Crop Productivity Response on Sandy Soils of Senegal Amended with Native Shrub Residues.

Over the past 10 years it has become evident that repeated additions of organic matter to soils of the Sahel are needed to maintain crop yields and soil quality. However, this semi-arid environment has limited organic resources due to low biomass and manure production. We have discovered an unrecognized resource, two native shrub species (G. senegalensis and P. reticulatum) that coexist with crops in Senegal and throughout the Sahel but their potential as soil amendments is largely uninvestigated. Consequently, an experiment was initiated in 2004 in Bambey (central Senegal) that had a split-split plot factorial design with fertilizer rate (0, 0.5 and 1 x NP recommended rate) as main plot factor and off-site residues addition (1,500 and 3,000 kg ha^-1) of each shrub species as sub-plot factor in four replicates. Crop sequences followed farmers’ practices of peanut (Arachis hypogaea)-millet (Pennisetum glaucum) rotation for four consecutives years. Yield data analysis showed a significant fertilizer effect on peanut pods, millet grain as well as aboveground biomass of crops. The increase in yield caused by application of mineral fertilizer was always > 150% relative to the control. Shrub residues application did not significantly affect crop yield until the 3^rd year of residue application. In year 4, there was no shrub species effect but rate of residues had a significant effect, with higher yields obtained with the highest residue rate of 3,000 kg ha^-1. Plots receiving the highest rate of G. senegalensis or P. reticulatum residue had 34 or 23% higher millet yield, respectively, than the control. These results suggest that returning unburnt shrub residues to soil along with minimum mineral fertilizer supply represent a step towards attaining sustainability of land productivity in semiarid Sahelian ecosystems. However, it may take a number of years before such a yeild response is observed.