Few smallholder farmers in the tropics/sub-tropics grow staple food crops at anything near their potential yield (e.g. maize potential yield is 7-9 tonnes/ha and the average actual yield in Africa is about 2 tonnes/ha). If this yield gap could be filled it would solve the issues of food security, indeed there would be an excess. Filling the yield gap requires better soil/land husbandry (improved soil fertility and soil ecological health), especially as poor farmers cannot afford to buy fertilizers and pesticides. The use of leguminous trees and shrubs as 2-3 year "Improved fallows" or "relay cropping" can partially restore crop yields (for maize to about 4-5 tonnes/ha), so freeing land for other crops, especially cash crops like fruit trees. The farmer then has the choice to purchase inputs to raise yields to the full potential.
African farmers have suggested that they would like to grow their traditional foods (originally gathered from forests) as new cash crops. After 20 years R&D this is now happening (10,000 farmers in 400 villages) with very exciting livelihood and environmental impacts (see attached), which include the creation of new business and job opportunities for young people and especially women in post-harvest value-adding, and local / regional trade in traditional markets. I have published a highly adaptable, generic 3-step model which I believe to be applicable in semi-arid to humid-zone regions. I believe that up-scaling this approach could resolve the agricultural sustainablity crisis. See also:
Leakey, R.R.B. 2014. Twelve Principles for Better Food and More Food from Mature Perennial Agroecosystems, In: Perennial Crops for Food Security, 282-306, Proceedings of FAO Expert Workshop, Rome, Italy, 28-30 August 2013, FAO. Rome.
Leakey, R.R.B. 2014. The role of trees in agroecology and sustainable agriculture in the tropics. Annual Review of Phytopathology 52: 113-133.
Leakey, R.R.B. 2014. An African solution to the problems of African agriculture, Nature & Faune 28 (2): 17-20.
Roger Leakey