Nitrogen Fixing Trees for Agroforestry
Nitrogen fixation is a pattern of nutrient cycling which has successfully been used in perennial agriculture for millennia. This article focuses on legumes, which are nitrogen fixers of particular importance in agriculture. Specifically, tree legumes (nitrogen fixing trees, hereafter called NFTs) are especially valuable in subtropical and tropical agroforestry. They can be integrated into an agroforestery system to restore nutrient cycling and fertility self-reliance.
On unvegetated sites, "pioneer" plants (plants which grow and thrive in harsh, low-fertility conditions) begin the cycling of nutrients by mining and accumulating available nutrients. As more nutrients enter the biological system and vegetative cover is established, conditions for other non-pioneering species become favorable. Pioneers like nitrogen fixing trees tend to benefit other forms of life by boosting fertility and moderating harsh conditions.
NFTs are often deep rooted, which allows them to gain access to nutrients in subsoil layers. Their constant leaf drop nourishes soil life, which in turn can support more plant life. The extensive root system stabilizes soil, while constantly growing and atrophying, adding organic matter to the soil while creating channels for aeration. There are many species of NFTs that can also provide numerous useful products and functions, including food, wind protection, shade, animal fodder, fuel wood, living fence, and timber, (see chart for specific species yields) in addition to providing nitrogen to the system.
Nitrogen: From the Air to the Plants
Nitrogen is often referred to as a primary limiting nutrient in plant growth. Simply put, when nitrogen is not available plants stop growing. Although lack of nitrogen is often viewed as a problem, nature has an immense reserve of nitrogen everywhere plants grow--in the air. Air consists of approximately 80% nitrogen gas (N2), representing about 6400 kg of N above every hectare of land. However, N2 is a stable gas, normally unavailable to plants. Nitrogen fixation, a process by which certain plants "fix" or gather atmospheric N2 and make it biologically available, is an underlying pattern in nature. (See box below for details on how nitrogen fixation works).
How to Use NFTs in a System
In the tropics, most of the available nutrients (over 75%) are not in the soil but in the organic matter. In subtropical and tropical forests, nutrients are constantly cycling through the ecosystem. Aside from enhancing overall fertility by accumulating nitrogen and other nutrients, NFTs establish readily, grow rapidly, and regrow easily from pruning. They are perfectly suited to jump-start organic matter production on a site, creating an abundant source of nutrient-rich mulch for other plants. Many fast-growing NFTs can be cut back regularly over several years for mulch production.
The NFTs may be integrated into a system in many different ways including clump plantings, alley cropping, contour hedgerows, shelter belts, or single distribution plantings. (See figure below). As part of a productive system, they can serve many functions: microclimate for shade-loving crops like coffee or citrus (cut back seasonally to encourage fruiting); trellis for vine crops like vanilla, pepper, and yam; mulch banks for home gardens; and living fence and fodder sources around animal fields.
Planting Nitrogen Fixing Trees
A survey of your area will be helpful in determining the habit and vigor of local NFTs. Some are small and produce edible shoots and pods, ideal for home garden use; others are large and fast growing for fuel wood or poles. Decide on what yields you want from your NFTs, and choose a diversity of species. For some characteristics of many nitrogen fixing trees, this chart may be of use.
Seed Pregermination Treatment (Scarification)
In many NFTs, the hard seed coat must be scarified in order to allow absorption of water, hence germination. There are several methods: hot water is the most common. Water temperature should be approximately 70-90 C° (160°F). The volume ratio should be 5-10 parts water to one part seeds. Seeds are placed in hot water for 1-3 minutes, then rinsed. Seeds may be soaked overnight at room temperature. A useful chart is given on the FACT Net website.
After scarification, a sticking agent such as vegetable oil or plain water is applied sparingly to seeds, and inoculum dusted into the mix. Seeds should be sown immediately. Do not expose inoculated seed to extremes in temperature or direct sunlight.
Plant material in the form of bare root seedlings, stump cuttings and branch cuttings should be kept moist and protected until planting. Punch a small hole in the ground with the same diameter as the plant material. Seedlings should be placed in the hole with the root/shoot collar of the tree at soil level. Stump cuttings are handled likewise. Branch cuttings should be scarified in several places with a sharp knife to promote rooting and put in the ground about one third of their length.
Initially NFTs require moisture and adequate nutrients, as well as protection from weed competition. The best way to achieve these conditions is to amend the soil and sheet mulch at the time of planting.
As the goal in agroforestry is to foster a productive and stable ecosystem, rather than simply to add nitrogen to the system, NFTs should be used with due care and oversight. Too many nitrogen fixing plants can overnitrify the soil and pollute ground and surface waters. NFTs are not a panacea. Most will not thrive in shade or fertile conditions. Because of their ability to thrive under poor conditions, they can easily become weedy. Therefore, if possible, use only NFTs which are already established in your area, or that have a history of not becoming weeds. NFTs can also become competitive for available soil nutrients, especially in arid areas-careful and informed management practices are advised.
Also, be aware that there are many other significant avenues for nitrogen fixation in nature, such as free-living nitrogen fixing bacteria, which should also be incorporated into a design.
How Biological Nitrogen Fixation Works in Legumes
Working with a group of bacteria called rhizobia, legumes are able to pull nitrogen out of the air and accumulate it biologically. The bacteria, which are normally free-living in the soil in the native range of a particular legume, infect (inoculate) the root hairs of the plant and are housed in small root structures called nodules. Energy is provided by the plant to feed the bacteria and fuel the nitrogen fixation process. In return, the plant receives nitrogen for growth.
There are thousands of strains of rhizobia. Certain of these will infect many hosts, certain hosts will accept many different strains of rhizobia. Certain hosts may be nodulated by several strains of rhizobia, but growth may be enhanced only by particular strains. Therefore, when introducing hosts to a new area it is extremely important to also introduce a known effective symbiotic rhizobia strain. Such effective strains have been identified for thousands of the important nitrogen fixing legumes, and can be purchased at low cost for the value returned. The best method for ensuring effective nitrogen fixation is introduce a known effective strain of Rhizobium to the potting medium at the time of sowing. Large, healthy nodules may also be used to inoculate seeds. To determine if the nodule is effective, it may be cut open. Effective nodules will have a pink to dark red pigment inside.
In conventional cropping systems it is estimated that 50-800 kg of nitrogen per hectare per year are accumulated by nitrogen fixing plants, depending on species, soil and climate, Rhizobium efficiency, and management. Equivalent quantities of manufactured nitrogen is produced using an energy intensive process, and the end product is high-priced nitrogen in a form which can be detrimental to soil ecology.
References and further reading:
FAO, 1984. Legume Inoculants and Their Use, FAO of the United Nations, Rome. Excellent practical handbook for inoculation.
MacDicken, Kenneth G. 1994. Selection and Management of Nitrogen-Fixing Trees. Winrock International Institute for Agricultural Development, Morrilton, Arkansas, USA.
National Academy of Sciences. 1979. Tropical Legumes: Resources for the Future, National Academy Press, Washington, D.C..
Nitrogen Fixing Tree Association (Currently the FACT Net). 1989-1994. NFT Highlights. Nitrogen Fixing Tree Association, Morrilton, Arkansas, USA.
Craig Elevitch and Kim Wilkinson
P.O. Box 428, Holualoa, HI 96725 USA
(Printed originally in the Permaculture International Journal, Issue No. 56)