Evidence for impact of green fertilizers on maize production in sub-Saharan Africa: a meta-analysis

A number of studies have tested the effect of woody and herbaceous legumes on soil fertility and maize yields in sub-Saharan Africa. Results have been mixed, however, generating debate about their effects on maize productivity. A meta-analysis was conducted with the aim of evaluating the evidence of yield benefits, or otherwise, from woody legumes and herbaceous green manure legume (HGML) treatments (Ts). Ninety-four peer-reviewed publications from West, East and Southern Africa had data complete enough to qualify for inclusion in the analysis. With unfertilized maize as the control (C) in all studies, 54 publications compared maize yield using HGMLs, 28 using non-coppicing woody legumes, 10 using coppicing woody legumes, 29 using natural fallows and 52 using fully fertilized maize monoculture. Mixed linear modelling of yield difference (D = T-C) and response ratio (RR = T/C) indicated that the yield response to legumes is positive. The mean yield increase over unfertilized maize was highest at 2.3 tonnes per hectare (t ha -1 ) for fully fertilized maize and lowest at 0.3 t ha -1 following natural fallows. The increase in yield over unfertilized maize was 1.6 t ha -1 using coppicing woody legumes, 1.3 t ha -1 using non-coppicing woody legumes and 0.8 t ha -1 using HGMLs. The coefficient of variation in D was highest using natural fallows at 229%, followed by HGMLs at 136%, non-coppicing legumes at 113% and coppicing legumes at 92%. Fertilized maize monoculture had the lowest variability at 70%. Doubling or better maize yields relative to the control (mean RR > 2) was recorded with coppicing woody species in 67% of the cases, non-coppicing woody legumes 45%, HGMLs 16% and natural fallows 19%. However, the doubling or better yields occurred only in sites with low-to-medium potential. Response was higher on Lixisols , which have few plant nutrients compared with Ferralsols and Nitisols. Amending postfallow plots with half of the recommended fertilizer dose further increased yields by over 25%. This suggests that organic inputs from legumes have synergetic effects with mineral fertilizer and that legume rotations can play an important role in reducing mineral fertilizer requirements. In all cases, the 95% confidence intervals did not include 0 for D or 1 for RR, indicating significant increase in yield response. It is therefore concluded that maize yield response to green manure legumes is significantly positive and yield is higher than in unfertilized maize and natural vegetation fallows

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