Fertile soils are critical to boosting cereal crop yields in Africa.

Flickr/Yosef Hadar/World Bank

Replenishing soil fertility with mineral and organic fertilisers could triple cereal crop yields in tropical Africa and achieve an African green revolution, says Pedro A. Sánchez from the Earth Institute.

Staple crop yields have not risen from one tonne per hectare in most of tropical Africa since the 1960s. The problem, according to Sánchez, is decades of farming without adequate fertiliser that have "stripped the soils of the vital nutrients needed to support plant growth".

The eighty 'Millennium Villages', set up in 2005 across ten African countries to promote community-led development, show the extent to which better soil fertility improves yields. Through mineral fertiliser applications, improved cultivars and the latest agronomic knowledge, maize yields have surpassed three tons per hectare in 78 per cent of village households, says Sánchez.

And at a national level, Malawi's fertiliser subsidy scheme — that gives farmers two-thirds off the market price of mineral fertilisers — has increased maize production from 0.8 tonnes per hectare in 2005 to 2.2 in 2007. The scheme has transformed the country from a receiver of food aid to a food exporter and donor, says Sánchez.

To build on these initial successes, African countries must now focus on adding organic fertilisers to their soils. "Only organic fertilisers add carbon, feed soil microbes and help to retain soil moisture", writes Sánchez. The best way of applying them, he adds, is to grow leguminous trees that capture nitrogen from the air and transfer it to the soil.

Such 'nitrogen-fixing' trees could capture 50–100 kilogrammes of nitrogen per hectare per year in tropical Africa — similar to the amount added by mineral fertilisers. They have the added benefit of providing fuel wood.

But to establish the nitrogen-fixing system, farmers have to forgo one crop, which makes it unattractive to most farmers in tropical Africa. Governments could overcome the hurdle by subsidising nitrogen-fixing trees, suggests Sánchez.

Link to full article in Nature Geoscience