In the past four decades, productivity in United States field crops has been transformed by the mechanical and fertilizer revolutions. Since input data are typically not available by crop, most investigations of productivity have been at the aggregate level. This paper develops a simultaneous equation, partial adjustment model of the demand for inputs, which generates estimates of the technical change parameters for wheat, corn, soybeans, and cotton. These estimates allow comparisons of the factor saving biases in technical change, leading to a novel test of the induced innovation hypothesis and the suggestion that the productivity slowdown may yet affect agriculture in the United States.