by Craig Elevitch and Kim Wilkinson P.O. Box 428, Holualoa, HI 96725 USA agroforestry.net copyright 1995,1998 (Printed originally in the Permaculture International Journal, Issue No. 56)
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.
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.