Why is Saudi Arabia buying up African farmland?

Via FP, here

"....Saudi Arabia recently purchased 500,000 hectares of land in Tanzania and Indian companies have bought land in Ethiopia, Kenya, Madagascar, Senegal and Mozambique....

... as governments or markets make land available to prospecting investors, large-scale land acquisitions may result in local people losing access to the resources on which they depend for their food security – particularly as some key recipient countries are themselves faced with food security challenges. ...."food is being shipped out and poor people are dying ..."