The combined application of organic resources (OR) and mineral fertilizers (MR) is increasingly gaining recognition as one of the appropriate ways to address soil fertility decline in sub-Saharan Africa (SSA). We conducted a meta-analysis of previously published studies that have examined the influence of OR and MR on crop yields. Our objective was to provide a comprehensive and quantitative synthesis of conditions under which OR, MR, and combined OR and MR influence crop yields in SSA. Four quality classes used were previously described, with classes I and II having >2.5% N while classes I and III have <4% polyphenol and <15% lignin.

Results/Conclusions

The addition of OR resulted in 44% yield increase over the control compared to 75% for both MR and OR+MR. Greater yield responses over the control were observed with class I OR with 64% yield increase compared to 5% with class IV OR while class III OR resulted in 40% yield increase. Surprisingly with MR, there were no differences in yield responses between < 30 kg N ha^-1 and > 100kg N ha^-1. There were greater yield increases with the combined application of OR with MR over sole OR in sandy (52%) than clayey soils (25%). Similar to sole OR, greater yield responses were observed with low OR quality (43%) compared to 24% for high quality OR when combined with MR. However, when yield responses were expressed per unit of N added there were negative responses to combined OR+MR compared to sole OR with -190% and -7% yield responses for low and high quality OR, respectively. The combined OR+MR compared to sole MR resulted in negative yield responses per unit of N added: low quality OR resulted in -14% compared to -45% with high quality OR. There were, however, no differences in yield responses between class I and II. These results suggest that there are greater yield differences following the addition of MR compared to OR. Furthermore, greater yield increase with combined OR and MR are observed when compared to sole OR than MR. There are greater yield benefits observed following the addition of class I and II. In conclusion, although there are greater yield increases following the combined application of OR with MR, they are not enough to cause a positive yield increase per unit of N added compared to the sole application of OR or MR.