Context and rationale
Where we live in Northern Tanzania, it is common to see fields prepped and planted year after year with little to no harvest. The ground does not yield as it did for previous generations, and many farmers we talk to cannot explain why. Agroforestry systems have the potential to improve crop production by rehabilitating the soil, increasing water retention, and reversing the effects of erosion.
Agroforestry combines agriculture and trees to enhance agroecosystems. Each agroforestry system has its own timeline, combination of species, and management strategy that helps maximize the system. At the Mavuno Village Farm in Mwanza, Tanzania, we have observed the impact of one specific agroforestry system using gliricidia (Gliricidia sepium) trees. As a leguminous species, gliricidia roots can be colonized by soil microbes that fix atmospheric nitrogen into plant-usable forms, resulting in increased soil nitrogen as the plant biomass is returned to the field 5, therefore reducing the need for fertilizer application when intercropping annuals such as maize or sunflower.
How we grow annual crops with gliricidia
Figure 6. Pruning gliricidia trees and stripping leaves (A). Dry mulch layer to plant into (B). Source: Will Caswell
- Plant the glricidia trees at a spacing of 2x2 meters (we planted our trees in 2017 from seed).
- Establish the trees by allowing them to grow without pruning for 2 years.
- Prepare the field for planting by pruning the trees at a height of roughly 0.5 meter. This can be accomplished with a sharp machete, although a good pair of pruners or loppers is helpful (Figure 6a).
- Cut removed plant material into shorter sections and strip the leaves off. Spread the leaves and small stems/branches evenly on the ground as mulch. Place thicker branches off to the side. These larger branches 6 are left in the field (Figure 6b).
- Plant the first annual crop of maize with the short rains in September (our first planting of maize happened in September 2019). Buds on the gliricidia trees will sprout and grow along with the crop.
- Harvest maize in late December or early January.
- Prepare the field again as described in steps 3 – 4.
- Plant a short-season crop (e.g., beans) in January.
- Harvest the beans in March.
- Prepare the field again as described in steps 3 – 4 and plant another maize crop or a third crop (e.g., sunflower). In Tanzania, we aim to interplant a green manure cover crop in April or May during the long rains and harvest in June/July at the beginning of the dry season. But the trees are not pruned again until September when the short rains begin (Figure 7).
Figure 7. Cropping cycle calendar for one whole year of the gliricidia system. Source: Stacy Swartz
It should be noted that we are set up to irrigate with sprinklers to supplement rainfall, which makes it possible to raise three crops per year. Without irrigation, I doubt more than two crops would be feasible.
Disadvantages/constraints
Because gliricidia has soft wood, poor pruning (rough cuts resulting in open wounds) will invite disease and shorten the lifespan of the tree. This also means that it makes poor firewood. Because of the trees’ ability to access water with deeper root systems, the trees will have the advantage over the intercropped annual species. Therefore, adequate rainfall and/or irrigation during the first couple weeks after planting is important for initial growth of the annual crops. Without enough water shortly after planting, it is possible that the plants closest to trees will be outcompeted. Another downside to this system is that it requires much more time during field preparations to prune trees and strip leaves.
Advantages/benefits
One of the advantages of this system is the opportunity to provide shade for the field in the dry season. Even without much rain the trees, at our spacing of 2 X 2 m, will regrow to shade everything below (Figure 8). This shade, along with mulch, helps maintain soil moisture and soil life throughout the dry season. Another advantage of adding gliricidia trees is the reduced weeding time. Once the trees are established and you’ve planted the first crops, you should see that there is very little initial weeding required before planting out the next crop. This benefit is provided by the trees’ ability to shade out and outcompete weeds. After trees are established, farmers experience reduced inputs towards weeding, spreading fertilizer, and watering throughout the cropping cycle.
Figure 8. Gliricidia regrowth at different points in the year. Tree regrowth as the maize crop matures (left) and at about one month before the first pruning of the year (right). Source: Will Caswell
While we have not recorded data from this plot and yearly managment has varied, it is clear through observation that crop health and vigor has improved and the quality of maize is better with gliricidia trees. Although the amount of rainfall was similar between the last two seasons, the maize health and yield were much better this year than last year. The difference is especially clear in the lower rows of the field, where a greater slope has caused erosion in the past. Once the gliricidia trees were established there, the topsoil started accumulating and the crops planted in these rows are now much healthier and productive. Improvements, therefore, could be due to a combination of soil-related benefits including increased soil moisture (a result of shade and mulch from the trees), improved soil health, (from the nitrogen-containing leaf mulch), and reduced soil erosion.
Future considerations
Gliricidia can be planted from seeds or stem cuttings. While planting by seed ensures that the plant has a tap root, cuttings also work well in our experience. Since we are irrigating to supplement rainfall, there is little worry that a tree planted from a cutting would die from lack of water once its root system is developed. If dependent on rainfall alone, you might opt for seedlings. An unknown is how long the trees will withstand continuous pruning before biomass yield declines.
In order to optimize this system, we have incorporated two pieces of machinery this year: a chainsaw and a wood chipper. The chainsaw allows for faster, cleaner cuts to the trees and enables us to prepare the field in less time than with a machete. The wood chipper saves us the steps of stripping leaves and cutting the branches into smaller sections. Instead, the whole branch is processed, leaving small wood chips behind. This is ideal mulch and easier to plant into. However, even without these tools you can manage this agroforestry system on small plots. I trust this article gives you an idea of how gliricidia can be integrated into annual crops, as well as variables such as tree spacing and tools that can be selected for your situation and context.
Reference
Bray, R.A, T. Ibraham, B. Palmer, and A.C. Schlink. 1993. Yield and quality of Gliricidia sepium accessions at two sites in the tropics. Tropical Grasslands 27:30-36.