Micro fertilizers yesterday, today and tomorrow ​  

In modern intensive crop cultivation technologies, the use of micro fertilizers has become an almost mandatory operation to increase crop yields and quality.  ​

​  Mineral salts as micro-fertilizers ​  

Currently, taking into account the low cost and accessibility, mineral salts or mixtures thereof in a single tank solution are often used as a single-component micro-fertilizer in farms. 

These are mainly copper, zinc, manganese and cobalt sulfates, ammonium molybdate, a solution of crystalline iodine, sodium or ammonium vanadate, boric acid or borax. The advantages of sulfates include the fact that, in addition to the trace element, they supply sulfate ion, the sulfur of which is well absorbed by the plant. However, as practice has shown, mineral salts of trace elements are inferior in their effectiveness to the more widely used chelated compound of trace elements: chelates in doses 2-10 times lower than mineral salts (in terms of trace elements) provide equal increases in yields of major crops. ​  

The use of micro fertilizers based on mineral salts, along with cheapness and accessibility, has serious drawbacks.:  

  - trace elements in the form of salts weakly penetrate through the waxy coating of leaves, are difficult for plants to access, and are effective only on slightly acidic and acidic soils;  

    - the soils are salinized by various anions and cations (Na+, Cl-);

  - mixing different salts in the same working solution often leads to their interaction with the formation of insoluble and inaccessible compounds to plants.    

   - with high water hardness of the working solution, fertilizers in the form of mineral salts interact with hardness salts, forming a precipitate

 - carbonates of the applied trace elements and insoluble magnesium sulfate. This reduces the effectiveness of the fertilizer and may cause blockage of the sprayer nozzles. ​ ​

Micronutrients in the form of phosphites, salts of phosphoric acid ​  

 An attractive idea is to create micro fertilizers containing digestible phosphorus compounds. It is known that plants absorb phosphorus only in the form of phosphates. But phosphates of all biogenic metals are insoluble in water. Therefore, some domestic manufacturers of micro fertilizers have decided to produce micro fertilizers based on phosphites. ​

  Phosphorous acid salts – phosphites, unlike phosphates, are highly soluble in water. When treating vegetating plants with aqueous solutions of phosphites, the latter from the leaves enter the root system and reduce the development of its root rot, stimulate the protective mechanisms of the plant itself. In the program of plant nutrition with phosphorus, it is impossible to talk about any replacement of phosphates with phosphites. Currently, there is no evidence that phosphites can be directly used by a plant as a source of phosphorus. Phosphite must be oxidized to phosphate. But so far, the scientific literature has not described enzymes that could carry out this process. Therefore, it is not correct to provide information on the content of digestible phosphorus in fertilizers on the labels of micro fertilizers with phosphites. ​

  Most foreign companies use phosphites as products with fungicidal activity, as this effect of the drugs is clearly visible when they are used, especially when protecting plants from powdery mildew and fungi of the genus Phytophtora. Among the phosphites positioned as biostimulants, the drug Nutri-Phite Magnum S from the German company Agroplanta is known. This drug enhances the growth of roots and their activity, has a beneficial effect on the laying of reproductive organs. The mechanism of its action on plants has not yet been studied. ​  

In addition to this drug, there are other products with phosphites on the market. These are Kafom (Spain, contains phosphites K, Mg, Zn, Mn and Ca), Quantum Phytophos (NPK Kvadrat, contains potassium phosphite), Kozyr (Sakura, Germany, contains phosphites K, Mn, Zn, Mo and Cu), etc. ​

  In EU countries, the use of fertilizers containing phosphites is prohibited. This is due to the fact that phosphites form toxic products with many organic substances in the soil or in the tank mixture. Even in extremely low concentrations, phosphites exhibit general toxic activity relative to all living organisms and are not a source of plant nutrients. ​ ​ Humic preparations ​  

As a rule, humic preparations in dry matter contain 60-65% humates, seven main trace elements – Fe, Mn, Zn, Mo, Cu, Co and B in the form of complex compounds with humic acids and a small amount of macronutrients. They are highly soluble. However, the concentration of trace elements in humates is very low, which does not allow them to be considered as a serious source of trace elements for plants. ​

  In fact, humic preparations are good organic fertilizers, not micro fertilizers. The high alkalinity of liquid humic preparations makes it difficult to enrich them with trace elements even in the form of classical chelates. ​ ​

Trace elements in chelated form ​

  In recent years, chelated micro fertilizers based on synthetic and natural organic acids have been in high demand in practice. The use of such fertilizers is a convenient, effective and economically justified way to provide plants with elements that promote their active growth and enhance immunity.  

