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Eneless Beyadi appears through a forest of maize clutching an armful of vegetables and flashing a broad smile.

Beyadi cultivates about half a hectare of plots in the village of Nankhunda, high on the Zomba plateau in southern Malawi. She gets up at 4 a.m. every day to tend her gardens, as she lovingly calls them, before heading off to teach at a school. In the afternoon, she returns to the gardens, which help to feed her family of six. As testimony to her efforts, the maize (corn) on Beyadi's land stands tall even in the lashing rain, whereas the stunted, yellowed stalks on a neighbour's plot bow low.

The strength of Beyadi's crop is down to more than her green fingers, though. It is also due to what she feeds the soil. Beyadi borrowed money from a European friend to purchase two 50-kilogram bags of chemical fertilizer for this growing season. Because a bag can cost up to 4,000 Malawian kwachas (US$24), it is beyond the reach of many Malawians, including Beyadi's neighbour, Catharine Changuya, an unmarried mother of four.

Fertilizers make such a profound difference here because the rusty red soil, as in many parts of Africa, is deficient in organic matter and in key nutrients such as nitrogen and phosphorus. By farming intensively without replenishing soil nutrients, farmers across sub-Saharan Africa have lost an average of 22 kilograms of nitrogen, 2.5 kilograms of phosphorus, and 15 kilograms of potassium per hectare annually over the past 30 years — the yearly equivalent of US$4 billions' worth of fertilizer. As a result, yields are meagre.

Agricultural experts worry that Africa's soil problems are heading towards a crisis. “The future picture is dire,” says Dennis Garrity, chief executive of the World Agroforestry Centre (ICRAF), headquartered in Nairobi. “Producing more food for a growing population in the coming decades, while at the same time combating poverty and hunger, is a huge challenge facing African agriculture.”

African governments, international donors and scientists all agree that farmers must revitalize their soils. But there is passionate debate about how to do it. Many African governments and agricultural scientists argue that large doses of inorganic fertilizers are the most practical solution. But others, such the Food and Agriculture Organization of the United Nations (FAO) in Rome, are pushing for greener, cheaper solutions, such as no-till farming that conserves soil and 'fertilizer plants' that boost the soil's nitrogen content organically. Researchers report that these latter techniques are beginning to raise yields and improve soil fertility. But farmers are slow to adopt such practices, which require significantly more labour.

Nature Podcast Natasha Gilbert talks about her trip to Malawi You may need a more recent browser or to install the latest version of the Adobe Flash Plugin.

Leading scientific and political figures will take the debate to the UN's Earth Summit in Rio de Janeiro, Brazil, in June. But whatever they recommend, the biggest test is what happens when Beyadi and other African farmers try to put into practice the grand plans of scientists, international donors and governments.

“Many people are promoting approaches without understanding the conditions in Africa and the communities and what works for them. They mean well, but they need to appreciate the realities of the smallholder farmer,” says Bashir Jama, director of the soil-health programme for the Alliance for a Green Revolution in Africa (AGRA), based in Nairobi.

Sub-Sarahan Africa is one of the poorest regions on Earth, in both living standards and soil fertility. The depleted soil has caused average yields of grain crops to stagnate at around 1 tonne per hectare since the 1960s. By contrast, yields now reach 2.5 t ha−1 in south Asia and 4.5 t ha−1 in east Asia, where chemical fertilizers have been widely adopted since the green revolution (see 'Uneven landscape'). Fertilizer use across Africa has remained at around 9 kg ha−1 of cultivated land over the past 40 years, whereas Asia uses 96 kg ha−1 of inorganic fertilizer.

SOURCE: IFDC

Cost is one of the biggest problems. Because of transport expenses, farmers in inland Africa pay more than twice as much for fertilizer as farmers in Europe. And supply is often unreliable because of poor distribution systems.

The World Bank and other major international donors helped to fund fertilizer use in sub-Saharan Africa in the 1970s and 80s, but they came to see such subsidies as a drag on private-sector development and cut them off, pushing African nations to cease offering them as well. However, when Malawi faced a major food crisis in 2005, President Bingu wa Mutharika, who was facing re-election, reintroduced subsidies for fertilizers and improved seeds.

