Bulletin #2251, Potassium: An Essential Element for Potatoes (PDF) By Steven B. Johnson, Ph.D., Extension Crops Specialist, University of Maine Cooperative ExtensionReviewed by John Jemison, ExtensionProfessor, Soil and Water Quality, University of Maine Cooperative Extension For information about UMaine Extension programs and resources, visit extension.umaine.edu.Find more of our publications and books at extension.umaine.edu/publications/. Figure 1.