In this study we assess the effects of a decentralized extension program and an additional video intervention on the adoption of integrated soil fertility management (ISFM) among 2,382 farmers in Ethiopia using a randomized controlled trial. ISFM should enhance soil fertility and productivity by combining organic and inorganic soil amendments. We find that both extension-only and extension combined with video increase ISFM adoption and knowledge. We further find evidence for increased adoption of ISFM practices among farmers in treatment communities that do not actively participate in the extension activities. The additional video intervention shows a significant complementary effect for these non-actively involved farmers, in particular regarding the combined use of the practices on the same plot. A causal mediation analysis reveals that increases in knowledge explain part of the treatment effects on adoption.