The advantages of such micro-fertilizers have already been appreciated by millions of farmers.   A number of different organic acids are used in the production of chelated micro fertilizers. The production of the vast majority of imported and domestic micro-fertilizers is based on two components - EDTA - (ethylenediaminetetraacetic acid) and OEDP - (oxyethylidenediphosphonic acid). The main properties of the most common complexon, EDTA, are the ability to bind ions of certain elements, including Ca2+, Fe3+, Cu2+, Ni2+, Co2+, and Zn2+ to form stable chelate complexes (from the Greek chele, crab claw). Of course, farmers are primarily interested in the effect of fertilizers with EDTA on plant productivity, soil biological activity and the economic efficiency of their use. And here lies a lot of problems that do not appear immediately. In particular, it has been proven that micro-fertilizers based on EDTA are not effective in root fertilization of plants on carbonate and alkaline soils (pH ≥8). ​

  In addition, EDTA is able to bind and take away calcium from any structures containing it, including plant cells. And calcium is an important macronutrient that participates in key physiological and biochemical processes of a living organism. The lack of calcium leads to a decrease in yields, deterioration in the quality of seeds and fruits, and a decrease in their shelf life. Therefore, after applying micro-fertilizers based on EDTA, it is advisable to carry out foliar treatment of plants with fertilizers containing calcium. ​  

It should be especially noted that EDTA is not absorbed by plants. After giving the plant a trace element, it enters the soil, concentrating mainly in its upper ten-centimeter layer. Since EDTA is a very stable compound, once in the ground, it does not break down, binds heavy metals, including mercury, cadmium, arsenic, lead and others. Heavy metals, being in the soil in the form of salts, are usually poorly soluble and poorly absorbed by plants. The formation of soluble EDTA complexes with heavy metals leads to more intensive and uncontrolled absorption by the root system. Studies have shown that this negatively affects the accumulation of heavy metals by plants even in the third year after its single application. As a result, the concentration of heavy metals in plants can rise to toxic levels, which leads to their depression, reduced yields and lower product quality. ​  

Large-scale use of EDTA increases the risk of its negative impact on the environment, as this chemical compound is poorly degradable in nature. Therefore, in a fairly short time, it has become one of the most widespread anthropogenic pollutants, including in the oceans. Thus, the use of micro-fertilizers based on EDTA is very dangerous for agriculture and the environment. In the long term, it will lead to increased pollution of agricultural land, soil degradation, reduced yields and deterioration of product quality. It is difficult, and sometimes impossible, to correct these consequences. In many countries, in particular, in Canada and a number of European countries, the use of micro-fertilizers based on EDTA is prohibited. ​  

The second chelating agent widely used in the production of micro–fertilizers is  EDF. All stable individual metal chelates and their compositions of various compositions can be obtained on the basis of this acid. In its structure, EDF is similar to natural compounds based on polyphosphates. When it decomposes, chemical compounds are formed that are not toxic to plants. Chelates obtained on the basis of EDF are effective on soils of various acidity (pH 4.5 – 11.0), have antiviral and growth-stimulating effects. The main disadvantage of EDF is its weak chelating ability relative to iron, copper and zinc ions. In the working solution, especially at high water hardness, these important biological elements are replaced by calcium, their effectiveness is significantly reduced. From an environmental point of view, EDF-based chelates are significantly less dangerous than EDTA-based chelates.  However, it must be borne in mind that under the influence of biological factors, OEDP-based chelates practically do not decompose. And their abiotic decomposition in natural conditions, especially in water bodies, under the influence of light leads to the formation of acetates and phosphates. ​  

There is no data in the scientific literature on the formation of phosphates from EDF in the plant organism and the absorption of phosphorus by plants from micro-fertilizers obtained on the basis of EDF. Therefore, it is not entirely correct to write in advertising and informational materials about the content of digestible phosphorus in such micro-fertilizers.

