are being formed. This is why we chop the legumes back into the soil before the pod forms if we hope to use the legume as green compost to enrich the soil. If the legume is a food crop, then we of course allow the plant to give us those delicious beans or peas!
Any stress, including transplanting, temperature or water fluctuations can reduce nitrogen fixation. At the time of pod fill, nodules on annual legumes often decrease their ability to fix nitrogen because the plant feeds the developing seed rather than the nodule. If additional stress is seen in the plants, it could be a nutritional stress that can be remedied by taking a soil sample in the middle of the growing season and adding additional needed minerals.
I’ve only touched on this subject. For a more scientific explanation of the wonder of this miracle, read and enjoy Jeff Lowenfels’ book, “Teaming with Nurtients: The Organic Gardener’s Guide to Optimizing Plant Nutrition”.
Below is the page where I gleaned such a clear explanation of this beautiful plant event: