PLANT LEAVES CONSUME NUTRIENTS IN THE SAME WAY THAT THEY CONSUME AIR.
A plant can absorb nutrients through it's leaves at a quicker rate than it can through it's roots. Nutrients pass through the stomata of the leaves. Stomata are pores on leaves that allow plants to “breathe.”
When stomata is open, water can pass through. When water loss occurs through leaves, the process is called transpiration.Not only can water leave through the stomata, it can also enter. And with that water that enters, nutrients that are dissolved will enter as well. When the stomata is closed, air, water and nutrients can not enter the leaf.
The nutrients provided through foliar spray will travel through the entire plant, all they way to the roots. Like carbohydrates produced through photosynthesis, nutrients from foliar spray will also disperse through the rest of the plant. In fact, up to 90% of a foliar-fed solution can be found in the roots of a plant within 1-hour of application 
HIGH EFFICIENCY FERTILIZATION
Foliar spraying is a highly efficient method for delivering nutrients. Agricultural scientist S.H. Witter pioneered some of the first scientific studies into nonroot plant feeding in the early 1950s at Michigan State University. He found that the uptake of various plant foods was from 100% to 900% more effective when the nutrients were applied to the leaves instead of to the soil. 
TOO MUCH OF ANYTHING IS A BAD THING
Foliar spraying can cause more harm than good if used in excess, high concentrations, or if applied under strong light leading to burn. Certain nutrients are more quickly absorbed through foliar sprays. Nutrient sprays can cause severe injury to fruit, leaves, shoots and buds. Therefore their use should be considered as hazardous. Do NOT apply unless a deficiency or low level of that specific nutrient is known to exist and has been confirmed by visual symptoms or tissue tests. Use dilute sprays and as low a rate as possible. Concentrates can cause serious injury.