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Nanomaterials in crop production

With the current rapid development of nanotechnology and the abundance of nanomaterials with unique properties, researchers' interest in the question of how nanomaterials affect plants, their productivity and other aspects is understandable. ​ The results of numerous studies in recent years indicate that nanoparticles of biogenic elements are characterized by ultrahigh reactivity, high permeability in plant tissue, and increased efficiency of physiological and biochemical processes. Nanoparticles have a positive effect on the development and productivity of major crops, as well as the bactericidal effect in "homeopathic" doses – 5-6 orders of magnitude lower than the same product in a macroscopic state. ​ Colloidal solutions of nanoparticles of many biogenic elements, ultrafine metal powders, carbon nanotubes, and fullerenes have been studied as micronutrients. Nanomicro fertilizers contributed to an increase in field germination of seeds, a significant increase in crop yields and an improvement in the quality of crop production. ​​

Recently, the development of nanotechnology has contributed to the development of new methods for chelating trace elements using reactively weak natural di- and tribasic food carboxylic acids – citric, succinic, malonic, tartaric, malic, etc. These acids are capable of forming chelate complexes with almost all biogenic metals, as well as bioorganic complex compounds with sulfur, iodine, and bromine. Previously, the synthesis of similar chelates could only be carried out in living cells. It has been proved by IR spectroscopy that citrates of biogenic metals are complex compounds with a chelated bond. Numerous studies have established that it is chelating agents that ultimately determine the effectiveness of fertilizers and the degree of absorption of trace elements by plants. According to a number of scientists, chelate complexes based on food acids are the most promising and safe for exposure to all types of biological objects. ​ ​ Chelated complexes of natural carboxylic acids

​ Chelates based on food-grade carboxylic acids are chemically identical to the natural components of living cells. They are integral participants in the metabolic processes occurring in plant and animal organisms, in particular in the Krebs and Calvin cycles. When faced with such chelate complexes, living cells perceive them as natural sources of not only essential biogenic trace elements, but also additional energy. And synthetic chelates are not found in wildlife, and when using them, a plant needs to utilize or remove a foreign synthetic ligand from its body with significant energy costs, whose role is to perform only the transport function of delivering a biogenic trace element. ​ When conducting a comparative assessment of the feasibility of using chelates of biogenic metals based on synthetic ligands and based on organic food acids as micronutrients, it is important to note the following scientifically proven facts. ​

The stability constants of chelates of basic biogenic metals based on EDTA range from 8.7 for magnesium to 25.1 for iron, and from 3.2 to 14.0 for chelate complexes based on citric acid, respectively. This suggests that citrate chelates, when ingested into a plant, are more than 2 times easier and faster to open, giving up their metal cations and carboxylic acid anions, which serve as a food element and a source of additional energy. New chelated complex compounds have a wider range of applications, not only in crop production. They are a universal tool for influencing all biological objects, including humans and animals. For example, magnesium citrate chelate is the basis of the well-known pharmaceutical preparation Magne-B6. The best proof of the safety of new chelates is the possibility of their use in medicines, food additives for humans and in preparations for animal husbandry. Citrate and succinate chelates of biogenic trace elements have already been certified as approved for use in organic farming. ​ For all applications of the new organochelate complexes, it is important that with the use of nanotechnology they can be obtained with metal ions in a low degree of oxidation. The uniqueness of trace elements in their low degree of oxidation lies in the fact that they can simultaneously be catalysts of biochemical processes, antioxidants, and antiviral substances. On the basis of such complexes for crop production, preparations can be created that simultaneously have growth-stimulating, stress-protective and protective effects on plants.

​ Using food-grade organic acids as ligands, chelated complex compounds with more than 30 chemical elements have been created, which makes it possible to develop a wide range of targeted drugs, taking into account the characteristics of various crops and to solve problems such as optimizing nutrition processes, increasing stress resistance, and plant protection.

Conclusion. ​

The purpose of this article was to review and objectively evaluate the main advantages and disadvantages of chemical compounds used in micro-fertilizers and forecast the development of the field. ​​ The main conclusion can be drawn as follows. Technologies for the production of trace element fertilizers are developing in the direction of increasing their efficiency, both environmentally and economically. It is with this focus in mind that synthetic chelates have replaced inorganic mineral micronutrients as more effective forms of trace element compounds. However, the environmental danger and the resulting restrictions on their use in a number of developed countries cast doubt on the prospects of using synthetic chelates in the near future. The new generation of chelates based on natural food acids has a number of obvious advantages in terms of safety, effectiveness and the ability to create a wide range of target products. It follows from the above that the process of replacing synthetic chelates with natural ones, although it depends on many factors, looks inevitable.