Inga (Ice-Cream Bean and Pacay)

What is Inga?

Open an Inga pod and you find the seeds bedded in a white, sweet, cottony pulp that melts in the mouth. That is where the English name ice-cream bean comes from. The genus Inga comprises a large number of tropical American trees, all legumes. Two of them are grown for this fruit: Inga edulis, the ice-cream bean of the Amazon basin, and Inga feuilleei, the pacay of Peru. The name Inga comes from the Tupi; edulis means edible.

Fruit is only half of what these trees offer. They fix nitrogen, resprout after cutting and drop a litter that covers the soil for a long time. They have long been planted as shade trees over coffee and cocoa. Among nitrogen-fixing plants they are the ones that grow into real trees and can carry a canopy.

Origin and growth

Inga edulis is a light-demanding tree of forest gaps in the lowland rain forest and along rivers. In the wild it reaches 30 metres, usually branching below 3 metres into a broad, flat crown. The pods are ribbed, often twisted, and up to a metre long; in Peru they are sold at roadsides. The pacay is at home in the coastal river valleys and Andean valleys of Peru, where it has been used for thousands of years; potters of the Moche and Chimú cultures modelled its pods in clay.

Requirements

Both are trees of warm climates. The World Agroforestry database gives Inga edulis an altitude range up to 1600 metres and a mean annual rainfall of 1200 mm, yet calls it remarkably resistant to drought and cold, occurring in regions with a six-month dry season. It tolerates acid, poor soils and even floodplains waterlogged for two to three months. Duke's handbook places it only where there is no frost. The pacay is described as more subtropical, with collections from the lowlands up to about 2800 metres in the Andes, but it too is damaged by low temperatures. Both are light-demanding trees.

Role in the soil: nitrogen, biomass and mulch

Inga edulis forms root nodules with Bradyrhizobium. A study by Leblanc and colleagues isolated fast-growing strains of this genus from its nodules; in sand cultures the young trees drew about 40 percent of their nitrogen from the air, in native soil about 10 percent. How well the partnership works therefore depends strongly on the soil. More on these partners is in the article on rhizobia.

What sets Inga apart is its litter. The leaves are rich in nitrogen, lignin and polyphenols and break down slowly. In the database's words this gives a long-term build-up of organic nitrogen and effective weed control; in trials weed growth fell more under Inga edulis than under other legumes. On cultivated slopes its mulch reduced soil erosion nearly to the level of secondary forest. Where leucaena feeds the soil quickly, Inga covers it for a long time. For biomass it is pruned: it regrows well, but not when cut below 0.75 metres, and the cut should sit a few centimetres above a node.

In the food forest

In a food forest Inga belongs to the canopy, the trees that will one day stand above the fruit trees and give them filtered shade. In agroforestry this is its classic role over coffee and cocoa. In Honduras the Inga Foundation has developed alley cropping with Inga as an alternative to slash-and-burn: rows of Inga are pruned, their prunings cover the soil between them, and crops grow in that layer.

In syntropic agriculture Inga is a long-lived support tree: it shades, fixes nitrogen and is cut back whenever the plants beneath need light. As a nurse tree it protects young, shade-loving species in their first years.

In the Mediterranean climate

The limits are clear. Both species are frost-tender, and the Mediterranean summer drought is longer and hotter than the dry seasons of their homelands. The pacay is the more promising of the two: it is described as more subtropical and has been grown as a fruit tree outside South America, for example in Arizona, where it needs irrigation and protection from frost.

Neither species is on the EU list of invasive alien species. The weed risk assessment for Hawaii (2013) does not rate the pacay as a high risk but as a case for further evaluation. Among its warning traits it names seed dispersal by birds and mammals and lists the species as naturalised on the island of Hawaii, while its summary states that it is not yet known to be naturalised there. For Inga edulis no naturalisation in Hawaii is known.

Propagation and care

Inga seeds are recalcitrant: they often germinate in the pod and keep for only about two weeks. Fresh seed must therefore be sown at once. The pacay can also be propagated from cuttings and by air layering. In the first months a clear circle of about a metre around the young tree helps.

How Terra Humana understands it

At Sa Medada the two Inga species are meant to become the later canopy: trees that will one day cast shade, feed the soil with slow litter and bear fruit. Whether two species from the tropics and the Andes can settle on a Sardinian slope is a real open question, and it is the kind of question Sa Medada is meant to answer.