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Foliar Application Investigation

 

Introduction

Foliar spraying or foliar fertilization is widely used in plant nutrition to improve yield, and it is a useful method in critical soil conditions that cannot provide elements. The foliar spraying method is a safe and suitable option for the optimal and balanced use of fertilizers or inputs in agriculture and to prevent environmental complications, which causes much lower costs due to the reduction in the amount of fertilizer used per hectare.
 
Faced with the following conditions, choosing foliar spraying is a method to respond to the needs of the plant:
     Stabilizing elements in the soil and reducing the availability of nutrients by the roots, which is caused by excessive use of chemical fertilizers or bad soil conditions, such as alkaline soils, light or sandy soils, lack of organic matter, low soil moisture, etc.

     special nutritional times during the plant growth period (such as early spring or bud swelling) and periods when the plant needs more nutrients for growth (such as seed filling or fruit volume)

     During drought or high humidity, which limits the absorption of elements through the roots and soil.

  Fertilizer efficiency (in terms of its effectiveness and composition) and compliance with the principles of foliar spraying (correct time and attention to environmental conditions) are very important to achieve the best results. Of course, it should be kept in mind that determining the influence of these factors in foliar fertilization is a complex matter that involves various aspects of physics, chemistry, environment, biology, etc. Some nutritional principles of foliar spraying have been proven; But there are many cases where the response of the plant in the physiological conditions of different species requires more research and investigation.

Plant leaf structure

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Cuticle in plants is a protective membrane that covers the epidermis of leaves, young shoots and other plant organs that do not have an outer skin. The cuticle consists of lipids and hydrocarbons covered with wax and is made exclusively by epidermal cells. Cuticle is hydrophobic and limits the two-way exchange of water, solutes and gases between the plant and the surrounding environment. As the surface of the plant acts as a barrier to the entry of water and solutes, it plays an important physiological role. Therefore, the importance of fertilizer formulation and foliar spraying conditions is very important to provide absorption conditions and facilitate the entry and passage of food.

Effectiveness in foliar application

The reaction of the plant to the fertilizers that are used through foliar spraying is probably affected by the properties of that fertilizer and determines the absorption and transfer of nutrients to the plant organs. On the one hand, the properties of different fertilizer formulas and environmental conditions (such as humidity and temperature) are considered effective factors in the performance of foliar spraying.

Solubility

The solubility of the fertilizer in water is important, because the absorption of mineral salts and salts occurs only when the prepared compound is dissolved in a liquid phase and can be covered and spread on the surface of the plant organs by foliar spraying.

Molecular Weight

The size of the fertilizer and nutrient molecule affects the rate of penetration into the leaf. The radius of cuticular water pores that allows the passage of water and solutes is approximately 0.3 to 0.5 nm in leaves and 0.7 to 1.2 nm in fruit. So, if the size of the fertilizer particles are large, their absorption from the pores will decrease. According to recent evidence, particles with a diameter of 1 micrometer do not enter the stomata, while particles with a diameter of 43 nm penetrate into the stomata.

Electric charges

Salts are electrolytes and when dissolved in water, they become free ions and the final solution is electrically neutral. Fertilizers are also ionized when dissolved in water, and when using salts or chelates and complexes (which are formed by mixing metal salts with chelating ligands) with anions and cations created in the fertilizer and water solution, they can penetrate the leaves. The nature of anions and cations in the solution-sprayed composition has physiological importance and their neutral, negative or positive charge is effective in leaf absorption.

pH

Considering that plant cuticles are polyelectrolytes, their ion exchange capacity changes with pH fluctuations. In research, it has been shown that cuticles have isoelectric points around pH=3, and when the pH of the solution is higher than this value, it makes the cuticle negatively charged, and cuticle carboxyl groups are easily attached to positively charged cations. Therefore, the pH of the combination of fertilizer and water that is used for foliar spraying is effective in its absorption and penetration into the leaves; But the response of the plant to these changes is not always the same and constant, and it seems that the pH of the solution alone does not predict and increase the absorption and penetration in the leaf, and it is significantly affected by the nutrients and fertilizer compounds used, as well as the type of plant. For example, the maximum absorption of urea in apple leaves was in the range of 4.5 to 6.6. (Cook and Boynton 1952) and the highest absorption of iron element by leaves was in the range of pH 5. (Fernandez and Ebert 2005)

POD

The drop point or dilution point is defined as the moisture threshold at which an ionic salt material dries in crystalline form on the leaf surface. When the humidity is above the dew point, the residual salt dissolves on the leaf cuticle and can be absorbed.
  The processes of hydration and salt dissolution are determined by its dilution point (POD), which is a physical characteristic associated with a fertilizer compound at a certain temperature; Therefore, the lower the dilution point of the salt, the sooner it dissolves when exposed to the relative humidity of the environment. When the relative humidity of the environment is higher than the drop point of the foliar spray compound, this compound will be dissolved and suitable for absorption by the leaves.

