Lightning provides nitrogen fixation that fertilizes crops, but there have been no estimates on how this natural provision of a critical input affects actual production and incentivizes farm behaviors. Based on a large panel of US counties with geo-referenced lightning activities, I exploit exogenous variation in lightning and find that a one standard-deviation increase in lightning flashes on average increases soybean yields by 2.2%, and the impact is larger in places with lower soil productivity. In contrast, there is minimal or even negative yield impact on corn as its nitrogen demand has been met by commercial fertilizer. On farm behaviors, I find fertilizer application is unaffected by the nitrogen windfall, but extra nitrogen carry-over provided by strong lightning last year significantly stimulates corn expansion. The results provide short-run implications on improving nitrogen use efficiency in agriculture, and they also imply potential long-run benefits as lightning co-evolves with climate change.