The classic Mitscherlich equation is based on Liebig’s law of the minimum. It describes the yield response of a crop to an increase in the main factor that is the limiting growth. The maximum or potential yield is an essential parameter in the Mitscherlich equation and it is assumed to be constant. This means that it is not affected by other factors that limit actual yields under field conditions. This assumption does not apply to rainfed agriculture in semi-arid areas because under such conditions potential yields vary with crop-available moisture. A theoretical framework for the application of the Mitscherlich equation to rainfed crop production is presented. Water-limited yield is assumed to be a linearly increasing function of available moisture; the quantity of nutrients required by a crop to achieve water-limited potential yield is assumed to be assumed to be a linearly increasing function of seasonal rainfall. The availability of nutrients is also thought to depend on the moisture available. All moisture dependent parameters are expressed as functions of annual rainfall and therefore the general form of the modified Mitscherlich equation for response to nutrients is simplified.