Little-known Acacias: Promising Alley-Cropping and Agroforestry Species

Little-known Acacia species, such as A. baileyana, have great promise for use in alley-cropping and agroforestry systems. According to M. Fukuoka's book: The One Straw Revolution , Acacia roots increase soil aeration and drainage by breaking it up . Acacia roots also "pump" other nutrients essential for plant growth from the subsoil to the leaves. The nutrient rich leaves of the Acacia add humus to the system, increase soil fertility, and make nutrients available in the topsoil for other plants. Like most other legumes, Acacias form a symbiotic relationship with a soil bacteria (Rhizobium) which absorb otherwise unusable nitrogen gas. The taproot of the Acacia, in a similar relationship with mycorrhizal fungi, transforms unavailable forms of phosphorus into forms available for plant uptake. Mycorrhizal fungi exploit other minerals in the subsoil as well.

Intercropped Acacias Protect Citrus

Because Acacia trees grow in all seasons they constantly put out new buds. Aphids feed on the Acacia buds, multiply in great numbers, and support a year-round Lady bug population. When pests such as mites, arrowhead scales, and cottony-cushion scales appear on the citrus, the Lady bugs immediately increase in number and move onto the citrus trees to feed on these insects. In additions to their effect as a natural biological control, the interplanted Acacias also reduce wind damage and improve soil nutrition. The result: citrus yields increased substantially, according to Fukuoka.

Three Candidates for Broader Use

Acacia baileyana is grown commercially for the production of cut flowers. Other qualities of A. baileyana make it valuable for agroforestry systems. It grows rapidly; re-sprouts after cutting; has hard wood and leaves good for fodder; and its flowers attract bees. A. angustissima, a little known acacia of Costa Rica is currently being studied by the Nitrogen-Fixing Tree Association in Hawaii. They describe it as fast-growing, and able to re-sprout after cutting. Although neither of these Acacias grow into large trees, they can be extremely valuable for use in alley-cropping systems, as pioneer species for rejuvenating degraded lands, and as a nurse crop for more-valuable tree species.

A better-known Acacia, A. mangium, is thriving on the acidic and poorly drained soils at Las Pavas, Panama. A.Mangium was introduced to Las Pavas to rejuvenate the nutrient-poor soils, and to halt the spread of Imperata, a tenacious weedy-grass introduced from Asia. The fast-growing Mangium has quickly out shaded Imperata while many trees planted by Dr. Gilberto Ocana, a biologist with the Smithsonian Tropical Research Institute, about three-and-a-half years ago, are now 42 feet high. Mangium has hard wood, and because of its height and form, can be used for construction. This species is fire resistant, re-sprouts after cutting, and is being used in alley-cropping systems.

Reforestation and Acacias

Large-scale tropical reforestation programs face the challenge of reversing massive environmental degradation. The need for massive reforestation to offset the carbon dioxide (CO2) problem is becoming increasingly evident, as is the need to develop agroforestry systems to sustain and increase agricultural production. Acacias, for the qualities described above, are especially appropriate for reforestation projects. The Smithsonian Institute is using Mangium alley-cropping systems for the creation of buffer zones around Las Pavas, a tropical forest preserve threatened by cattle ranching and slash-and-burn agriculture.

Further Research Needed

There are several areas of continued research that need greater attention, including:

  • A better understanding of the ecosystems in which Acacias thrive, so that species would be planted in appropriate environments;
  • a reliable seed source to insure the production of the desired qualities (tall strait trees for construction, short bushy types for windbreaks);
  • information on the potential uses and products of different Acacias (re-sprouting, for wood, stakes, protein content and digestibility of the leaves intended for fodder; and
  • a data base containing this information must be made available for potential users.

    Michael D. Benge, Agroforestry Officer, Bureau for Science and Technology, Office of Forestry, Environment, and Natural Resources, Agency for International Development, Washington, D.C. 20523