Micro-nutrients
Some elements are directly involved in plant metabolism (Arnon and Stout, 1939). However, this principle does not account for the so-called beneficial elements, whose presence, while not required, has clear positive effects on plant growth. Mineral elements which either stimulate growth but are not essential or which are essential only for certain plant species, or under given conditions, are usually defined as beneficial elements.
Read more about this topic: Plant Nutrition, Functions of Nutrients