Advice & Education :: About Foliar Sprays
Foliar spray application is widely used to supply specific nutrients to many crops.
The goal of foliar uptake is to increase the tissue nutrient content.
Nutrient foliar sprays are used to correct micro-nutrient problems.
The rationale for foliar sprays are:
- Only small amounts are needed to correct the deficiency
- The absorption of micro-nutrients via the roots is often too low because they are limited within soils.
Advantages of foliar spraying (compared to soil fertilization):
- fast results
- combination(s) of micro-nutrients are possible
- Only low doses can be applied and the results are temporary.
How does it work?
When the spray is applied, the micro-nutrients have to pass several barriers (Figure 1):
- the outer waxy hydrophobic cuticle layer covering the epidermal cells
- the wall of the epidermal cell
The absorption of nutrient solutions by plant surfaces occurs via the cuticle, stomata, trichomes and lenticels.
By a diffusion process the nutrients pass the cuticle. Permeation through the plasma membrane of the epidermal cells occurs by active transport, a process requiring energy.
The role of stomatal penetration is not yet fully understood, but it is important as well as the role of other epidermal structures such as trichomes and lenticels.
Other sites for entry are insect punctures, leaf cracks and tears.
Young leaves have been shown to have a better absorption rate.
Environmental factors can influence cuticle development. One example is that shade and humidity can cause thinner cuticles.
The role of macro-nutrients?
The use of macro-nutrients in foliar spraying has hardly any discernable benefits. The quantities required are to large for foliar application and even when large applications are used there is lack of response (phosphorus). Also the results of using large applications of macro-nutrients is that leaves will suffer from phytotoxicity (as with magnesium).
The mix of nutrients must be done very carefully because nutrients can interfere with the absorption of other important nutrients required for plant development.
Possible deficiencies of the micro-nutrients zinc, manganese and boron can be readily corrected through nutrient foliar application. Timing is critical due to the limited duration of effectiveness.
Additives and chelates show little or no benefit, but surfactants do. Surfactants will increase nutrient uptake.