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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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