By Mark Kinver Science and nature reporter, BBC News

The project will focus on small-scale farmers, most of whom are women

A $180m (�90m) five-year project to revive sub-Saharan Africa's depleted soils has been launched in Nairobi.

The Alliance for a Green Revolution in Africa's (AGRA) Soil Health Program will work with 4.1 million farmers to regenerate 6.3m hectares of farmland.

Organisers hope the scheme will boost farmers' incomes, improve crop yields and protect the region's soils.

Initial funding has been provided by the Bill and Melinda Gates Foundation and the Rockefeller Foundation.

"Currently, farm yield in Africa is one-quarter of the global average, and one-third of Africans face chronic hunger," said Dr Namanga Ngongi, AGRA's president.

"We know that the use of high quality seeds, combined with the rejuvenation of African soils, can begin to turn around this dismal situation."

Focus on women

The programme will give particular attention to women, who make up the majority of small-scale farmers, who are best placed to know how various crops fare in local soils, explained Dr Akin Adesina, AGRA's vice president for policy and partnerships.

Improving soil fertility will boost crop yields and incomes for farmers

"No one size can fit all," he said. "We will work with farmers and researchers to develop locally adaptable soil interventions."

Unsustainable practices in recent decades had led to soil degradation in the region. For example, continuous cultivation of land without replacing nutrients taken up by crops had led to a fall in soil fertility.

Degraded soils were also prone to erosion and were unable to retain moisture.

Researchers said the poor conditions meant that farmers were more likely to clear forests and savannahs as they searched for arable land.

"AGRA's goal of enabling small-scale farmers to produce more on less land will have multiple social, economic and environmental benefits," said Achim Steiner, executive director of the UN Environment Programme, which is one of the project's partners.

""It can reduce the pressure to clear new land for agriculture, which in turn can assist in countering deforestation, conserving biodiversity and triggering improved management of Africa's wealth of natural and nature-based assets."

The Soil Health Program will form one aspect of the organisation's work to improve the plight of the region's farmers in the global agriculture sector.