Four functional forms were investigated to determine their usefulness in estimating the relationship between soil erosion and crop yields. These functional forms were linear, polynomial, Cobb-Douglas, and Hitscherlich- Spillman. The Mitscherlich-Spillman is shown to better meet the theoretical requirements. A variable defined as the mechanical composition of the plant rooting zone is shown to be a superior predictor variable to topsoil depth. Soybean yield data for Georgia Cecil soils were fitted to a Mitscherlich- Spillman function.