"Fertilizer Tree" May Revive African Farmlands

Ochieng' Ogodo in Nairobi, Kenya
for National Geographic News
September 3, 2009
For one and a half decades, Johannes Mutisya, 54, a farmer in Kenya's Makueni district just east of Nairobi, has done what he could to eke out a living—but with little success.

He has bent wearily and scratched the dry, hardened earth, sprinkling a few maize or bean seeds and hoping for the best.

"These days we are only trying," Mutisya said, a distant look on his face.

"It is not like two decades ago and beyond, when people could be sure of bounty harvest after the rains."

Mutisya's situation is occurring across Africa, where farmlands are severely degraded and production is down.

African farmers on average apply only 10 percent of the soil nutrients, such as fertilizer, used in other parts of the world.

Coupled with effects of climate change, such as extreme droughts, the situation seems bleak.

But growing trees—especially acacia— on farms can improve the lot of some African farmers, said Dennis Garrity, who heads the Nairobi-based World Agroforestry Center.

Garrity spoke at the Second World Congress of Agroforestry held in Nairobi in August, which convened more than a thousand international experts to discuss the importance of growing trees on farms.

Nitrogen Fixer

The tall, long-lived acacia tree Faidherbia albida could serve as a free source of long-lasting and crop-boosting nitrogen, Garrity said.

A nitrogen fixer, the tree species could limit the use of polluting chemical fertilizers while also providing animal feed, construction material, and even medicine for farmers across sub-Saharan Africa.

"This is a fertilizer tree with reverse leaf phenology, which makes it become dormant and shed off its nitrogen-rich leaves during the early rainy season and at planting time when seeds need nitrogen," he said.

"And [then the tree] regrows the leaves at the beginning of the dry season, and thus does not compete with crops for light."

The tree species also acts as a windbreaker, provides wood for fuel and construction, and checks soil erosion by making the soil loose for water absorption during rainy season, Garrity added.

Acacia, iconic trees of the African landscape, are well adapted to a wide array of climates and soils, from the deserts to the humid tropics.

"Fertilizer Trees"

Scientists first observed farmers in Africa's Sahel region growing the trees in their sorghum and millet fields about 60 years ago.

Today the practice is still seen in Senegal, Mali, Burkina Faso, Niger, Chad, Sudan, and Ethiopia, and in parts of northern Ghana, northern Nigeria, and northern Cameroon. (See a map of Africa.)

In Zambia preliminary research has found that unfertilized maize yields in the vicinity of acacia trees averaged nearly three times those of crops grown nearby but beyond the trees' canopy.

Nobel Peace Prize laureate Wangari Maathai told the agroforestry conference that Africa must return to sustainable agriculture that embraces planting of "fertilizer trees" such as acacia.

(Related: "Food of the Future to Be More Diverse?")

"We have done great damage to the ecosystems through less sustainable agriculture practices like monoculture that has contributed to food insecurity in Africa," Maathai said.

"We need to encourage farmers to grow many food crops to reduce vulnerability of communities."

Acacia trees may also give small farmers benefits in the lucrative carbon-storage market, said Archim Steiner, executive director of the United Nations Environment Program (UNEP).

The World Agroforestry Center and UNEP are developing a standard method for measuring carbon storage on all types of landscapes, which could provide a basis for providing farmers with a financial incentive to increase tree cover on their farms.

Climate change talks scheduled later this year in Copenhagen will consider a new strategy that could include such a program.

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