Journal or Book Title
Sulfur is often classified as a “secondary” essential element, mainly due to a smaller plant requirement, but also because it is less frequently applied as a fertilizer compared to N, P, and K. This has certainly been the case in Iowa, where research had not documented S deficiency or fertilization need for optimal crop production. However, if deficient, S can have a dramatic effect on plant growth and crop productivity – more than the classification “secondary” would imply.
International Plant Nutrition Institute
Sawyer, John E.; Lang, Brian J.; and Barker, Daniel W., "Sulfur Emerges as a Nutritional Issue in Iowa Alfalfa Production" (2011). Agronomy Publications. 63.