Legumes are often grown as green manures because of their role in fixing nitrogen. But are they actually doing the job? It’s possible to grow a field of clover but not add any nitrogen to the soil. This is more likely to happen when there is already high levels of nitrogen in the soil or if there is no symbiotic bacteria (either present in the soil or added as an inoculant).
To check if your legume is fixing nitrogen, dig it up just as it starts to flower. You should find nodules on the roots—these can be round, cylindrical or lumpy structures. Cut a few open. If the interior is red, the plant is actively fixing nitrogen.
To learn more, see pictures of several types of nodules, and find an interpretation guide, see “Examining root nodule activity on legumes” by Reidun Pommeresche and Sissel Hansen. The Technical Note released by the Research Institute of Organic Agriculture (FiBL) in 2017 is available as a free download (4-page pdf) from orgprints.org/31344.
- Janet Wallace