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Advice for farmers

Practical notes on how to do Inga alley cropping

Summary

Inga Alley Cropping requires decent rainfall and does not function at heights above 1600m. Inga edulis and Inga oerstediana seem to be the best species to use.  The seeds need collected and distributed fairly quickly before they stop being useful or are eaten by wildlife, and then they need planted quickly in order to remain fertile. The seedlings must be weeded and watered, and it is beneficial to give them some shade. They are then planted out in rows with spaces, the alleys, between the rows. In between 8 months and about two years they are pruned at about 1½ m height. The leaves and small branches make mulch within the alleys to rejuvenate the soil. The big branches make good firewood. In about 4 – 6 weeks after pruning crops can be sown. Maize, bean and many other crops can be grown. When the crops have been harvested the Inga Trees regrow before they are pruned again for the next crops. 

What kind of environment is suitable?

Inga alley cropping is an effective, sustainable alternative to slash and burn agriculture in the rainforest and former rainforest lands, including degraded and acid ones. It requires a fair bit of rainfall, 1200 mm on more, and it can be grown at elevations of up to 1600 m.  It has been grown at greater heights but did not do well for alley cropping. 1

Which species of Inga is suitable?

Several species of Inga have been tried.  The conclusions should not be regarded as final.   Local variation may be important, and there may be an advantage in trying a local species that is known to thrive in the region.  But after a substantial amount of practical investigation Inga edulis and Inga oerstediana have proved to be the most reliable. The most commonly used species is Inga edulis. Inga vera has sometimes performed well too, but not always.

Inga edulis trees. Photo by Gaston Bityo

Inga nobilis, jinicuil cocleensis and punctata are not recommended, and some trials with Inga marginata and samanensis have also not done well2

What inputs are needed?

The original research listed these inputs: Inga seed, black plastic bags and soil or compost to grow the seed in before planting out, fencing, and tools, including  good sharp ones for pruning the Inga. Mycorrhizal inoculum may be required, and an initial application of rock phosphate can make a big difference to start off the system on degraded soil.  As a rough guide, in past trials one $8 bag of rock phosphate has been sufficient for a hectare of land. A hectare of land requires 5000 Inga trees.

However, we have not found all these inputs to be necessary.  We have not used rock phosphate in either Cameroon or Ecuador, and the crops have done very well without it.  So it looks like this is only necessary when the soil is very badly degraded.  Also, we are experimenting with bare root nurseries, which do not require plastic bags, saving expense and inevitable litter.  Also we have had good results from planting the seeds directly in the ground where the Inga is to grow.  For a discussion of the pros and cons of this latter method see   Lessons learned.

 Germination is generally good (but see the next section below) so one should not need many extra seeds at the start.

Inga edulis produces seed from about three years old, but sometimes much sooner.  One large Inga tree produces about 1000 seeds, maybe more. So planting a hectare of land would require 3 to 5 trees.  Or one tree might supply 2 to 4 farmers with enough seed to plant a 1/10th hectare plot each. In Cameroon we have started many farmers off with much smaller plots, sometimes even with as few as 50 Inga trees.  That is enough to show that the system works.

Collecting Inga pods. Photo Pastor Nsandah, Kenya.

For the first year, the farmer needs to put in a lot of work. He has to pre-slash the site to keep down the weeds, plant the Inga, and slash the weeds again to keep an area of about a metre diameter clear round the base of each tree till the young trees are fully established at about six months. This might come to over 100 man-days for a hectare.  However most farmers would want to start much smaller than that, for example with a tenth of a hectare, and this would make the work quite manageable.  More land could then be put under Inga in subsequent years when the farmer has seen good results.

However from the second year on the alley cropping system requires very much less work than slash and burn, about half the man hoursa.  And as the alley produces for many years the farmer saves a considerable amount of time.

Seed collection, viability and planting the seeds

The pods go bad if left on the tree for too long, or they get eaten by animals and do not remain viable for any length of time after collection – two weeks at the outside, but it is best to plant them within five days.   For example when seeds were collected on Monday and podded into a bucket of water on Tuesday, many were putting out roots before they went into the ground on Thursday.

