Struggling with tired soils and costly fertiliser? Agroforestry in Malawi offers one answer: nitrogen-fixing “fertiliser trees” grown among maize. Farm studies show steady, modest gains, and the best results come when trees are paired with some fertiliser.
| Item | Value |
|---|---|
| Long-term trials began | 1991, in Malawi and Zambia |
| Best Malawi trial result | Trees plus fertiliser at half the recommended amount |
| Farm-plot yield gain with trees | 190 to 890 kg per hectare (6% to 32%) |
| Average Malawi cereal yield, 2009 to 2013 | Around 2 t per hectare |
| Estimated attainable maize yield | 5.4 t per hectare |
These figures come from a ScienceDaily research summary and a paper in Experimental Agriculture. Figures can change, so check the sources before you rely on them.
Agroforestry in Malawi: what are fertiliser trees?
Fertiliser trees are legumes that fix nitrogen from the air and add it to the soil. One widely used example is gliricidia, which was built into maize fields in long-running experiments.
Gliricidia “fixes” nitrogen, drawing it from the air, changing it into a form that plants can use for their own growth, and inserting it into the soil. Its shed leaves also return organic matter, increasing the soil’s “structural stability, erosion resistance and capacity to store water”.
The same idea is promoted across the continent, as the overview of agroforestry in africa explains.
What do the long-term trials show?
Three coordinated experiments, begun in 1991 in Malawi and Zambia, found that farms mixing nitrogen-fixing trees and maize have consistent and relatively high yields year after year. That steadiness matters where yields in many areas are stagnating or declining.
In Malawi, the highest average maize yield came from fields that combined fertiliser trees with inorganic fertiliser, applied at just half the standard recommended amounts.
The lesson is not “trees instead of fertiliser”. It is that trees can make a smaller dose of fertiliser go further.
Which tree practices were promoted?
A farm study in Malawi measured five agroforestry practices for soil fertility improvement that were promoted widely in the country. They involved maize combined with these species:
- Faidherbia, inter-planted with maize.
- Gliricidia.
- Pigeon pea.
- Tephrosia.
- Sesbania.
Each one asks something different of the farmer, so it is worth checking the details of any option before choosing it.
The faidherbia story
Faidherbia is unusual because it sheds its leaves during the early rainy season and remains dormant during the crop-growing period. As a result, it “does not compete with them for water, nutrients, or light”, according to a ScienceDaily summary of conference evidence from November 2010.
That summary reports a typical doubling or tripling of maize yields in Zambia and Malawi, and says Malawian farmers increased yields by up to 280 percent under the canopy.
The farm-level study in the next section found much smaller average gains. Treat the big numbers with care, since they may not match what an average field will do. A sensible farmer or adviser would ask which result matches their own soil, rainfall and labour.
How well does it work on real farms?
Research stations are tidy, but farms are not. The farm study found the mean effects were “modest but positive”. Plots with trees averaged 190 to 890 kg per hectare (6% to 32%) higher yields than those without.
The study put the gains from 0.32 to 1.04 t per hectare more maize with agroforestry, which is 21%, 25%, 17%, 34% and 11% higher yield with gliricidia, faidherbia, pigeon pea, tephrosia and sesbania, respectively.
The authors also describe a large gap between the gains seen on research stations and the uptake of agroforestry by farmers across the country.
Why does variation between farmers matter?
An average gain hides winners and losers. The authors say that a farmer trying these options faces a risk, with a 33%, 20%, 18% and 30% chance of obtaining less maize than without agroforestry, depending on the practice.
They add that those communicating with farmers need to be honest and clear about this risk. They also note that up to 30% of soils in sub-Saharan Africa are degraded and non-responsive, so some farmers see no immediate gain from fertiliser.
- Expect gains to differ by field and by year.
- Match the tree option to the farm’s soil and labour.
- Treat tree fertiliser as one part of a wider plan, not a guarantee.
Why maize and climate raise the stakes
Maize accounts for more than half of the cropped area and the calories consumed in many countries in sub-Saharan Africa. The ScienceDaily summary of the trials calls the small-scale maize system “in crisis”, with yields in many areas either stagnating or declining.
It adds that for just 1°C of warming, more than 75 percent of the present maize-growing areas in Africa are predicted to experience at least a 20 percent reduction in yield under drought conditions. A crop system that holds up in poor years is therefore worth more than one that only wins in good years.
This is why the steadiness found in the long-term trials is as important as the size of the average gain.
What holds farmers back?
The farm study points to several limits. It says closing yield gaps is not necessarily a sound economic route for smallholder farmers with other livelihood options.
It also stresses that research on agroforestry for soil fertility has been going on for a long time in Malawi. Various forms have been practised by farmers and promoted over the last quarter of a century, yet uptake remains below the potential.
- Gains vary widely from farmer to farmer.
- Degraded soils may not respond quickly.
- Farmers with other income options may not see the trade-off as worthwhile.
- Honest advice must state the risk, not only the average.
Better targeting of each option to site and farmer circumstances is the study’s proposed answer, since understanding the sources of variation reduces the risk a farmer faces on adopting the innovation.
How big is the food security gap?
Cereal yields in Malawi consistently averaged around 2 t per hectare from 2009 to 2013, while attainable maize yield was estimated at 5.4 t per hectare. Closing even part of that gap would matter for households.
The paper also notes that almost half (47%) of children under five were stunted, citing national data from 2010. It ranks Malawi 171 out of 187 countries in the 2011 UNDP Human Development Index.
Neighbouring regions face similar soil problems, and readers can compare approaches in agroforestry in ethiopia and agroforestry in tanzania.
Frequently asked questions
What is a fertiliser tree?
Show answer
It is a nitrogen-fixing legume, such as gliricidia, grown in or beside crop fields to improve soil fertility.
Can trees replace fertiliser completely?
Show answer
The Malawi trials found the highest average maize yield when trees were combined with fertiliser at half the standard recommended amount.
Do all farmers gain the same?
Show answer
No. Farm-plot gains averaged 6% to 32%, and the authors stress that performance varies across farmers.
When did the research begin?
Show answer
The coordinated Malawi and Zambia experiments began in 1991.
