The land rush, which is still accelerating, has been triggered by the worldwide food shortages which followed the sharp oil price rises in 2008, growing water shortages and the European Union's insistence that 10% of all transport fuel must come from plant-based biofuels by 2015.
In many areas the deals have led to evictions, civil unrest and complaints of "land grabbing".
The experience of Nyikaw Ochalla, an indigenous Anuak from the Gambella region of Ethiopia now living in Britain but who is in regular contact with farmers in his region, is typical. He said: "All of the land in the Gambella region is utilised. Each community has and looks after its own territory and the rivers and farmlands within it. It is a myth propagated by the government and investors to say that there is waste land or land that is not utilised in Gambella.
"The foreign companies are arriving in large numbers, depriving people of land they have used for centuries. There is no consultation with the indigenous population. The deals are done secretly. The only thing the local people see is people coming with lots of tractors to invade their lands.
"All the land round my family village of Illia has been taken over and is being cleared. People now have to work for an Indian company. Their land has been compulsorily taken and they have been given no compensation. People cannot believe what is happening. Thousands of people will be affected and people will go hungry."
It is not known if the acquisitions will improve or worsen food security in Africa, or if they will stimulate separatist conflicts, but a major World Bank report due to be published this month is expected to warn of both the potential benefits and the immense dangers they represent to people and nature.
Leading the rush are international agribusinesses, investment banks, hedge funds, commodity traders, sovereign wealth funds as well as UK pension funds, foundations and individuals attracted by some of the world's cheapest land.
Together they are scouring Sudan, Kenya, Nigeria, Tanzania, Malawi, Ethiopia, Congo, Zambia, Uganda, Madagascar, Zimbabwe, Mali, Sierra Leone, Ghana and elsewhere. Ethiopia alone has approved 815 foreign-financed agricultural projects since 2007. Any land there, which investors have not been able to buy, is being leased for approximately $1 per year per 2.5 acres.
Saudi Arabia, along with other Middle Eastern emirate states such as Qatar, Kuwait and Abu Dhabi, is thought to be the biggest buyer. In 2008 the Saudi government, which was one of the Middle East's largest wheat-growers, announced it was to reduce its domestic cereal production by 12% a year to conserve its water. It earmarked $5-billion to provide loans at preferential rates to Saudi companies which wanted to invest in countries with strong agricultural potential .
Meanwhile, the Saudi investment company Foras, backed by the Islamic Development Bank and wealthy Saudi investors, plans to spend $1-billion buying land and growing seven million tonnes of rice for the Saudi market within seven years. The company says it is investigating buying land in Mali, Senegal, Sudan and Uganda. By turning to Africa to grow its staple crops, Saudi Arabia is not just acquiring Africa's land but is securing itself the equivalent of hundreds of millions of gallons of scarce water a year. Water, says the UN, will be the defining resource of the next 100 years.