Podding Inga seeds, Cameroon. Photo by Linus

So while seeds have been successfully transported from a nursery to a plantation over many miles (and as long as 5 days) generally one wants to have a tree with ripe fruit reasonably close. Once the tree has been found one has to keep a watch on it to be ready to collect the seeds as soon as they are ready. Once the seeds are collected they have to be removed from the pods, cleaned of the ‘fluff’ to prevent them going bad or mouldy very quickly, and kept moist until planted, for example in a bucket of water.

Inga seeds in a bucket of water. Photo by Antony Melville.

To ensure the formation of root nodules and infection with the mycorrhizal fungi the seeds can be soaked in water with broken up pieces of root nodules and soil taken from a mature Inga tree for twelve hours.

They are planted into soil or compost, local forest soil containing organic matter being very suitable, in black plastic bags not more than 2 cm below the surface. Usually, one seed is planted per bag, but if there are plenty of seeds two may be sown and later the smaller one removed. They are grown in partial shade until they are ready to be planted out two to three months later when they have grown to a height of 30 to 40 cm. The shade should be progressively reduced and removed prior to transplanting. They need plenty of water both as seedlings in the nursery and when planted out.

Seedling nursery, Ecuador.

Inga seeds can also be planted straight into the soil. Sometimes  this has been less successful, but some have claimed that it has produced better growth than planting in bags first (see Lessons Learned, www ….) Planting initially in a nursery has been the usual recommendation.

Planting out

Planting out Inga seedlings. Photo Linus.

Before planting out the seedlings the farmer has to slash the weeds to clear the ground.

It is best to do the planting out when there is likely to be some cloud cover and a reasonable amount of rain, so local climate conditions should be considered. Some January plantings in Honduras when the rainfall was low and there was little cloud cover to shield the young plants from the hot sun did not do at all well. Had they been planted in May/June they would have had less exposure. In Honduras, there is less seed loss to birds in April than in October/November when fewer alternative foods are around for the birds. Parrots love the seed.

A poorly grown seedling planted out in January with little rain, photo was taken in April. Honduras. Photo by Antony Melville.

Until the trees are established the weeds around them need to be kept down by slashing, but once they are big enough they hold their own against the weeds, and when they eventually close their canopies across the alleys they suppress the weeds very effectively by shading, shedding leaves and then by the mulch formed when they are pruned.

Row on young Inga following the contours of the land. Photo by FUPNAPIB .

The young Inga is planted out in rows, like hedgerows with alleys in between.  A distance of perhaps half a metre might be left between adjacent plants and a distance of 4m as the alley width, that is, the distance between the rows, but this can be varied.  The wider the alley the less competition there will be between the trees and the crops, but if the alley is too wide the trees may not provide enough shade and mulch to cover the alley and smother the weeds.  The best alley cropping trees, Inga edulis and Inga oerstediana can probably function with an alley width of 5m. However, 4m have been generally used.

On hillsides, the Inga are planted along the contours, and pruned branches can be laid across the stems to stop erosion.  Such Inga alleys have been found to be efective in this.  Otherwise, the alleys might be aligned so as to minimise the shading of  crops by the trees, that is, taking account of the direction of the sun.

Rows of young Inga planted out following the contours. Photo by Chanchamaya project, University of La Molina, Peru

Pruning

The time to prune the trees is when they have closed their canopies, generally in about two years, so that they have shut out the light in the alleys and thus killed most if not all of the weeds. In good conditions, this may take less than two years but if conditions are not good, for example, there is insufficient rain or the soil is exceptionally poor, three years may be required.

The canopies have closed over, and the weeds have been suppressed in the alley. Photo Tiiu Miller

How the pruning is done is very important.  If they are pruned too low down the trees are apt to die.  One should not remove absolutely all the foliage. The minimum height for pruning might be about 1m and a maximum height of 1.75m. Obviously, the higher the pruning the quicker the regrowth.  Lower pruning will let in more light and there will be less competition from the trees to the crops.  Pruning should be done with a clean careful cut with a good sharp tool a little way above a node, at an angle to allow rainwater to drain off.

Woman Pollarding Trees

Natalie pruning the Inga. Ecuador.

