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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 we believe other minerals) so it gets recycled and not washed out of the soil.  Thus leaf fall, and the leaves and small branches that are left in the alleys between the Inga rows, contain nutrients for the crops and fertilize the soil.

Root nodules on a young Inga seedling

As the Inga regrows the fertility of the plot is maintained.  Not only that, but planting Inga alleys on previously much used and degraded land can re-fertilize it so that good harvests can be obtained. Thus good crops are obtained without the need for chemical fertilizers, pesticides or herbicides and without the 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 in many of the farms we worked with was hard and compacted, and of a light, reddish colour, as it is indeed in many of these tropical countries.  That shows it had little carbon in it.  But 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 two months two weeks for those with Inga trees while those without Inga trees needed three months and a week.

Coffee and Cocoa

Besides using Inga for alley cropping it makes a great shade tree for coffee and cocoa, providing both shade and increased soil fertility and therefore increased yields.  Fewer Inga trees are used than for alley cropping, and they are not pruned as closely, but only as much as is necessary from time to time to adjust the amount of sunlight.  This has the extra benefit of the Inga trees within a coffee or cocoa plantation serving 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. They 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 and 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 compelling.

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

The Amount of Nutrients Present in Inga Tree Prunings

In Ecuador Dr. Guillermo Valle (our Honduran partner) assessed the amount of nutrients that the prunings from an Inga tree would supply 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 brought home the value of the Inga technique as an organic 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.