The experiments upon which this study is based were designed to allow (1) estimation of the fertliizer-crop production surface and (2) specification of economic optima in level of fertilization and combination of nutrients. Two nutrients were varied on each experiment.

The corn experiment, on calcareous Ida silt loam soil, included nine rates each of N and P₂O₅. Red clover and alfalfa were on Nicollet and Webster loam soils with P₂O₅ and K₂O as the variable nutrients. Each experiment included two replicates of 57 different nutrient combinations - 114 completely randomized observations.