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Inga restores soil fertility

Inga is a legume. Legumes have nitrogen fixing bacteria in root nodules (i.e. they can make the nitrogen in the air available to plants). They also have mycorrhizae (root fungi) that take up phosphorus (and possibly other minerals) so it gets recycled and is not washed out of the soil. Thus the leaves and small branches that are pruned and left in the alleys between the Inga rows, fertilize the soil with all nutrients crops need.

Root nodules on a young Inga seedling

As the Inga regrows the fertility of the plot is maintained.  Planting Inga alleys on previously used and heavily degraded land can re-fertilize it, so that good harvests can be obtained again. There is no need for chemical fertilizers, pesticides or herbicides. Nor is there a need to keep on clearing more forest.

Some of our field work

Crop Yields

In Cameroon, Gaston Bityo, our Cameroon partner and Coordinator, did a little ‘rough and ready’ experiment traveling to nine farmers’ plots in different locations to compare yields from an Inga plot and an adjacent no Inga plot of the same size, sown with the same maize seed at the same time, all on degraded land. These were small plots of 300 m2, with 200 Inga trees. The comparison plot yields were very poor. Yields from the Inga assisted plots were much better, ranging from 3 times to 15 times more. (See table below). (Various disasters befell: for example a tree fell down on the plot of farmer GB in the table).

‘ROUGH AND READY’ COMPARISONS ON DEGRADED LAND OF MAIZE YIELDS BETWEEN PLOTS WITH AND WITHOUT INGA’ Comparing maize yields from Inga plots with yields from otherwise comparable plots without Inga. The entries between the red lines represent the different farmers. Within each of these sections the black lines divide each year’s harvest. The yellow bars represent the harvest from the Inga plots, and the grey from the adjacent no-Inga plots. The crossed ones are failed harvests due to various problems, like sowing too late so that the rains finished before the crop was well grown. All the plots were 300m2 with 200 Inga trees in 6 rows

Plot with Inga
Adjacent comparison plot without Inga, sown at the same time with the same maize seed, but a bit more seed as there were no trees to take up some space.

In Ecuador we have gained some impressive results. Here are two photos.

‘Mega cassava’, weighing 38 lbs. from an Inga alley in Ecuador. Most cassava weigh about 15 lbs.
Typical maize from outside an Inga alley on the left and from inside on the right. Ecuador.

Soil Improvement

Improvement in the soil within the Inga alleys is clearly apparent. For example in Ecuador much of the soil in many of the farms we worked with was hard and compacted. It was of a light, reddish colour, as it is in many tropical countries. The colour shows it had little carbon in it. However, inside the Inga alleys it became soft and manageable, and black.

Compacted, infertile orange soil without Inga, and black soft soil inside the Inga alley

Speed of Crop Growth

Not only do we find consistently that crops grown with Inga are bigger and better than crops grown without Inga, but also they are ready for harvest sooner. In Cameroon our colleague, Linus Arong, says that maize and groundnuts were ready for harvesting in 10 weeks for those with Inga trees while those without Inga trees needed 13 weeks.

Coffee and Cocoa

Besides using Inga for alley cropping, it makes a great shade tree for coffee and cocoa. In addition to shade, it leads to increased soil fertility and therefore increased yields. Fewer Inga trees are needed than for alley cropping, and they do not have to be pruned as closely. Nonetheless they should be pruned from time to time to ensure sufficient sunlight for the crops. An extra benefit of using the Inga trees within a coffee or cocoa plantation is that it can serve as a seed source, unlike the trees within an alley plantation where they are pruned before they flower.

Cacao is grown extensively in Cameroon by the subsistence farmers. Atanga Wilson, one of our Cameroon partners, reports that after about 15 years the cacao trees started dying because of the loss of soil fertility and lack of shade. He has supplied Inga seedlings to plant in cacao farms to improve the lost soil fertility, and provide shade and firewood for drying the cocoa beans. He found that cacao under Inga grows better and faster.  For example, cacao farmer Julius Awah says that his cocoa trees “were dying” but after planting Inga in the cacao plot they “stopped dying and are growing very green now”.

Atanga did some assessments in the farmers’ plots, to compare the productivity of cacao with and without Inga.  There were 9 farmers growing cacao both with and without Inga. He selected five of them randomly for the assessment. He found that with Inga the trees produced more pods with bigger beans in them, as seen in the table below.

Farmer Number of cocoa pods per tree with Inga Number of cocoa pods per tree without Inga Weight of 500 beans (gms) grown with Inga Weight of 500 beans (gms) grown without Inga
1 56 20 1082 822
2 54 16 1085 825
3 52 18 1087 824
4 46 13 1109 831
5 52 13 1087 831
Average 52 16 1090 827

This was not a very precise experiment but nonetheless the results are very interesting.

Another potentially very interesting and important observation from Ecuador, is that cacao grown in Inga alleys appears to be resistant to pod rot (Moniliopthera).

The Nutrients Present in Inga Tree Prunings

In Ecuador Dr. Guillermo Valle (our Honduran partner) assessed the nutrients that the prunings from an Inga tree supplies to the soil. He visited Ecuador at pruning time to assist with the establishment of teaching Inga alley cropping there with several farmers.

He collected the leaf matter of the first two trees pruned from three plots (from two separate parts of a plot), bagging it up and weighing the result. From this he calculated the average leaf matter across a hectare, and compared the nitrogen content of the leaves to the recommended nitrogen application for growing 1 ha of maize with artificial fertilisers. A rough average across the 3 farm plots was 150Kg of N per Ha, almost double the recommended application of 80Kg. This really brings home the value of the Inga technique as a highly productive organic farming process.

Dr Guillermo Valle calculating the nitrogen content of pruned Inga leaves from one tree. Ecuador.
Bagging the pruned Inga leaves for weighing. Ecuador.

In Summary

All our project leaders are telling us how much the Inga is improving yields, and as we process the data we will report on more.  These are rough experiments so far. We are also doing some soil analyses, investigating aspects of Inga alley cropping such as why some Inga plots are much more productive than others, or whether one can grow more than one crop in an Inga alley, and if so, which and how.  We are also talking to some universities and hope to get further research done in collaboration with them.