I have been trying to estimate how much private investment is going into African agriculture. I can't find a comprehensive source, but on the private equity side, Reuters has a decent summary of private financial flows into agriculture.
The flows from institutional investors - sovereign wealth funds and the like - seem harder to track. I suspect that some small-scale investment by Chinese farmers is also dipping below the radar. This article in the People's Daily provides an interesting clue: 70% of Chinese investment in Africa is by private companies. However, I'm not sure I trust their numbers: can there really be only $32.3m of Chinese investments in Africa? Per day perhaps?