1/12/2021 - By Lynn Griffith

Did you know plants absorb nutrients through their foliage, but only while the spray is wet on the leaves? Once the spray dries, absorption ceases until the leaves are moistened again, either by dew, irrigation, rainfall, or fog. To get the most out of your foliar nutritional program, I have put together a guide below.

Some plants absorb nutrients through their leaves better than others

Plants with thinner, less waxy cuticles tend to absorb better. Varieties such as roses, impatiens and poinsettias will generally absorb better than plants such as rhododendron, conifers and ligustrum. Finer textured turf grasses such as bermudagrass and ryegrass will absorb better than more succulent turf varieties such as St. Augustine.

Foliar absorption may take place through stomates

Some plants have equal numbers of stomates on the upper and lower leaf surfaces. Others have more stomates on the underside of the leaves. Therefore, getting nutritional sprays on the undersides of the leaves may increase nutrient absorption through the stomates. By spraying the undersides of the leaves, they also stay wet longer than on upper leaf surfaces in most cases. Absorption can therefore be greater on the underside, even if stomates distribution is uniform.

Timing of nutritional sprays is important

Try to spray early in the morning when possible. Mornings are typically cooler and the stomates tend to be open. The sprayed leaves will stay wet longer and absorption is increased. Cloudy cool days are preferable to hot sunny days.  Wetting agents or surfactants may increase nutrient absorption. They improve the amount of contact between the spray and the foliage. More leaf surface area is covered, and there are fewer drops and droplets. Harrell’s has several excellent products for this. The use of stickers may help keep the materials on the leaf longer in rainy areas or frequently irrigated situations.

Adding urea or potassium nitrate may increase absorption of other nutrients

In field and vegetable crops, some studies show that 50% of applied urea and 50% of foliar applied potassium nitrate can be absorbed within eight hours. Micronutrient absorption is much slower and less efficient. In most plants, foliar urea is absorbed more efficiently and rapidly than nitrate or ammoniacal nitrogen, though plants can absorb all three. You can get more micronutrients into plants if you spray major nutrients with them.

Increasing the application rate does not necessarily increase foliar absorption

The limiting factor is simply how much the leaf is able to absorb. Increasing the dosage can waste material and increase the chances of phytotoxicity. Occasionally, nutrient toxicity can occur when spraying at high rates. Often, the difference between maximum absorptive spray rate and the rate that may burn is large. Growers and superintendents are better off spraying more frequently with moderate rates than with very high doses.

Younger leaves often absorb better than older leaves

Young leaves have thinner cuticles for easier penetration. Older leaves may have thicker cuticles and a harder leaf surface. Most micronutrient deficiencies occur in the newer leaves. Nutrient distribution within the plant can be different with foliar absorbed nutrients as opposed to root absorbed nutrients.

Foliar nutrition is a safe, useful, economical, and an efficient way to provide supplemental major, secondary and micronutrients to plants. It is not a substitute for a good controlled-release or liquid feed program, but it is an excellent technique for supplemental fertilization and for correcting nutrient deficiencies.

To find out more tips on foliar absorption, you can contact your Harrell’s Sales Rep or check out this article written by Dr. Raymond Snyder.