Myths and reality of foliar nutrition
Foliar fertilization is a common practice to provide mineral nutrients to crops, especially in limiting soil nutritional conditions. Compared with soil applications, foliar nutrition offers several potential advantages as it provides nutrients directly to the foliage, regardless of the soil, so it acts much faster and prevents loss of nutrients by leaching or immobilization in the soil. Thus, foliar nutrition allows to correct or prevent nutritional deficiencies in a fast and friendly manner with the environment. However, there are also potential disadvantages and risks associated with this practice.
Among the disadvantages and risks is that a sub-dose of nutrients may not achieve the correction of the deficiency while an over-dose can burn the leaves and impact production. In foliar fertilization, the fertilizer absorption rate is affected by factors such as environmental conditions, the type of fertilizer salt and, of course, the crop.
In his prominent keynote delivered at the New Ag International conference in New Delhi, German professor Dr. Thomas Eichert, a world leader in foliar nutrition research, addresses some of the most important misunderstandings or 'myths' regarding the application of mineral nutrients to the foliage is concerned.
To optimize foliar nutrition, it is necessary to know the principles of foliar absorption of nutrients and control parameters. Fortunately, considerable progress has been made in understanding these processes and many previously accepted concepts and theories must have been abandoned or at least modified, but some of the old visions still remain entrenched in the minds of researchers and producers.
Myth 1: foliar fertilization is a "natural" alternative to soil applications
In foliar fertilization, as a common practice, it should always be considered that the aerial organs of plants are more designed to minimize the exchange of matter with the environment than to absorb mineral nutrients. Therefore, the surface of the leaves is covered by a lipophilic cuticle more or less water repellent, which resists the penetration of hydrophilic solutes with nutrients.
On the other hand the stomata are protected against the infiltration of liquid water. Therefore, the main challenge is to overcome the leaf surface barrier so that the nutrients are absorbed by the leaves at an adequate rate or high enough to correct the nutrient deficiency but low enough to prevent burning of the leaves. leaves. The fact that the foliar absorption of nutrients by the surface of the leaf is a purely physical process and governed by the laws of diffusion increases the difficulty.
Myth 2: leaves actively absorb foliar nutrients
In contrast to root absorption, the processes involved in foliar absorption are passive and therefore the penetration of nutrients on the surface of the leaf is not selective. This means that the nutrients applied to the leaf will penetrate independently of the physiological requirements of the plant. In this context it is important to mention that the old and refuted 'ectodesmata' hypothesis can still be found in current study texts and publications.
According to the 'ectodesmata' hypothesis, the cytoplasmic extension of the epidermal cells of the cuticle, where it is believed that it actively participates in the foliar absorption of hydrophilic solutes. However, it was subsequently shown that the ectodesmata was the result of experimental errors.
The passive nature of the absorption process has an important consequence: any substance present on the surface of the leaf will penetrate as long as there is a concentration gradient across the surface of the leaf as the driving force of the diffusion. Without the resulting penetration rate is very high and incompatible with the metabolism of the plant, the result will be the burning of the leaf, a phenomenon frequently observed in practice.
The effectiveness of foliar spraying therefore faces two challenges, on the one hand a low rate of absorption due to the repellent properties of the leaf surface and on the other an excessive rate caused by the passive absorption process. So the main challenge of foliar spraying is to apply the optimal dose of nutrients to correct or prevent nutritional deficiencies without burning the leaves.
Myth 3: the cuticle is the only available way of entry
Since the beginning of the mechanistic investigation of foliar absorption, evidence appeared pointing to the stomata They played an important role in the penetration of the blade. It was observed that the absorption rates correlated with the presence, frequency or opening of stomata. On the other hand, it was clear that infiltration through the stoma of the solution applied to the leaf was not produced by mass flow (unless the surface tension was decreased by a very effective active ingredient on the surface).
This paradox was resolved when it was demonstrated that nano particles penetrated the stomata by diffusion on the surface of the stomatal pores and not by mass flow of the solution. It was found that the percentage of stomata involved in foliar absorption is often small while at the same time the quantitative contribution of stomatal penetration to total absorption can be substantial. The evidence suggests that, especially for mineral nutrients, stomata may be the most important passageway.
Myth 4: the cuticle is (always) the main way of absorption
The cuticle is inert and hydrophobic skin that covers most of the foliage of plants. The backbone of the cuticle is provided by a three-dimensional polymer formed of cutin in which there are amorphous or crystalline lipids (waxes) embedded.
While small lipophilic solutes can easily penetrate the cuticle by dissolving in waxes and by diffusion through gaps in the cutin grid, the penetration of hydrophilic solutes such as mineral fertilizer salts is strongly hampered by their very low solubility in the cuticle. For example, the solubility of NH4NO4 in the cuticle is more than 107 times lower than in water (estimated by the partition coefficient of octanol water), and estimates based on other salts gave similar values.
