This paper proposes a method for building general models based on continuous analysis of data from on-farm research (OFR) in order to derive recommendations for farmers. The data requirements are not excessive for practitioners of OFR in developing countries, and the paper analyzes problems involving only one or two generic nutrients. Following a brief introduction (Chapter 1), the paper reviews the basic technical and economic concepts related to the continuous analysis of fertilizer response data (Chapter 2) and proceeds to a discussion of the criteria involved in choosing among various functional forms (Chapter 3). Chapter 4 presents a method for constructing a general model that can be used to derive recommendations for farmers. The final chapter covers special topics, including the use of spline functions and how to determine optimum levels of fertilizer for multiple outputs.viii, 103 pages