Abstract: The mathematical models of large area economic fertilization based on fertilizer yield response experiments were developed, by using orthogonal trend analysis. The theory and the method to develop the models were analysed and the properties of the fertilization models were discussed. The relationships among fertilizer requirements, crop yields and soil nutrient supplying amounts were established in the fertilization models. The economic rational fertilizer requirements can be estimated by the predition of yield response functions determined through testing soil nutrient supplying amounts. The models were proved by large area fertilizer field experiments for rice to have a good accuracy and sensitivity.