Overstory #4 - Nitrogen Fixing Trees--A Brief Introduction
Nature has many ways of creating abundance under adverse conditions. Imagine plants that accumulate their own fertilizer, grow extremely quickly, tolerate harsh climatic conditions, and are prolific. Many nitrogen fixing plants fit this description, and have numerous uses in gardening, farming and forestry.
Although many nitrogen fixing plants lend themselves to agroforestry, nitrogen fixing trees (NFTs) have perhaps the most use in sustainable systems. The uses for NFTs include windbreak, shade, fodder, organic matter production, mulch, fuel, timber and food. Chances are that if you live in the tropics, there are several NFTs already growing as pioneers in your area.
Nature has devised a unique path of nutrient cycling used by these trees. Air consists of approximately 80% nitrogen gas, which is normally unavailable to plants. NFTs utilize this atmospheric nitrogen. Through an association with Rhizobium, a bacteria which is hosted in the root system of NFTs, these plants biologically accumulate nitrogen, pulling this essential nutrient out of the air for their own use, and, if managed, making it available to other crops as well.
In addition to their ability to make use of atmospheric nitrogen, NFTs are often also deep rooted, which allows them to seek out nutrients in deep soil layers. This allows NFTs to fulfill many of their own needs in infertile, harsh conditions, and therefore have the ability to quickly "pioneer" bare or degraded lands. Pioneer plants are nature's first endeavor to establish vegetation wherever it is lacking. In addition, many NFT species can be pruned back frequently, and the nutrient-rich leaves used as a high-fertility mulch, or for animal fodder. In this way, NFTs support many other life forms on the farm.
The long list of useful characteristics of NFTs includes their ability to:
- establish readily
- grow rapidly
- tolerate poor soils
- tolerate drought
- thrive in full sun
- enhance overall fertility
- regrow easily from pruning
Selecting an NFT for a certain function can be tricky. As a start, a survey of your area will be helpful in determining the habit and vigor of some NFTs. You may notice some exemplary specimens which could be useful in a windbreak or for shade. Information may be available at the your local agricultural extension service, or you may contact one of many international organizations that have agroforestry programs (see FACT Net highlight below).
If possible, use only NFTs which are already established in your area, or that have a history of not becoming weeds. For example, if an Albizia species is already naturalized on the site, it may be useful as a source of organic matter, fodder, shade and even a light weight timber.
There are also many NFTs which are not considered weedy and are extremely useful. The common pigeon pea, a perennial edible "pea tree," has demonstrated little potential for becoming a weed in Hawaii, and makes excellent fodder, quick windbreak, green manure, as well as many other uses.
For more detailed information on NFTs and how to design them in a farm system, please see Nitrogen Fixing Trees: Multipurpose Pioneers at or download the free pdf document, Nitrogen Fixing Tree Start-Up Guide.
Some useful books on nitrogen fixing trees:
- 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. Comprehensive treatment of NFTs and their uses.
- National Academy of Sciences. 1979. Tropical Legumes: Resources for the Future, National Academy Press, Washington, D.C.. A very useful reference at a reasonable cost.
- Nitrogen Fixing Tree Association. 1989-1994. NFT Highlights. Nitrogen Fixing Tree Association, Morrilton, Arkansas, USA. Each highlight covers an NFT species in detail.
Organization Highlight: FACT Net (Farm, Community, and Tree Network)
For information on nitrogen fixing trees, you may wish to contact Farm, Community and Tree Network (FACT Net, formerly the Nitrogen Fixing Tree Association) which is an international organization dedicated to stimulate the use of multipurpose trees. Among the many publications FACT Net offers at a reasonable cost, you will find comprehensive fact sheets on many important NFTs as well as research reports, and agroforestry information. Address: FACT Net, Winrock International, Rt. 3, Box 376, Morrilton, Arkansas 72110, Email: firstname.lastname@example.org.
Seed Source: AgroForester Tropical Seeds can fill your requirements for a wide range of high quality nitrogen fixing tree seed, as well as pure rhizobia inoculants.
About the Authors
Kim M. Wilkinson is the Education Director for Permanent Agriculture Resources and editor of The Overstory. She has B.A. degrees in Anthropology and Ecology from Emory University.
Craig R. Elevitch is an agroforestry specialist with more than ten years of public and private sector experience in tropical agroforest and forest management. He has a M.S. degree in Electrical Engineering (Dynamical Systems) from Cornell University.
Related Editions to The Overstory
- The Overstory #65--Biological Nitrogen Fixation
- The Overstory #61--Effects of Trees on Soils The Overstory
- The Overstory #42--Improved Fallow
- The Overstory #29--Tropical Green Manures/Cover Crops
- The Overstory #22--Pioneering
- The Overstory #16--Multipurpose Trees