Over the next few years, that policy reaped strong agricultural gains, which came to be known as the Malawi miracle. As fertilizer use in the country almost doubled between 2005 and 2009, maize yields surged from around 1 t ha−1 in 2005 to just under 3 t ha−1 in 2009–10, according to government figures. The agricultural subsidy programme cost the Malawian government $461.4 million over five years, and comprised 13.5% of the national budget in 2009, the most recent year for which figures are available.

Obtaining accurate data on yields is difficult, says Andrew Dorward, an agro-economist at the School of Oriental and African Studies in London, who has analysed the Malawi subsidy scheme, but he is convinced that the subsidy is having a positive effect. And he is not alone. “Before the subsidy the country was a patchwork of yellow maize. Today it's all green,” says Stephen Carr, a retired consultant on African agriculture based in Zomba, who has worked there since the end of the 1980s.

The rise in crop yields helped to convince the World Bank to soften its stance in 2007, when it announced that subsidies “may be justifiable on a temporary basis to stimulate increased fertilizer use in the short term”.

Other African nations have watched Malawi's experiment with interest, and some — including Rwanda, Zambia and Mali — have ramped up their own schemes. Rwanda began subsidizing fertilizer transport costs in 2006. Fertilizer imports to the country nearly tripled between 2005 and 2007, and wheat and maize yields have increased by 16% and 73%, respectively, over the past five years. The Rwandan system differs from Malawi's by encouraging the development of a fertilizer distribution industry, which is more attractive to donors and is supported by the World Bank.

An international collaboration of researchers is hoping to improve the use of fertilizers by developing digital soil maps covering 42 African countries south of the Sahara. Started in 2009 with an $18-million grant from AGRA and the Bill & Melinda Gates Foundation to the Tropical Soil Biology and Fertility Institute in Nairobi, the maps will provide up-to-date information on soil properties, derived from satellite measurements and sampling at 60 sites across Africa. Keith Shepherd, a soil scientist at ICRAF who has worked on the maps, says that the analysis will inform agronomists and agricultural extension services about soil health and what nutrients are lacking. “Until now there was no unbiased sampling at this scale so there was no reliable data on acute problems,” he says.

AGRA is also helping to bring some of Africa's better-quality phosphate deposits into production, which will provide sub-Saharan countries with a cheaper source of locally produced phosphate fertilizer.

N. Gilbert

Even so, fertilizer use in Africa is at the mercy of precarious politics. Although Rwanda's fertilizer programme is growing, Malawi's has started to fall apart as the country's economy has collapsed and its international relations have deteriorated. Many of Malawi's biggest donors, including the UK government's Department for International Development, suspended budgetary support to the nation last year because of concerns about governance and the Malawian government's refusal to devalue its currency as recommended by the International Monetary Fund.

Although the United Kingdom reinstated some funding to help transport fertilizer, many Malawians couldn't purchase it this year. Changuya walked for an hour and a half to the depot in town, only to find that all the subsidized fertilizer was gone and she would not have been able to afford it anyway.

Greener solutions

With fertilizer-subsidy schemes in trouble, many researchers and donors are supporting more-sustainable methods of boosting yields. They argue that long-term fertilizer use is not only too expensive but also degrades the environment, in particular by releasing the greenhouse gas nitrous oxide.

“The conventional approach has been to focus on improved seeds and chemical fertilizers, but I think there is plenty of evidence to show that there are alternatives that demand significant attention,” says Garrity.

One green solution gaining attention is to plant high-protein nitrogen-fixing legumes such as pigeon pea, peanuts and soya beans. With help from bacteria in their roots, legumes capture nitrogen from the atmosphere and convert it into compounds that can be used by plants. They can add up to 300 kg N ha−1 to the soil in a season. Farmers can plant legumes next to grain crops or they can be alternated by season. Studies in experimental plots in Malawi showed that the legumes increased maize yields by 116%.

Although that strategy looks great on paper, poor farmers cannot generally give up much of their limited land to grow legumes, which require extra labour. So, many of the leguminous plants are underperforming in the field. On the average smallholding in Africa, they often fix less than 8 kg N ha−1 per year.

Ken Giller, an agronomist at Wageningen University in the Netherlands, is hoping to tackle these problems through a four-year research programme called N2 Africa, which he helped to start with $22 million from the Bill & Melinda Gates Foundation and the Howard G. Buffet Foundation. The project aims to breed edible legumes with increased yields and better nitrogen-fixing ability, and to help spread legume crops across Malawi and seven other African nations.

