This is a great piece on the subject in The Economist:
Something akin to Asia’s rural development may, at last, be happening in parts of Africa. Since 2002 the proportion of African workers employed in agriculture has fallen from 66% to 57%. Yet the real value of agricultural production has grown at an average pace of 4.6% a year, double the rate between 1970 and 2000. Even so, the region is lagging behind. Most of the increase comes from using more land, rather than improved productivity.
A good deal of the divergence in agricultural productivity after the 1970s is driven by African states failure to increase the use of fertilizers.
More broadly, smart policy to increase agricultural productivity must focus on market reforms (to ensure most of the surplus in the sector goes to farmers and to intensify financialization), rationalization of farm subsidy regimes, addressing the question of farm size (increasing urbanization may reduce the political cost of land consolidation in the region), and investing in logistics to reduce wastage between farms and markets (including transportation and storage).