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.
Heady, Earl O.; Pesek, John T.; and Brown, William G.
"Crop response surfaces and economic optima in fertilizer use,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 32
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol32/iss424/1