African Tree Makes Soils Rich for Crops

Aug. 28, 2009 -- A unique African tree could dramatically improve the yield of crops planted under its canopy by providing natural, renewable fertilizer, says a new study.

The tree has the potential to aid farmers throughout Africa, South America, and much of south and Southeast Asia, according to the researchers.

"Soil fertility is one of the major constraints to food production in sub-Saharan Africa, and nitrogen is one of the most limiting elements," said Lou Verchot of the Center for International Forestry Research in Bogor, Indonesia, who was not a part of the new study.

Conventional agriculture provides nitrogen through nitrogen-containing, man-made fertilizer, but it can be prohibitively expensive for subsistence farmers.

Enter Faidherbia albida, a close cousin of the acacia -- the iconic, flat-topped tree that dots the African Savannah. Faidherbia is one of several trees that can capture nitrogen from the air through its roots and incorporate it into its leaves.

But what makes it unique is that it grows in the dry season and drops its leaves come the rainy season, when crops start to grow.

"These trees drop their leaves right when the plants are needing nitrogen," Verchot said.

"It acts like a fertilizer factory," said Dennis Garrity, Director General of the World Agroforestry Center in Nairobi, who authored the report. "You get this rich leaf material which is great organic fertilizer produced free of charge in the fields."

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"The trees turn absolutely skeletal in the wet season so that they don't compete for sunlight or water or nutrients," he added. Meanwhile, the leaves and nutritious pods serve as food for livestock during the dry season when everything else has shriveled up.

Presenting at the World Congress of Agroforestry in Nairobi this week, Garrity reported a three- to four-fold increase in maize yields underneath the Faidherbia canopy compared with crops outside the canopy in studies in Malawi and Zambia. Faidherbia also increased yields in sorghum, millet and cotton fields.

Garrity hopes to spread the word about the Faidherbia tree throughout the African continent and beyond. The tree is compatible with the climate and farming practices in India and the rest of south Asia, in Southeast Asia, and in parts of South America.

In Zambia and Malawi, where Faidherbia trees are most widely used, farmers plant the trees in a checkerboard pattern every 30 feet throughout the field.

Although the Faidherbia tree has a unique life cycle, it is just one example of the important role trees in sustainable agriculture, Garrity said.

In other findings released at the Congress, the World Agroforestry Center reports satellite measurements showing that half of the world's agricultural lands, home to more than half a billion people, contain more than ten percent tree cover.