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.
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.
Micronutrients, as the name suggests, are required by plants in very small quantities. Zinc uptake by most plants, for instance, is 0.5 pounds per acre or less. The most common foliar micronutrients are zinc, boron and manganese (the latter primarily for soybeans). Less commonly supplied micronutrients included molybdenum and copper.
Foliar fertilizers are almost always marketed in liquid form at application rates of a quart to a few gallons per acre. On a cost per pound of plant nutrient basis, foliar fertilizers are usually much more expensive than the same micronutrient in granular form. Granular forms of micronutrients can be applied as a broadcast, in a fertilizer band (zinc for corn), or as a topdress (boron for alfalfa).
Since the goal is to have the nutrients remain on the leaf, plant stage is important when considering foliar applications. For instance, a broadcast application of a foliar fertilizer to corn that's only a foot tall would result in over half of the fertilizer winding up on the soil surface.
To avoid runoff of the fertilizer solution from the leaves, a low rate of water or other carrier is also important. This may prevent the addition of micronutrients to a fungicide application since higher volumes of carrier are normally needed for fungicides. Nutrients are absorbed by the plant only when they are in solution. Once the solution dries, the fertilizer becomes a solid again and must be re-wet by dew. However, rain can wash the fertilizer from the leaves, at which point it becomes a soil-applied fertilizer.
One advantage that foliar applications have is that they aren't affected by soil properties that can inhibit the uptake of soil-applied fertilizers. Very high levels of phosphorus, for instance, can reduce the uptake of zinc by the corn plant's root system, especially when soil zinc levels are low to begin with.
Another consideration to bear in mind . . . since foliar-applied micronutrients are not translocated in the plant, repeated applications may be necessary. For instance, one or more applications of zinc sulfate or zinc chelate may be needed to correct a zinc deficiency in corn. While it's more economical to apply zinc fertilizer either preplant or with the starter fertilizer, sometimes zinc deficiencies aren't noticed until after the corn plants have emerged. In these cases, an emergency application of zinc as a foliar spray may be economical.
Micronutrient discussions often fail to mention manure as a source of nutrients. The typical dairy manure contains a whole alphabet of major, secondary and micronutrients. In most cases, regular manure applications are sufficient to prevent most secondary and micronutrient deficiencies. Two possible exceptions are boron for alfalfa and zinc for corn. In many dairy farming areas the most common micronutrient deficiency, even where manure is used, is boron for alfalfa and other forage legumes.
At Miner Institute, we found a direct relationship between past manure applications and soil test zinc levels: The more manure applied in the previous five years or so, the higher the soil test zinc level. The grains and other nutritional supplements that dairy farmers purchase represent imports of nutrients (including micronutrients), often from outside the region. For this reason, dairy farms that apply manure to their fields at least once during the crop rotation are less likely to encounter micronutrient deficiencies