The primary way a plant takes up nutrients is through its root system. Nutrients that are needed at rates of 10 to 100 or more pounds per acre must be taken up via this route. Nothing happened over the millennia to change this simple mechanism.

Meanwhile, leaves are designed to absorb sunlight, photosynthesize and transpire water and gases. Transpiration is a movement of materials out of the plant, not into the plant. It occurs through stomata which make up about 10 percent of the leaf surface and have been called the "lungs" of a plant.

Supplying nutrients through leaf uptake is a challenge because that's not the primary purpose of plant leaves. However, even though nutrient uptake through the leaves isn't a "natural" process, that doesn't mean it's not possible.

Foliar-applied nutrients can enter the plant either through the stomata or by penetrating the waxy cuticle on plant leaves. But only very small amounts of nutrients can enter this way which is why major or secondary nutrients such as nitrogen, phosphorus, potassium, sulfur, calcium and magnesium are seldom applied as foliar fertilizers. Foliar applications of major nutrients, primarily nitrogen, have been tested in university trials with little success.

What about micronutrients?

Most foliar applications are used to supply micronutrients. Some agronomists recommend using a nitrogen solution as a carrier for summer fungicide applications to corn. However, as noted in the previous paragraph, research has found very modest yield improvements from foliar-applied nitrogen (N). The gains are certainly not enough to pay for the cost of the application, so most of these agronomists recommend foliar nitrogen only if it's applied as the carrier for a fungicide. One study showed that uptake of foliar-applied N would be enough to supply the corn plant's needs for less than a day.