Research into agroforestry, a sustainable farming technique which combines traditional farming techniques with cutting edge knowledge and introduces trees into farmland systems, has revealed a multitude of benefits for maize production.

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Research in Malawi and Zambia has shown that planting rows of "fertiliser trees" in maize fields can help farmers across sub-Saharan Africa cope with the impacts of drought and degraded soils. The 12-year study revealed the benefits of agroforestry in reducing the need for expensive inputs, improving yields and providing environmental protection.

Researchers from the World Agroforestry Centre (ICRAF) began three coordinated trials in 1991. They found that farms that mix nitrogen-fixing trees and maize have consistent and relatively high yields year after year.

Maize mono-crops grown with inorganic fertilisers were shown to yield higher in some years, but in the long term their yield was much less reliable and the cost of fertilisers does not provide the other benefits associated with agroforestry; in Malawi, the highest average maize yield was found in fields that combined both fertiliser trees and inorganic fertilisers, but applied at just half the standard recommended amounts.

Akinnifesi, former regional coordinator for the World Agroforestry Centre and co-author of the study, commented, "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. Moreover, they need to know which farming systems will be both stable and sustainable as the environment and climate changes."

Sustainable approach essential in alleviating poverty

In sub-Saharan Africa, yields are around a quarter of the global average. Variable rainfall, drought and degraded soils all affect yields in the region, where one in three people is chronically hungry. Continuous farming without replenishing the organic matter and nutrients in soil has led to erosion and reduced soil fertility, and there are signs of increasing soil acidity in some areas exposed to prolonged use of inorganic fertilisers and the burning of crop residues.

The ICRAF researchers said their study is the first to analyse long-term yield stabilising benefits of a sustainable farming system, which will be able to better withstand environmental change than conventional practices.

As the most dynamic environmental change between years is found in rainfall, this will directly impact maize yields on Africa's largely non-irrigated, rain-fed farms. In these areas, maize accounts for around half the cropped area and calorie intake of local populations.

Experiment yields overwhelmingly positive results

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 fertilisers. The leaves shed by gliricidia also return organic matter to the soil, increasing its structural stability, erosion resistance and capacity to store water.

Lead author Gudeta Sileshi said, "Growing maize with legume trees has increased yields in many parts of sub-Saharan Africa. Now we know this is not just a temporary phenomenon. For maize farmers who can't afford fertilisers, agroforestry with nitrogen-fixing trees offers a stable increase in production, allowing them to feed their families and replenish the soil."

Past long-term studies have shown that continuous maize cropping with inorganic fertilisers 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 year on year.

In addition to fertilising fields, studies in Europe and other global regions have concluded that agroforestry plays a key part in reversing biodiversity declines. By providing habitat for threatened insects which pollinate and control pests in arable farming systems, the method can potentially tackle falling biodiversity. Furthermore, in many cases the trees themselves can provide an extra source of income for farmers.

Research and trials are increasing in France, where the National Agricultural Research Institute (INRA) has championed agroforestry, and studies from the UK and United States have shown threatened farmland bird populations may be protected by rolling out tree farming systems.

However, until now the expense associated with planting and protecting the trees until they are mature, as well as a blinkered approach to agricultural research and development, has hampered uptake of the approach. Nevertheless, experts across a number of continents have expressed grave concern over the disappearance of trees from agricultural landscapes and the knock-on effects this trend will have.   

The ICRAF study was published in this month's issue of Agronomy Journal of the Soil Science Society of America.

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