“Dilute your fertilizer and spray it on the leaves”
This recommendation is widely spread within the internet, when it comes to foliar fertilization. There might be some positive effects visible using this method. But, professional foliar fertilization is a fast and target-oriented tool to cure nutrient deficiency, stimulate the vegetative and/or generative growth or spray pesticides. Using only specific nutrients in well created formulation is the pre-requisite for successful and reliable results.
Since the early 19th century, foliar fertilization is a common agricultural practice and became of scientific interest in the last century. In the last decades foliar fertilization is gaining importance, due to the possibility to reduce the amount of water and nutrients in plant production.
A short introduction to foliar fertilization:
Under conditions, such as drought or high precipitation, under low root activity during generative phase (flowering period) or low transpiration rates (deficiency of immobile nutrients, such as Ca, B, Mn), the effectiveness of soil fertilization can be significantly reduced. Foliar fertilization could be a target-oriented alternative with high economic benefit to correct or avoid deficiencies.
Foliar fertilization during the generative stage is already a common practice in commercial fruit orchards and vegetable production. It is mainly used to increase quality traits or reduce symptoms of deficiency.
In Hawaii, within pineapple production foliar fertilization is used for feeding the plants. The requirement of total nitrogen is covered by 70-80%, phospor and potassium by 40-50% only via foliar fertilization.
Plant nutrient uptake by roots is highly influenced by the physical and chemical properties of the soil. Even with optimal nutrient concentrations in the soil, nutrient deficiencies can appear.
In general, nutrient uptake by leaves follow mainly the same principles, as via roots. In roots, there are active absorption processes regulated by root exudates. But most uptake follows a passive absorption via diffusion. There are no active processes for foliar nutrient uptake.
The biggest difference is a barrier on the leaf surface, called the cuticula. The amount of nutrients which enter into the leaf tissue is strongly dependent on the structure and composition of the cuticula, number of stomata and the size and charge of the molecule applied.
There are three defined pathways for nutrients to enter the leaf surface, namely the lipophilic, the hydrophilic and the stomatal pathway. Detailed description of the different pathways are described in the Blog: The science of foliar fertilization
When creating a foliar fertilizer the specific barriers for the different pathways need to be considered. There are different additives used to change the physical-chemical properties of the fertilizer solution… details about the different additives are described in the Blog: Additives in foliar fertilizer solutions
For example, fertilizer without additives have a high surface tension and as soon as the solution hits the leave it builds up to a drop. The drop lies on top of the open stomata but will not enter (pic. 1). With the addition of additives lowering the surface tension of the solution, the fertilizer will flow into the open stomata.