Northeast Research and Demonstration Farm
A long-term experiment has been conducted since 1979 at the ISU Northeast Research and Demonstration Farm, Nashua, Iowa, to study the effects of phosphorus (P) and potassium (K) fertilization on soil-test values and grain yield of corn and soybeans grown in rotation.
Iowa State University
Mallarino, Antonio P. and Pecinovsky, Kenneth T., "Phosphorus and Potassium Fertilization for Corn and Soybean Grown in Rotation for 30 Years" (2014). Iowa State Research Farm Progress Reports. 2048.