A Meta-Analysis of Integrated Soil Fertility Management Effects on Maize Productivity in Sub-Saharan Africa.

Integrated Soil Fertility Management, which necessarily includes the combined application of organic resources (ORs) and fertilizers, aims to increase crop yields, agronomic efficiency and address soil fertility decline in sub-Saharan Africa (SSA). A meta-analysis was conducted to provide a comprehensive and quantitative synthesis of conditions under which ORs, N fertilizers, and combined ORs with N fertilizers positively or negatively influence crop yields, agronomic efficiency and soil organic C (SOC) in SSA. Four OR quality classes were assessed; classes I and II had >2.5% N while classes III and IV had <2.5% N and classes, I and III had <4% polyphenol and <15% lignin. On the average, yield increases over the control were 62%, 85% and 115% following the addition of ORs, N fertilizers and ORs + N fertilizers, respectively. There was a general increase in yield responses with increasing OR quality and OR-N quantity, both when ORs were added alone or with N fertilizers. Surprisingly, greater OR residual effects were observed with high quality ORs and declined with decreasing OR quality. The greater yield increases with ORs + N fertilizers than either resource alone were mostly due to extra N added and not improved N utilization efficiency because negative interactive effects were, most often, observed when combining ORs with N fertilizers. Mixing fertilizers with manure resulted in the highest agronomic efficiency values [36 kg (kg N)^-1] while organic inputs of medium quality also showed significantly higher N-AE values compared with the sole fertilizer treatment. While ORs and ORs + N fertilizer additions increased SOC by at least 12% compared to the control, N fertilizer additions were not different from control suggesting that ORs are needed to increase SOC. Thus, the addition of ORs will likely improve nutrient storage while crop yields are increased and more so for high quality ORs.