Soil fertility and crop yields under improved-fallow systems in southern Mali

Improved fallows involving short-rotation woody and/or other perennial spec ies are increasingly being tried as a means of sustaining crop production i n impoverished farming systems of sub-Saharan Africa. The soil-improving po tential of two tree species [Gliricidia sepium (gliricidia) and Pteocarpus erinaceus (pterocarpus)] and a shrubby legume Stylosanthes hamata (stylosan thes) was investigated in a four-year study on farmer's field and in a rese arch station in the Koutiala region in southern Mali (12.25 degrees N, 5.42 degrees W; 650 to 850 mm annual, unimodal rainfall; on a Typic Plinthustal f). Two strategies for soil improvement were tested: improved fallow sensu stricto and biomass transfer, the former 'on-farm' and the latter 'on-stati on.' In the on-farm experiment, maize (Zea mays) yielded best when it follo wed a two-year fallow of gliricidia alone or in association with stylosanth es. On the research station, maize dry matter production and grain yields d uring two seasons following the application of ex-situ grown biomass were s imilar and highest in the stylosanthes- and manure-applied treatments. The biomass decomposition rates of the three species were in the order: glirici dia > stylosanthes > pterocarpus. The treatments did not have any significa nt effects on the soil fertility parameters that were monitored, possibly b ecause of the short-term nature of the study. The treatments effects on cro p yields were, however, significant. Gliricidia + stylosanthes in combinati on appeared to be particularly promising as short-rotation-fallow species i n the region.