Most of our basic growing methods rely on ground fertilizer applications to give plants the nutrients needed for healthy development. However, at one time or another we may have tried applying nutrients to correct deficiencies or accelerate growth ("green up" our plants), with mixed results.

Those "mixed results" may be because in order for the plants to absorb the foliar applied elements, we have to understand how they work in order to apply them effectively.

If we just spray them on at random, chances are that we are just throwing our money and effort away, so here are a few basic points that will help us to get the foliar micronutrients to work.

Macro vs. Micro:

Macronutrients (N,P,K, calcium, magnesium and sulphur) must be applied on the ground as the plant requirements are higher than what could be supplied through the foliage. They are also difficult to absorb through leaf tissues.

Micronutrients (including zinc, boron, copper, iron, manganese, molybdenum, etc) are usually required in small amounts and can be applied through foliage as you can normally cover the plant’s needs with two or three annual applications.

However, for the leaves to absorb them, they must be produced in a foliar spray version, which usually means that at least the zinc and the iron are chelated.

Foliar Fertilizer Application:

One of the reasons for foliar application of micronutrients is that in soils with pH over 7.5 most micronutrients become unavailable and therefore practically useless.

If you have alkaline soils, don't waste your time putting down micros with the exception of moly, which seems to be unaffected. If you want to adjust the soil pH, bring it down to the ideal at 6.5.

Another consideration is the possible phytotoxicity of the material being sprayed and this is dependent on the concentration of the product and the ambient temperatures affecting the crops. At times, the required concentrations are too high to apply as they will cause leaf burn, and so to be able to cover the nutritional needs of the plants, you will have to make several applications.

Do the math and decide if this is for you.

Leaf Uptake:

Leaf absorption (uptake) is critical for foliar feeds to work and this changes with each variety of plants. Check with your university or extension agent on the intake properties of the plants you grow.

Speaking of leaves, remember that many of them are not designed for liquid nutrient solution uptake, and that, in general only about 15 to 20% of the materials actually are taken in.

The stomates, through which absorption takes place, are found mainly on the undersides of the leaves. The upper surfaces of the leaves are usually coated with waxy substances, which protect that stoma from the elements. As the leaf ages, this cuticle becomes thicker. Also, the hotter the climate, the heavier the cuticle.

However, don't confuse the hot climate with the ideal times for absorption, which are usually in mid morning when the cuticles are still soft from the overnight humidity and the leaf metabolism is active because of the warm temperature.

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