Environmental conditions

Environmental factors such as humidity and temperature play an important role in the performance of foliar spraying and absorption of fertilizer solutions by leaves. The effect of these factors is either directly on the solution and the combination of fertilizer and water before spraying, or through the effect on growth processes such as changes in photosynthesis, opening of stomata, respiration, etc. in the plant. Humidity mainly affects absorption by influencing the size of foliar spray droplets, persistence on the leaf surface, swelling of the cuticular membrane, and improvement of fertilizer dissolution. Temperature also affects various physical and chemical factors of fertilizer composition such as surface tension, solubility, viscosity. In general, increasing the temperature range (for example, from 0 to 40 °C) is effective in increasing the solubility of active ingredients and excipients, but decreases viscosity, surface tension, and pour point (dilution). In addition, high temperature also accelerates the rate of evaporation of the solution and reduces the drying time of the solution, effectively reducing penetration into the leaf.
  Other environmental factors such as light intensity or precipitation may also affect the performance of foliar spraying. For example, iron chelate breaks down when exposed to sunlight. On the other hand, the occurrence of precipitation in a short period of time after using foliar spraying may quickly wash and destroy the fertilizer solution from the surfaces. Consequently, weather forecasting should be considered prior to foliar application to avoid conditions that can reduce moisture and increase drying speed, such as high winds, heavy rain, or extreme temperatures at the time of foliar application.

Fertilizer Formulation

Fertilizers sprayed on the leaves of plants usually consist of at least two main components, which are the active substance and the neutral (or auxiliary) substance. Additives and surfactants are an important and widely used group of auxiliaries that reduce surface tension and increase the coverage of fertilizer solution on the plant surface. As a result, improving the spread and coverage (moistening) and durability (preservation and adhesion) of active materials and fertilizer compounds on the leaf surface leads to an increase in their absorption rate. Additives are also effective in reducing the toxicity of fertilizer compounds in plants by increasing efficiency.

Mobility of Elements

Nutrient elements have different conditions in terms of movement in plants that affect their absorption rate. Mobility and dynamism for nutrients and elements that are mobile or non-mobile in solving deficiencies that appear suddenly or develop secretly and then appear in foliar spraying is important, especially in the periods of plant growth when the deficiency is removed from the method. Soil or root absorption cannot be fixed and the time of absorption of elements does not provide the opportunity to solve this deficiency except by foliar spraying.
  Therefore, food materials are divided into 3 groups according to their mobility and dynamism in the plant, the first group which has high mobility in the plant, which includes elements of nitrogen, phosphorus, potassium, sulfur and magnesium. The second group has moderate or variable mobility in conditions, which includes iron, zinc, copper, boron and molybdenum elements. The third group, which includes manganese and calcium elements, are much less active in the plant.
  Determining the mobility of the affected elements is the ability of nutrients to enter the plant and move inside the phloem and finally reach the tissues. The degree of mobility of an element can be different during the life of the plant and the type of plant. For example, the amount of mobility of iron element varies in the growth stages of plants and in different species. or molybdenum element has high mobility in soy, rice and beans; But it is less active in other species. Also, boron element has fast absorption and high mobility in most species.

Foliar Time

According to the stages of plant growth and the effectiveness of different elements during the growth period, it is important to use fertilizer or the right element and use it at a specific time in the performance and obtain the desired result in foliar spraying. For example, zinc has a special effect on cell division, nucleic acid metabolism and protein synthesis; As a result, its function in growing tissues and general conditions of spring season is important in many species, and most plants show zinc deficiency in the early growing season. zinc element in most garden products; It has a great effect on rapid vegetative growth and increased flowering; Therefore, the application of this element in the form of foliar spraying at the time of swelling of the buds and before flowering and at a time interval of 2-3 weeks of this period improves the yield and increases flowering.
  Phosphorus foliar application has also been reported to have beneficial effects for increasing fruit formation, improving fruit growth and improving fruit quality during the growing season.
  Late season (after harvest) foliar application is a common practice in many trees with the rationale that it can be effective for the next season's flowering period. But one should pay attention to the conditions and the ability of leaves and trees to absorb elements after harvesting. Generally, for trees such as cherries, grapes, apricots, and peaches, which have an earlier harvest, foliar spraying after harvest is more effective; But late-ripening trees and species such as almonds, pistachios, walnuts, apples, and pears have a weaker performance in absorbing elements after harvest.
  On the other hand, the mobility of elements is also important in their use for foliar spraying after harvesting. According to the evidence and research, it seems that nitrogen and potassium elements are more suitable for use after harvesting than non-mobile or low-mobility elements such as calcium, iron, manganese and even zinc.

Toxicity

Sometimes foliar spraying with compounds that have high salt, toxic substances and pollution, the acidity of the solution causes toxicity in the plant. Poisoning can be the result of osmotic, elemental or metabolic disorders and can have different degrees, two common symptoms of poisoning are:
  1) Necrosis spots when the spray drops dry, which occurs as isolated spots.
  2) Burns of the leaf margin and tip due to the gravity flow of the sprayed solution to these areas
  The occurrence of necrotic or marginal lesions can lead to a decrease in the photosynthetic area of the leaves and thus to a decrease in productivity, and finally can neutralize the growth stimulating effects.
One of the common symptoms of post-foliar poisoning is burning, which may be the result of cell rupture due to the large difference in osmotic pressure across the cell wall when a very concentrated fertilizer solution is applied to the leaf surface. This type of damage and leaf burn occurs due to the use of compounds with a high salt index. For example, spraying potassium chloride alone is not recommended due to its relatively high salt index.
  Direct ionic effects are an important factor in determining the toxicity of foliar spraying of compounds that contain zinc, copper, iron, and manganese elements, but generally, burns are not obtained by observing the appropriate dose of foliar spraying.
An increase in the concentration of elements in the cells also disrupts the natural growth of the plant. For example, although the copper element has favorable effects for fungicides; But using it with high concentration and repeatedly causes poisoning.
To reduce toxicity in foliar spraying, it is important to use fertilizer that is optimized by formulation and addition of auxiliary materials. Also, the recommended dose and concentration in each product, which is the result of several tests, should be observed.

 

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