Marginal economic analysis can be applied to the relationship between nitrogen fertilizer as an input factor and crop yields to determine the rate of application of fertlizer thet will maximize returns from its application. This method of analysis is supplemental to other analyses of experimental data by methods specified by the designs of agronomic experiments. This bulletin reports on the application of a method of economic analysis illustrated by experimental results fom plots designed to test the application of nitrogen fertilizer on irrigated corn in Oregon, Washington, and Nebraska. Under existing information, the exponential function (Y1 = M - ARxi) is a logical function to represent the relationship for the application of a single input. This functionis used to represent the yield response curve of eperimental data. Unless the economic implications of the most profitable rate in terms of the comparative returns to nitrogen are observed, the relative importance of different rates may be overlooked. The most profitable rate of application at which returns to nitrogen will be maximized may be identical for two different situations but the returns to nitrogen may differ greatly. The comparative returns to nitrogen form the primary criterion by which one situation can be selected over the other. Differences between the most profitable rates of nitrogen application for an estimated yield curve and its confidence limit curves may be substantial, but the differences in the return to nitrogen are small. The (...).