Effectiveness
"Dr. H.B. Tukey, renowned plant researcher and head of the Michigan State University (MSU) Department of Horticulture in the 1950s, working with research colleague S.H. Wittwer at MSU, first proved conclusively that foliar feeding of plant nutrients really works. Studies were conducted using radioactive phosphorus and potassium to verify its veracity. Plant foliage was sprayed and measured with a Geiger counter to observe absorption, movement, and the utilization of plant essential nutrients. Results from the experiments quantified plant nutrients moving at the rate of about one foot per hour to all parts of the plants."
Foliar feeding is considered especially useful for introducing trace elements, or for "emergency" feeding of plants which are found to have a specific shortage. But in some cases, with tomatoes, for example, it is believed that foliar feeding during flower set causes a dramatic increase in fruit production.
Read more about this topic: Foliar Feeding