In this article that we are going to comment on today in Gardenprue we are going to discuss a technique widely used both in the home garden and in large agricultural operations. It is about using foliar applications to introduce nutrients in a more effective way in the plants.
However, there are many factors that can reduce the performance of the application, such as temperature, leaf size, phenological state, etc.
Therefore, we may wonder if foliar applications of nutrients are really effective or if it is better to use the well-known fertigation system.
FOLIAR APPLICATION OF NUTRIENTS IS NOT A MODERN THING …
Not much less. There is documentation that confirms that iron sulfate was applied in 1844 to correct iron chlorosis in vines destined to produce wine in France.
What has been modernized is the efficiency of the nutrients that we provide foliarly, since we can currently take advantage of the advantages offered by the latest chelates or complexes that are quickly absorbed and reduce losses in their application.
Although it is commonly thought that this is the case, there is a scientific aspect that rejects that the absorption of foliar nutrients is done through the stomata. They claim that the stomata only have the function of absorbing gaseous elements (gas exchange) and that the water droplets are usually several times larger than the size of the stomatal opening.
Counting on the surface tension of the water, it is difficult for it to enter through us.
On the contrary, it is argued that nutrients are absorbed through the pores of the leaf (they are not stomata) through the concentration gradient, passing through different layers of the leaf by various absorption routes.
THE STAGES OF FOLIAR APPLICATION OF NUTRIENTS
When we apply a nutritional product or even a phytosanitary to combat a plague or disease, the absorption process goes through several stages. Specifically 3 stages.
STAGE 1: RETENTION OF NUTRIENTS OR ACTIVE MATERIALS IN THE LEAF
When we apply a nutrient or an active material (phytosanitary) in a foliar way, part of these substances are retained in the leaf, others are lost.
In fact, an important condition is that the product we apply has the chemical property of being retained on the sheet for at least a few hours (the best is more than 3 hours).
This guarantees the maximum absorption of the product, so it is very convenient to look at the weather forecast and observe that it will not rain or wind in the following hours.
A tip: the more humidity there is (measured in% relative humidity), the better the permeability of the cuticle of the leaves and the greater the absorption. Contrary to what we might think that because it is dry it will absorb more, as if it were thirsty.
It is also advisable to use surfactants or products that increase the solubility, miscibility and reduce the droplet size of the nutrients. With this, it is achieved that the foliar application is more effective, measured in a higher percentage of product absorbed by the leaf.
STAGE 2: TRANSPORT OF THE SUBSTANCE INTO THE LEAF (CELLS)
At this stage, the nutrients have been absorbed from the surface of the leaf into the layers, moving towards the interior progressively.
Epidermal cells act as natural barriers that prevent certain substances from being absorbed or not and passing the “natural filter” imposed by plants. The rate of absorption will depend on the conditions of the plant and the concentration of the solute. That is, the concentration of salts in the aqueous medium.
STAGE 3: MOVEMENT OF THE NUTRIENT TO THE ORGANS (SUCH AS FRUITS)
Nutrients or compounds move through cells through 2 movements. Between the spaces of the cells (intercellular spaces), known as the apoplast , or through the interior of the cells (intracellular spaces), known as the syplast .
The substances applied foliarly are directed to the organs where there is the greatest demand for the nutrient. For example, if we make a foliar application of iron, it will slowly move to the young leaves that present a chlorotic state due to a lack of this element.
In the case that we make an application in fertigation to correct a deficiency , it has the disadvantage that the displacement is greater (from the roots to the aerial part, in the case of an iron deficiency). For this reason, foliar applications are often used , although special conditions must be taken into account depending on the product.
THE SPEED OF FOLIAR ABSORPTION DEPENDS ON THE CHOSEN NUTRIENT
Not all nutrients have the same chemical properties or the same molecular size. Therefore, it is to be expected that its absorption rate will be different.
Of the known nutrients, this would be the classification from highest to lowest absorption of 50% of the application.
- Nitrogen: 30 minutes to 2 hours
- Phosphorus: 5 to 10 days
- Potassium: 10 to 24 hours
- Calcium: 1 to 2 days
- Magnesium: 2 to 5 hours
Likewise, the different nature of them also depends on each of these nutrients. For example, the case of potassium, whose contribution can be made with potassium nitrate, monopotassium phosphate , potassium sulfate , etc.
In this image we see the example of the absorption rate after 24 hours of application. We see that potassium nitrate has an efficiency lower than 50%, but much higher than the rest of solutions rich in potassium applied in a foliar way.
In the case of potassium sulfate , less than 20% of the salts applied by foliar spray have managed to be absorbed. In addition, of that 19.4%, there are still losses, since only 13.2% is translocated from the leaf to organs such as fruits.
This image represents the difference in the distribution of the iron ion on the leaf under different conditions. In case A, with a healthy green leaf, and in case B, with a stressed leaf with transport problems and restricted growth.
WHEN IS IT ADVISABLE TO MAKE FOLIAR APPLICATIONS?
As a rule, foliar applications do not have any type of restriction as long as they are suitable to apply.
The best use of aerial treatments will be made when we have problems in the soil and a low rate of root assimilation.
This can be due to problems with the pH, soils blocked by nutrient antagonisms, low percentage of organic matter, waterlogging, extreme drought, etc.
+ Modify the soil pH
+ Contribution of organic matter to the soil
We must also count on the fact that foliar fertilization is more efficient than fertigation, which is why it is fully recommended in situations of nutrient deficiency. This is because by applying the required nutrient through the leaves it goes much more easily to the parts where it is needed, and does not have to go the long way from the roots to the organs.
Foliar treatments are recommended in times of greatest need, when the roots do not have enough energy to assimilate all the nutrients it needs.
Keep in mind that the absorption of nutrients by the roots is not done at zero cost and has to spend energy to take another. In the case of cations, it releases H + to compensate for the assimilation of positively charged ions and to always be in ionic equilibrium.
Something to keep in mind is that nutrients have different ways of moving, so you can differentiate between nutrients with high mobility and those with low mobility .
HIGH MOBILITY NUTRIENTS
They are nitrogen, phosphorus, potassium, magnesium, etc. Applications both in fertigation and in foliar treatments are well transported within the plant and the corrections of deficiency states usually take a short time (several days at most).
LOW MOBILITY NUTRIENTS
However, we have many micronutrients (iron, manganese, zinc, copper, etc.) with low mobility that depend on many conditions to be transported. Movement is slow and correction of chlorosis usually takes a few more days.
A clear example is that of iron chlorosis and correction, which can take more than a week with repetitions in the applications.
All the best. Agromatic.