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Nature’s Nitrogen

Free Food for Your Garden

By Virginia Hayes

Suppose you never had to fertilize your garden again? Suppose
all you had to do was include a few special plants in your garden
design to produce all the nutrients your garden needed? Sound too
good to be true? It’s not. Farmers all over the world, from India
to Mexico to Hawai‘i, have been doing just that for ages. In fact,
not only have these farmers been feeding their plant crops, they
are supplementing the diet of their livestock, creating living
fences and windbreaks, even feeding their families from the fruits
of these amazing plants. What’s the trick? It’s called
nitrogen-fixing and there are a slew of plant species that do
it.

Nitrogen is a “colorless, odorless, tasteless, insoluble, inert
diatomic gas comprising 78 percent of the atmosphere by volume.” It
is also a constituent of numerous biologically important compounds,
such as proteins, DNA, and RNA, to name a few. As such, it is one
of the most important plant nutrients, an element that plants
cannot survive without. Plants themselves are unable to take
nitrogen directly from the air; it must be combined with other
elements to form compounds that can be absorbed through the roots
of the plant.

Luckily, a number of different bacteria have evolved a way to
capture gaseous nitrogen and form ammonium and many of them have
also evolved in a symbiotic relationship with host plants,
providing them with a source of nitrogen in exchange for
carbohydrates for their own metabolism. They inhabit sections of
plant roots called nodules and directly capture nitrogen from the
tiny pockets of air that exist in healthy, aerated soil. Research
has also suggested the nodules on nitrogen-fixing trees transfer
nitrogen directly to the roots of other trees growing nearby, so
you get double-duty from planting these special species.

Pretty amazing, right? But how can we profit from this
phenomenon in our gardens? Plants with the capacity to form
symbiotic relationships with nitrogen-fixing bacteria abound. Many
of them are to be found in the legume family. Common vegetable peas
and pole beans are among them, and you may be familiar with the
bacterial inoculants available for treating them as you plant. More
importantly, some of the larger, woody legumes can be planted just
to harvest their foliage and wood. This is how they can become
players in our own gardens. A living fence or hedge of one or more
of these species can be harvested repeatedly for its nitrogen-rich
clippings (all without any additional fertilizer inputs from
you).

These can be used directly as mulch or composted, whereupon a
cadre of other microbes convert the stockpiled nitrogen to make a
nutrient-rich soil amendment. If you keep chickens or other small
livestock (even cows and goats!), there are leguminous crops you
can harvest to feed them. And if you rely on a wood-burning source
of heat in winter, there are woody species that can provide more
than the nitrogen from green, leafy parts, but can also be cut for
their wood. Other common uses are as living windbreaks, erosion
control, habitat for wildlife, and sources of nectar and pollen for
bees and other beneficial insects.

Some of the species for consideration may be new to you. Some
are also considered very weedy, so careful consideration is
important. One such tree, black locust (Robinia pseudoacacia),
grows so prolifically from seed as well as root sprouts that it is
a noxious weed in many southern states. Some of the less invasive
species may have their uses, though, if you are willing and able to
undertake their management cautiously. Those known to re-seed
prolifically can be sheared and clipped just prior to flowering
time to reduce the amount of seed produced. Many others, though,
are perfectly suitable for small homeowner gardens without worry of
their escape into the surrounding area. It may require a little
stretch of the imagination to find them places in a more
conventional landscape. Let’s look at a few candidates and their
possible uses.

Erythrina species are in the pea family, a fact you can note on
close examination of their flowers. Many Erythrina grow happily
along the South Coast. Many are also quite large and prolific in
their growth habits. Once you see that vigor as a boon instead of a
headache, you can appreciate them more. Instead of waiting until
that Erythrina caffra or E. corralloides reaches 30 feet in height,
begin pruning it young and often. Use the softest branches and
leaves for compost and mulch. If you do let it grow woody, its
wood, though somewhat soft, can be dried and burned in the wood
stove. Other Erythrina species are smaller and can be used as
hedgerows for wildlife refuges and occasionally sheared for the
compost pile.

Another large tribe of useful nitrogen-fixing trees that foster
the helpful bacteria as well is the genus Acacia. From this
widespread genus there are many different forms from which to
choose for your site. Prune for foliage or harvest young saplings
for fencing or firewood. Some of them, like A. melanoxylon, also
make fine-grained wood suitable for furniture and other woodworking
purposes, should you have the space and time to devote to them.

It may surprise you to know that some common landscape plants
are among the ranks of such useful species. Members of the Albizia
(mimosa), Calliandra (powder puff or fairy duster), and Gleditsia
(honey locust) genera all have this amazing ability. So do redbuds
(Cercis species), desert ironwood (Olneya tesota), tipu tree
(Tipuana tipu), and mesquite (Prosopis species). From this list
(and there are many more), you can see there are nitrogen-fixers
growing in many different climate zones. The tropics support a
large majority of them, but desert and temperate regions have their
own nominees. There are also some non-leguminous plants that form
the same valuable symbiosis. Alders (Alnus species), Causarina (and
the closely related Allocausarina), Eleagnus, and even Ceanothus
species are part of this amazing troop.

For more information about how you can tap into the bounty of
nitrogen-fixing plants, visit agroforestry.net/pubs/NFTs.html
and v1.winrock.org/forestry/factnet.htm
or do your own Internet search.

Virginia Hayes, curator of Ganna Walska Lotusland, will answer
your gardening questions. Address them to Gardens, The Independent,
122 W. Figueroa St., S.B., CA 93101. Send email to vahayes@lotusland.org.

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