WOODFALL WILD IMAGES/PHOTOSHOT

The FAO is promoting other green ways of raising yields, in particular an approach called conservation agriculture, which involves covering fields with mulch and not tilling the soil. The FAO says that this improves soil fertility while reducing erosion and labour. In June 2011, the FAO launched a programme called Save and Grow, currently funded at around $7 million per year, which over the next 15 years aims to promote research, training and resources for this type of agriculture. But critics argue that conservation agriculture can actually decrease yields and that few farmers in sub-Saharan Africa are willing to use these techniques. The promotion of conservation agriculture “is wholly misplaced”, says Giller.

In experimental fields outside Lilongwe in Malawi, researchers are studying another type of green approach as part of an ICRAF project run by Gudeta Sileshi, an agricultural scientist at the Chitedze Agricultural Research Station. At the far end of one field, mature Faidherbia albida trees stand some 4 metres high, their leafless branches feathery against the sky. Smaller, younger trees dot the fields. They are all 'fertilizer trees', which fix nitrogen and improve the nitrogen content of the soil. Past agricultural-improvement schemes have tried unsuccessfully to sell the idea of these trees to African farmers, but there is renewed interest from researchers and governments, because tree planting is seen as one way of sequestering carbon and combating global warming.

Sileshi is testing several types of trees, but he is betting that the top performer will be F. albida, which is indigenous to Africa. Besides fixing nitrogen, it has deep roots that draw nutrients from far below the surface and store them in the tree's spiky leaves. When the leaves fall, nitrogen is returned to the top layer of the soil for use by crops planted beneath the tree's canopy. Faidherbia albida sheds its leaves early in the rainy season when crops are beginning to grow, so it doesn't compete with them for light, nutrients or water; its summer canopy of leaves reduces transpiration from crops underneath it and evaporation from the soil. In a trial in Zambia, maize yields under F. albida reached 4 t ha−1 compared with 1.3 t ha−1 outside the canopy.

In the Chimbalanga 2 village near the shores of Lake Malawi, a healthy crop of maize grows beneath the naked branches of F. albida. “If everyone planted trees, we would reduce the problem and return the soil to how it used to be,” says Beather Kandaya, a farmer in the village. Kandaya was trained in fertilizer-tree husbandry by Malawi's agricultural extension service and she passes the knowledge on to her neighbours. But the practice is not catching on quickly. In Malawi, just over 1% of farmers grow the trees.

“The fertilizer subsidy is a matter of life and death.”

This could soon change. Malawi, Niger, Kenya and Rwanda are among the African countries promoting the use of trees within farming. And international donors are starting to support tree programmes.

Beyadi's experience, though, shows how much work there is still to do. She and her neighbours care for a small experimental nursery filled with some 200 spindly seedlings of leguminous plants and trees. Of the 240 F. albida seeds that the group planted last year, only one seedling has survived. It is a common problem because the roots of F. albida are very fragile and easily damaged. And it will be 6–10 years before an F. albida sapling is ready to fertilize crops. At the Chitedze research station, ICRAF and the agricultural non-governmental organization Total Land Care, based in Lilongwe, are trying to find ways of cultivating F. albida more efficiently.

Proponents of fertilizer trees say that they deserve a chance and that they have received far less funding and promotion than other sustainable farming techniques and inorganic fertilizers. “The simplest, cheapest techniques are the ones that are ignored as they are not of interest to the private sector,” says Garrity. “It's time we no longer ignored them.”

But many agricultural experts and farmers conclude that green approaches are not enough — that Africa can't solve its soil problems without chemical fertilizers. “I continue to believe leguminous trees and plants have got to play a part in maintaining soil fertility and food security, but they can't replace inorganic fertilizer,” Carr says. “The fertilizer subsidy is a matter of life or death.”

Giller argues that to win over farmers, new techniques for increasing crop yield must bring extra benefits, which is why he is focusing his research on edible legumes. The acid test is whether farmers continue to use the new approaches after aid support ends.

Beyadi acknowledges that many of her fellow farmers will drop the new green techniques when aid goes. And although she says that she will continue to care for the fertilizer trees, she doesn't regard them as her best hope for the future. Like her neighbours, she sees inorganic fertilizers as the key to growing more food. Surveying her lush green gardens, Beyadi wonders whether she will be able to buy enough fertilizer next year to ensure an equally bountiful harvest.