Rapid growth, particularly before the first pruning, or later if the pruning is infrequent, can result in there being little foliage low down. This means that if the  Inga was to be pruned down to the usual optimum height bare stems with neither leaves nor leaf nodes would be left.  This would make regrowth slow, or even kill the trees.  So a two-stage pruning may be better.  There are two ways to do this. In one method in the first stage,  the central stem is cut out to encourage branching from lower down, so that when o the second pruning is done there are some leaves low enough to be left on the stem after the pruning.  However this will only be successful if enough light reaches the lower stem for it to grow branches.  Failing this, one can cut out all the side branches higher up and leave the main stem. This will encourage more branching lower down, and when that has happened the central stem can be pruned

The exact timing of pruning will depend not only on when the canopies have closed but when the crops need to be sown.  A first pruning might be done 4 to 6 weeks before planting time, and a second, if needed,  within a fortnight before planting.  If the trees are growing too vigorously and competing with the crops some light pruning can be carried out as needed while the crops are growing. But whatever way the pruning is done it is important not to totally remove the foliage.  The Inga trees can withstand a lot but not that.

Larger branches, which are particularly plentiful on the first pruning, make good firewood.  The smaller twigs and the leaves are however left to lie in the alleys to form mulch.  The thick leaves of the Inga form a protective layer over the soil, essential in the hot sun, which would otherwise dry out the soil and the roots of the crops. In succeeding years the mulch in the lower layers rots down for the crops to feed on, while the newest prunings form a protective layer on top. When the trees are pruned less often there is more woody growth and so more firewood, but also more competition from the trees with the crops.

On hillsides some branches are laid across the Inga stems to prevent erosion.  The Inga system has been found to be very resistant to erosion.  YOU HAVE SAID THIS ALREADY UP ABOVE

Pruned stems laid across the rows of Inga to prevent erosion. Photo by Marco

Planting the crops

The crops are planted into holes the farmer makes into the mulch. Maize and beans have big seeds that are strong enough to then grow up through the mulch, whereas the smaller weed seeds cannot manage that.

Maize, beans, vanilla, pepper, and pineapples are examples of crops that have been successfully grown in the Inga alleys.  As the farmer can reclaim degraded land next to his home and use it year after year he can keep an eye on his expensive cash crops, and his family can help. 

Vanilla and pepper can be grown on living support such as Gliricidia sepium in the middle of the alleys.

Pepper planted out and thriving on a living support in an Inga alley. Photo by FUPNAPIB.

After the Harvest

When the crops have been harvested the Inga is allowed to regrow until it has again closed the canopy and the cycle can be repeated over and over.

Pineapples thriving in an Inga alley. Note the Inga regrowing on either side. Photo by FUPNAPIB 2006.

References and Further Reading

  1. Hands, M.R.  The uses of Inga in the acid soils of the rainforest Zone: alley-cropping sustainability and soil-regeneration. In Pennington, T.D. & Fernades,

 E.C.M. The Genus Inga: Utilization. The Royal Botanic Gardens, Kew, 1998.

  1. Hands, M.R. Alley-cropping as a sustainable alternative to shifting cultivation:  phase III.Inception report.Commission of the European Communities directorate general 1.   North-South relations.   Tropical forests budgetary line project HND// B7‑6201 / IB / 97 / 0533(08) May 1998

  1. Hands, M.R. Alley cropping as a sustainable alternative to shifting cultivation:  Phase IIIFinal Report.Commission of the European Communities directorate general 1.   North-South relations.   Tropical forests budgetary line project HND// B7‑6201 / IB / 97 / 0533(08) June 2002.

  1. Hands, M.R.  Presentation given at Kew.   September 2005.

  1. Hands, M.R.   The uses of Inga species.  Unpublished manuscript c.2003

  1. Hands, M.R.   The Inga project 2005 and onwards. c. 2005

  1. León Geldres,JaimeThe Chanchamaya  Project. News Archives ,  onthis site.

A very good account of how to grow it, descriptions of the tree and its habitat, and of uses and problems encountered in growing it, with several further references. Recommended as additional reading

  1. Melville, A.Some thoughts on expanding the use of Inga alley cropping News Archives, on this site.

9. Pennington, T.  Inga Management. In Pennington, T.D. & Fernades, E.C.M. The Genus Inga: Utilization. The Royal Botanic Gardens, Kew, 1998.   An excellent account of Inga management, covering pretty well what one would want to know when one was planning on using the system: very useful tips on topics like seed collecting and preservation, other means of propagation, sowing, transplanting, species selection and even to what uses the Inga has been put.

Last updated: July 31, 2022