The Inga Tree

The Genus Inga contains around 300 species in tropical America, which are thought to have all evolved within the last 2 million years. Artefacts in the shape of Inga seeds pods have been found in Peru and elsewhere dating back thousands of years and the tree is an important part of the local agro-economy. The trees produce edible fruit and were part of the basic diet of the indigenous pre-Columbian inhabitants of Peru. They have also been used as shade trees for coffee, cacao and tea in the past.

Ten reasons why Inga is an essential part of successful alley cropping:

1. High species diversity and great ecological range

Inga is a huge genus of around 300 species widely distributed throughout humid lowland and montane regions of tropical America. Each locality has its own set of species adapted to the local conditions. Using local species avoids dependence on a single species with the associated problems of pests and diseases.

2. Rapid Growth

Many Inga species are fast growing, light-demanding plants which have the ability to compete successfully with fast-growing secondary vegetation.

3. Rapid Germination

Inga is easily grown from seeds, with normal germination rates of 95-100%. Given moisture and shade, the seeds germinate within a week or two of planting.

4. Tolerance of poor soils

Many Inga species are well adapted to infertile, red acidic soils, such as those found over wide areas of the humid lowland tropics of Amazonia. Such species as I. Edulis and I. Marginata flourish under very low pH conditions, which other legumes cannot tolerate, while other Inga species do well on poorly drained or periodically flooded sites. In addition, they are very successful at restoring compacted pasture land, which can then be reclaimed using the alley cropping systems.

5. Improving soil fertility through nitrogen fixation and mycorrhizal activity

All species of Inga tree we have investigated produce root nodules containing nitrogen fixing bacteria. Crops grown in combination with Inga benefit from the release of nitrogen and also from a sustained release of nutrients from the slowly decomposing leaf mulch. The permanent mulch beneath Inga trees causes rooting to be raised up into the surface layers of the soil and thus above the region of aluminium toxicity. The mulch helps to mimic the conditions of a natural forest and also reduces the soil surface temperature to the levels found in natural forest. This is key for the germination of crop seeds sown within it.

Inga roots also form associations with mycorrhizal fungi, which probably provide the means by which Inga plants are able to recycle phosphorus which is unavailable to non-mycorrhizal plant species on the same soils.

6. Shading by Inga controls weeds

All Inga species have essentially the same branching pattern and when allowed to grow in an open situation will give rise to the characteristic broad umbrella shaped crown. This shape makes Inga an excellent shade tree for such crops as coffee, cacao and tea, which require a partial shade with sun flecks. This also makes them suitable for alley cropping as they are an effective weed control and good for recuperating abandoned pasture. Some species combine very fast growth with very large leaves. The leaves which fall throughout the year are relatively slow to decompose and soon form a long-lasting mulch below the tree. Combined with the shading effect of the Inga crown, this mulch soon depresses the growth of all vegetation below. Within a year or two weeds are eliminated, producing a clean forest soil which can then be brought into productive use by the local farmers.

7. Coppicing ability

All Inga species investigated have the ability to withstand coppicing from an early age. The high quantities of biomass produced by regular coppicing can be used as green manure and for weed control, as well as a large amount of firewood.

8. Fuelwood

Throughout Central and western South America, where a large proportion of the population still rely on wood for cooking, Inga species are usually cited as a preferred fuelwood. This is for three reasons; Firstly it is fast growing, secondly it has a high tolerance of coppicing (vital to the alley cropping system) and thirdly it burns well without producing a lot of smoke. It is highly likely that Inga would also be good for charcoal production.

9. Biological interactions

All Inga species have small nectar producing glands on the leaves. These attract a wide range of insects to the plant, especially ants. The direct effect of these visiting insects is that they protect the Inga plant against herbivores. However, there is also an indirect benefit in that the visiting insects may also parasitise pests living on crop species grown among the trees in the alleys. In this way, Inga has been successfully used as a nurse crop for other timber species such as Mahogany (Swietenia macrophylla) which is normally heavily parasitized by a shoot borer, Hypsipla.

10. Edible fruit

All species of Inga have edible fruit and many are protected and cultivated for this reason. At certain times of the year the fruit can sold in the local markets providing a useful source of additional income for farmers and peasants. Each country in Central America and the Andean region has a unique group of species with edible fruit extending from the lowlands up to 3000m in altitude.