*Research Shows that Legume Trees Can Fertilize and Stabilize
Maize Fields, Generating Higher Yields*
*NAIROBI, KENYA* (15 October 2012)—Inserting rows of “fertilizer trees”
into maize fields, known as agroforestry, can help farmers across
sub-Saharan Africa cope with the impacts of drought and degraded soils,
according to a 12-year-long study by researchers at the World Agroforestry
Centre
The study, *Can Integration of Legume Trees Increase Yield Stability in
Rainfed Maize Cropping Systems in Southern Africa?*, by Gudeta Sileshi,
Legesse Kassa Debusho and Festus Akinnifesi, was published in this month’s
issue of *Agronomy Journal* of the*Soil Science Society of America*.
Three coordinated experiments, begun in 1991 in Malawi and Zambia, found
that farms that mix nitrogen-fixing trees and maize have consistent and
relatively high yields year after year. In Malawi, the highest average
maize yield was found in fields that combined both fertilizer trees and
inorganic fertilizers, but applied at just half the standard recommended
amounts.
Maize mono-crops grown with inorganic fertilizers may have higher yield in
some years but the yield is less reliable in the long run. Mono-cropping
without replenishing soil nutrients in any way—the de facto practice of
resource-poor maize farmers—was the least productive and most unpredictable
of all.
“To grow their way out of poverty, Africa’s small-scale farmers don’t just
need a good harvest for one or two years, they need long-term stable,
high-yield harvests,” said coauthor Akinnifesi, former regional coordinator
for the World Agroforestry Centre, Southern Africa Regional Programme.
“Moreover, they need to know which farming systems will be both stable and
sustainable as the environment and climate changes.”
In sub-Saharan Africa, where more than one in three people is chronically
hungry, variable rainfall, drought and degraded soils all cut into yields
that are one-quarter of the global average.
While previous studies have shown the short-term benefits of such
agroforestry practices, today’s study is the first to analyze long-term
yield stability in the face of environmental change. Year to year, the
most dynamic environmental change is found in rainfall, which directly
impacts maize yield on Africa’s largely non-irrigated, rain-fed farms.
Maize accounts for more than half of the cropped area and the calories
consumed in many countries in sub-Saharan Africa. The small-scale maize
farming system, covering 10 percent of the region, is in crisis, with
yields in many areas either stagnating or declining. As a result of rapid
population growth, average farm sizes have fallen to less than 0.5 hectares
in many areas.
Continuous farming without replenishing the organic matter and nutrients in
soil has led to erosion and soil infertility, and there are signs of
increasing soil acidity in some areas exposed to prolonged use of inorganic
fertilizers and the burning of crop residues.
With climate change, maize cropping systems are expected to experience even
more dramatic reductions in yield. 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.
The experiments incorporated a tree called gliricidia into maize fields.
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. This alleviates the need for big doses of manufactured nitrogen
fertilizers. The leaves shed by gliricidia also return organic matter to
the soil, increasing its structural stability, erosion resistance and
capacity to store water.
“Growing maize with legume trees has increased yields in many parts of
sub-Saharan Africa,” said Gudeta Sileshi, the lead scientist on the study
and regional representative for ICRAF’s Southern Africa Program. “Now we
know this is not just a temporary phenomenon. For maize farmers who can’t
afford fertilizers, agroforestry with nitrogen-fixing trees offers a stable
increase in production, allowing them to feed their families and replenish
the soil.”
Application of fertilizer without the addition of organic matter may not be
sustainable because only organic matter helps to retain soil moisture, adds
calcium and feeds soil biota, contributing to soil health and structure.
Past long-term studies have shown that continuous maize cropping with
inorganic fertilizers in Nigeria resulted in significant yield declines
over a 16-year period. Likewise, in Pakistan, a 14-year study showed
declining rice yields even when the recommended level of nitrates,
phosphate and potassium were applied.
In the United States and Europe, researchers have conducted long-term
studies ranging from 20 to 120 years, monitoring the impacts of cropping
systems on dynamic soil processes. Such long-term studies have been
virtually nonexistent in sub-Saharan Africa.
“We need well-designed long-term trials that will allow scientific
assessments of different cropping systems with a changing climate in
Africa,” said Debusho, a senior lecturer at the University of Pretoria.
“Such information can guide the exploration of technological alternatives
and the development of policies to improve the adaptability and
sustainability of cropping systems.”
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The *World Agroforestry Centre*, based in Nairobi, Kenya, is the world’s
leading research institution on the diverse role trees play in agricultural
landscapes and rural livelihoods. As part of its work to bring tree-based
solutions to bear on poverty and environmental problems, centre researchers
– working in close collaboration with national partners – have developed
new technologies, tools and policy recommendations for increased food
security and ecosystem health. For more information, visit
www.worldagroforestry.org.