Foliar Spraying Techniques
Foliar sprays can be categorized as “systemic” or “non-systemic”.
Systemic spays, like sprays with silicic acid (and other micronutrients), are absorbed into the plant via the surface of the cuticle layer and via the stomata.
Thereafter the nutrient is transported via the vascular system to the remainder of the plant where they are needed.
Non-systemic sprays are used to treat problems located on the surface of the leaf. This means that most pesticides and some fungicides are non-systemic. They are used to treat infections on the surface of the leaves (like mildews).
A wetting agent is important. These additives are surfactants used to improve the capacity of the spray to “wet” foliage.
This is achieved by lowering the surface tension of the foliar spray solution. Due to lower surface tension, the spray droplets spread across the surface so that greater coverage occurs. Where systemic sprays are being used, the spray is able to cover a larger area which increases the opportunity for absorption (Fig. 1).
In the absence of a wetting agent, large droplets will form on the leaves. These will either roll off the leaf thus being wasted or may cause some burning due to a magnifying effect when intense sunlight is present.
Some leaves (such as ‘waxy’ leaves) are harder to treat properly.
Wetting agents also have “hygroscopic” properties. These properties extend the duration that the foliar spray remains wet on the leaf. This is important because absorption of the active component only occurs while the spray remains wet.
Quantities of nutrients in foliar sprays
Unlike roots, foliage is not adapted to absorb large amounts of nutrients.
In spite of this, foliar spraying is able to take advantage of the large ‘surface area’ of leaves on a plant: “The leaf surface of a 12-year-old apple tree in Washington State is equivalent to one-tenth of an acre, even though that tree only occupies about one-hundredth of an acre (dr-Tukey). So there is a large feeding area and a large opportunity for nutrient input.
Next to silicic acid, iron, zinc, potassium, boron, molybdenum and other trace elements can be absorbed through foliage.
Radioactive tests show that micro-nutrients, once sprayed, are in the sap stream within one hour concluding foliar sprays are effective for correcting nutrient deficiencies.
Silicic acid foliar sprays with specific micro-nutrients formulations are very useful to influence plant characteristics such as fruit set, fruit size and pest and disease resistance.