Agriculture Reference
In-Depth Information
Legumes
Although there are several non-legume nitrogen-fixing plants, often working in partner-
ship with a group of bacteria called Actinomycetes, most of our nitrogen-fixers are from
the legume family, working with Rhizobium bacteria. It's usually a good idea to use a
legume as a cover crop, such as vetch or clover. This is especially important when you're
going to be planting crops that are considered to have a high demand for nitrogen, such as
corn or other grasses, but almost all vegetable crops benefit from a legume. In fact, it's a
great idea to grow one interspersed among your beds.
The root hairs of a legume search the soil for the right species of Rhizobium bacteria
with which to partner. Certain strains of Rhizobia provide optimum nitrogen production for
each legume. If the plant roots don't find their ideal match, they will work with the other
strains, but they won't be able to produce as much nitrogen. The root encircles the bacteria
colony and creates a nodule as big as a kernel of corn where the bacteria live. The bacteria
make an enzyme that converts gaseous nitrogen to ammonia and the plant uses that ammo-
nia to build amino acids. In return, the plant gives carbohydrates and other organic sub-
stances to the bacteria.
When the plant goes to seed, the nodules are discarded and the bacteria remain viable in
the soil for three to five years waiting for the next legume to come along. Unfortunately,
the bacteria may not be numerous enough to provide optimum nitrogen fixation when that
next legume is planted. Since we don't know if we have the right bacteria in our soil and
whether or not they are numerous enough, it's a very good idea to add them in with the
seed or buy seed that has already been inoculated.
Each legume needs a different inoculant. Ideally, the garden center will have the right
one or a mixture. If the folks in the garden center don't seem to know what you need, you
may have to order it online. The two main forms of inocula are solid and liquid, with solid
being the most common. Either of them can be mixed with the seed before you plant or ap-
plied in the furrow directly with the seeds.
Even though they're planted earlier than grasses in the fall in order to get them estab-
lished before winter, legumes don't do much until spring, when they put on their growth
and fix most of their nitrogen. Most of the nitrogen fixed by a legume isn't released to oth-
er plants until the legume decomposes. Even then, if the legume is allowed to produce
seeds, much of the nitrogen will end up in the seed. This is great if you want the plant to re-
seed on its own, or if you want to leave it as a groundcover such as in an orchard, but it's
not great if you want that nitrogen to be available to other plants.
What we often do, therefore, is cut the crop down right in the early-to mid-stages of
blooming in the spring before we seed or transplant our vegetables. If you want to save
