The header of the Article “Optimum Nutrient Solutions for Plants” by D. R. Hoagland, published in Science Vol 52, Issue 1354, pp. 562-564 on 9 Dec 1920. DOI: 10.1126/science.52.1354.562
While this started Skoog's mission to identify this 2nd factor, he interrupted his search for a postdoc with Dennis Hoagland, a pioneer in #PlantNutrition who was first to formulate a growth solution with exact nutrients: the now famous Hoagland solution. The second important mentor for Skoog.