However, mineral fertilizer salts can be absorbed when applied to the surface of the leaf (without stomata), indicating that the model of dissolution and diffusion in the cuticle, which is valid for lipophilic apolar solutes, can not satisfactorily explain the Cuticle absorption of hydrophilic solutes. For this reason, the polar water pore model was developed. According to this 'pore model', water can be absorbed by the cuticle and form internal aqueous sets.
If sufficient water is absorbed, the assemblies can form an aqueous bridge within the lipophilic cuticle in which hydrophilic solutes can diffuse between the outer surface of the leaf and the epidermal cells. Since hydration status depends on the development of cuticle pores, it is clear that relative humidity (RH) controls permeability. It has been shown that with a decrease in the RH of 100 to 90% the permeability of the cuticle of pears decreases by a factor of 10, and that with 50% of RH the permeability decreases even more, by a factor of 100. From this it can be concluded that the lower the HR, the lower the permeability of the cuticle. Under these conditions the contribution of stomatal absorption can be particularly important.
Myth 5: the more relative humidity the better
Frequently it is established that a high RH favors the absorption of the nutrients applied to the leaf. However, this presumption is generally based on intuition rather than facts. As noted above, the cuticle patency actually increases with the increase in RH and it could also be assumed that at high RH the probability of stomatal opening is itself greater than at low RH.
Therefore, at high HR the permeability of both foliar absorption pathways would be greater than at low RH. However, in addition to permeability, there is another parameter that controls foliar absorption rates, such as the concentration gradient across the surface of the leaf. This gradient is controlled by the concentration of salts on the surface of the leaf, which in turn is directly controlled by the HR.
The concentration of nutrient salts in applied foliar fertilizers is generally not in balance with the humidity of the atmosphere. Consequently, the applied solution will evaporate until equilibrium is achieved. It has been shown that the balance of concentration of the solutes applied to the leaf depends on both the environmental RH and the hygroscopicity of the solute.
The degree of hygroscopicity of a solute can be expressed by the RH above which the salt dissolves in the water absorbed from the atmosphere. This humidity threshold is called "deliquescent relative humidity" (HRD) or "deliquescence point" (PD). Each salt has a constant specific DRI at a given temperature. The interaction between HR and HRD of the solute controls whether the solution sprayed on the leaf surface will dry out (if HR <HRD) or not (if HR> HRD).
If the RH is less than the HRD of a given salt, the solution of that salt will evaporate completely leaving a residual dehydrated salt on the surface of the leaf from which absorption of nutrients is impossible. If HR = HRD, the saturated saline will remain on the surface of the leaf and the concentration gradient across the surface of the leaf will be at the maximum. With HR> HRD the equilibrium concentration of the salt decreases constantly until saturation (HR = 100%) reaches a theoretical concentration of zero.
The availability of nutrients will depend on the different DRIs of the salts, which increases in the following order: chlorine <nitrate <sulfate. This allows the resulting nutrient absorption rate to be adjusted by selecting the type of salt according to the prevailing RH levels. Foliar fertilizers often contain additional compounds such as adjuvants or other nutritional salts. What should be considered since these mixtures change the DRI of the applied solution.
The permeability of the leaf and the concentration of salt on the surface of the leaf are the two key parameters that control the rates of absorption of nutrients. If the permeability of the sheet increases with increasing RH, the concentration decreases with increasing RH, at least above the RH of the respective salt.
This antagonistic behavior supports that the assumption that high values of HR lead to a high penetration rate is wrong. The calculation models show that the maximum penetration rates are generally expected at intermediate levels of HR. At intermediate levels of RH, both the leaf permeability and the salt concentration at the surface of the leaf are in an intermediate range and none of them near zero.
Often an advantageous alternative to ground application
In many situations foliar nutrition can be a promising and advantageous alternative to soil fertilizer applications. Optimizing the effectiveness of foliar nutrition requires at least basic knowledge of the factors that regulate the absorption of nutrients applied to the foliage. It must be considered that absorption processes are exclusively passive and governed by the laws of diffusion.
However, plants do not have a direct way of controlling this process, so it is very important to carefully select the appropriate type of fertilizer. It has been established that both the cuticle and the stomata are able to absorb nutrients. An important difference between both pathways is that the permeability of the cuticle is highly dependent on HR, whereas the stomatal pathway is probably less affected by changes in HR. High levels of RH generally increase the permeability of the leaf surface but at the same time the effective concentration of nutrient in the leaf is low. Therefore, high absorption rates are expected at intermediate levels of relative humidity.
To optimize foliar nutrition, it is necessary to know the principles of foliar absorption of nutrients and control parameters. Dr. Thomas Eichert has made considerable progress in understanding these processes, thanks to which many previously accepted concepts and theories had to be abandoned or modified. The clear thing is that in many situations foliar nutrition can be a promising and advantageous alternative to soil fertilizer applications.
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