We use plot-level data from the National Panel Survey to estimate maize - N respon se rates and the profitability of inorganic fertilizer use. We find that average smallholder maize-N response rates are not even 50% of those from zonal center trials, implying that there is a considerable gap between actual and potential returns from fertilizer use. Fertilizer use on maize is only marginally profitable for farmers with average response rates, even in higher potential zones. Farmers who used improved maize seed, fallowed a plot more recently and/or received an extension visit have higher response rates and more profitable fertilizer use, yet fallowing is infrequent and extension does not reach most farmers.