Improved fallows come of age in the tropics

Improved fallows are the deliberate planting of fast-growing species - usually legumes - for rapid replenishment of soil fertility. Improved fallows are rapidly spreading in several regions of the tropics as a sensible way for in situ accumulation of large quantities of N in vegetation and soil, as well as for providing sustainability enhancing services. Research on improved fallows increased after the mid 1980s with the development of what is known as the second soil fertility paradigm, which is based on sustainability considerations. Many lessons have emerged from short-term improved fallows (< 5 years duration). These include the diversity of farm sizes where improved fallows are used, the advantage of sequential versus simultaneous systems, the utilization of dry seasons unfavorable for crop production, the comparative advantages of woody versus herbaceous leguminous fallows, the magnitude of N accumulation, the strategic use of N fertilizers, and the importance of P. Other key services provided by fallows include fuelwood production, recycling of nutrients besides N, provision of a C supply to soil microorganisms, weed suppression, Striga control, and improved soil water storage. Natural fallows of non-legume shrubs belonging to the Asteraceae family, collectively called 'daisy fallows', may provide lessons for the development of improved fallows. The maintenance of genetic diversity in fallows is paramount. The main limiting factor in Africa is clearly the supply of germplasm of improved fallow species. This must be overcome though large-scale seed orchards and nursery development before impact at the scale of millions of farmers can take place.