https://blogs.scientificamerican.com/observations/farming-degrades-land-farming-can-also-bring-it-back/

Conventional farming can be hard on land: clearing forest for crops, excessive tilling and continuous cultivation all cause the earth to lose fertility, productivity and the ability to retain water—all of which endangers crops and therefore farmer incomes. Degraded land is a worldwide crisis: The FAO estimates that a quarter of the world’s land is highly degraded or in danger of becoming so. In Africa, where 80 percent of farms are small plots of about five acres or less, 65 percent of arable land is degraded. In economic terms, the effects of land degradation are equivalent to $6.3 trillion a year of impaired ecosystem function—increased costs for fertilizers or money spent on irrigation equipment, for example. 

How can we revitalize degraded land? Agroforestry, the practice of growing trees with crops or livestock, provides one elegant solution. The benefits of agroforestry are numerous: tree roots anchor soil, preventing erosion; leaves and pruned branches from the trees become a mulch that reduces soil runoff and erosion, eventually decomposing into an organic litter layer that enriches soil. And many trees species used in agroforestry are nitrogen-fixing, replenishing this key nutrient to the soil. Finally, soils rich in organic matter hold more water, which permits more and healthier plants. These remarkable changes brought about by the inclusion of trees—improvements in ecosystem services like water retention, soil fertility, and reduced erosion—are key to restoring a landscape to productivity.