Legumes are widely known as having the capabilities to “fix” nitrogen; i.e., take nitrogen from the “air” and, through bacteria that live in root nodules, turn it into nitrogen that is usable by the plant. While this is very true (and some legumes can fix very large amounts of nitrogen), there is a lot of misunderstanding about how the fixation process actually works, and this can result in farmers seeing less benefit from legumes than is possible.
One misconception is that legumes fix nitrogen from above-ground air. Legumes do take nitrogen from air- but it is air in the soil, not from above ground. So, the more compacted the soil is, the less likely it is that significant amounts of nitrogen are being fixed by the plant. Legume nitrogen production will increase as overall soil health increases.
Another misconception, and one that is very widespread, is that legumes are actively and generously providing neighboring plants with lots of nitrogen. The reality is that nearly all of the nitrogen released by legumes is released when the plant (or part of the plant) dies. Many people plant annual legumes next to annual food crops expecting to see a significant increase in plant health and production, and are disappointed by the results. Those legumes are releasing some nitrogen, but is an insignificant amount. They will release more eventually, but in an annual system, it is highly likely that it will not be in time to benefit the other plants.
So, how do we actually realize the benefit of legumes?
It is always important to remember that a plant is more than just what you can see. The above ground biomass is only part of the plant. Many plants are, roughly, 50% above ground and 50% below (this is a huge generalization, as this number depends on many different factors, but it is easy to picture in your head). When plants are pruned, trimmed, or grazed, they shed roots in correlation with the loss of the above-ground biomass. When a legume sheds some of it’s roots, the nitrogen in those roots will begin to become available to other plants.
This is why it is much easier to see the benefits of legumes in a pasture/grazing system. The plants are being “pruned” by the animals on a regular basis, releasing nitrogen from the roots, then growing back, and being grazed again. Not only that, but some of the nitrogen that makes up the leaves and stems of legumes is returned to the soil and made available to the other pasture plants. Finally, and importantly, some legumes in pasture systems are perennial. Because they are always there, and go through daily and seasonal environmental conditions, they are always alternately growing and senescing, and releasing nitrogen as they do so.
When growing annuals for our own consumption, we can mimic the “grazing effect” on legumes in order to benefit our gardens. By pruning our legumes, we can help make more nitrogen available to our plants during the growing season. Planting longer-living, more vigorous, or even perennial legumes is a good choice (you don’t want to kill the plant by pruning too much or too soon, but you still want to prune it during the life of the plants around it).
You can also plant legumes in the off season- this is why many GMCCs are legumes. They finish their growing season before other crops are planted, and as the roots and leaves from the GMCC decompose in and on the soil, that nitrogen is available to the newly planted crops (so don’t pull out your GMCCs or remove the foliage; that defeats the whole purpose!).
That’s my two cents. For more info on nitrogen fixation and legumes, check out this ECHO Development Note: (Selecting Legumes as Green Manure/Cover Crops | ECHOcommunity.org). What are some experiences you have had with legumes that were successful? Or not successful? Does anyone have a favorite legume to grow in your garden?