There is actually a lot of good, arable land in Africa; the problem is that ag productivity in Africa and other poor countries is hindered by the failure of farmers to use modern inputs (i.e., fertilizer, drought- or flood-resistant seed varieties, etc.) These gains from new technology are not only a reduction of downside risk to the farmer (i.e., crop loss) but also come from the crowding-in of other investments that increase productivity in normal years too.
Parts of Southern Africa are wonderful for farming.