Many attempts have been made to understand the plant response to water using plant growth models. The effects of fertilizer on crop yield are greater than those of water. Therefore, some models that have been developed for the evaluation of water-yield relationships can also be used for fertilizer-yield relationships. The purpose of this study was to develop an equation to use in fertilizer applications similar to one used to evaluate crop production under adequate and deficit water supply regimes. The equation can be rewritten for the fertilizer consumed by plants instead of the evapotranspiration (ET). Sample applications showed that the resulting ET-yield response factor equation can be used successfully in the evaluation of fertilizer-yield relationships. Thus, the yield loss per unit of fertilizer deficiency can be determined more precisely by the equation.