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Inga edulis was selected after considerable research with many different trees, particularly other trees in the genus Inga. The other good tree for the alley cropping was a related species, Inga oerstediana, but Inga edulis is used more and is more available.
It is a legume, which means it has root nodules with bacteria that fix nitrogen and mycorrhizae to recycle phosphorus (and other nutrients).
It withstands annual regular pruning (above chest height).
Inga trees close their canopies across the alleys so well that lack of light kills the weeds.
It grows within a broad altitude range.
It germinates quickly, with a high rate of germination (when the seeds are planted soon after they are harvested).
It survives on and re-fertilizes severely degraded soil.
The time to reach the pruning stage is usually 18 months to 3 years.
It has large thick leaves that don’t rot down too fast so they form a permanent mulch to protect the soil. The soil is therefore never left bare. The mulch builds up year after year and retains moisture, allowing the crops to tolerate some drought.
It has good resistance to pests and diseases. Nectaries on the leaf nodes feed ants that are thought to protect it from other damaging insects.
It is not poisonous.
It provides good firewood. This valuable by-product helps make it more acceptable to new farmers.
Firewood from a small Inga plot (200 trees). Cameroon. Photo Gaston Bityo.
It has edible pulp round the seeds, another characteristics that may not be essential, but can help make it more acceptable to new farmers.
It does not have a bad reputation as an invasive plant.
Inga edulis is a native of Western Amazon, but has been exported to many other tropical countries, mainly as a shade tree for coffee and cocoa. It provides both shade and increased fertility and therefore increased yields. A collaboration between Kew Gardens and the Inga Foundation failed to find a native African tree that was as suitable for the alley cropping. There may be some other Asian trees that might be good, though none have been identified yet.