Micronutrients & Evolution of Technology: Plant Nutrition of the 21st Century

While soils may contain sufficient amounts of micronutrients for plant growth, there are several factors which affect availability to the plant. These include soil pH, soil organic matter dynamics, redox potential, soil texture, mycorrhizal fungi, interaction with coexisting ions, and chelates. For example, micronutrient deficiencies usually occur in soils with low organic matter whereas greater active organic matter increases their availability. The availability of Fe, Zn, Cu, Mn, Ni, and B increases with a decrease in soil pH whereas the reverse is true for Mo. A deficiency of micronutrients is more likely in light-texture sandy soils. Fe, Cu, and Mn are more available under waterlogged than aerated conditions. Mycorrhizal fungi increase the uptake of several micronutrients, particularly Cu and Zn. Chelates play a significant role in micronutrient uptake. They bond with metals, particularly Cu, Fe, Mn, and Zn, increasing the solubility of the metals and affecting their supply to plant roots. Shortage of the micronutrients in the crops is compounded by low crop use efficiency for micronutrients (<10%) compared to macronutrients (20 to 80%). In many situations, this scenario is further complicated by the inadequate replenishment of soils through fertilization of micronutrients biologically mined from the soil by high-yielding crops such as corn, rice, soybean, and wheat over many decades. Moreover, farmers are often more focused on the management of macronutrients N, P, and K even with the knowledge that micronutrients are as important as macronutrients for sustainable farming.

The application of micronutrients in various forms individually or mixed with fertilizers has been found to correct deficiencies and physiological disorders resulting in enhanced crop growth and productivity. The extent of benefit varies from 10 to 70% depending upon the micronutrient, the soil and the crop. The benefits in fruit and vegetable crops are greater as the application of micronutrients increases their marketable yield and enhances nutritional quality, size, color, taste, and earliness, and reduces the dependence on plant protection chemicals. Still, there have been several reports of a significant reduction in plant growth and yield of various crops of agricultural importance in soils deficient in one or more micronutrients even after the application of micronutrients. Therefore, it is important to understand the role of each micronutrient in plant growth and development, the barriers to their availability and uptake, and the best formulations to increase their use efficiency for higher crop yields per unit area for sustainable agriculture.

A. Role of micronutrients

Physiological roles

Micronutrients play many complex roles in plant processes and including photosynthesis, respiration, enzyme function, formation of hormones, metabolic processes, nitrogen fixation and reduction of nitrates to usable forms, cell division and development and regulation of water uptake. For example, Zn, the first micronutrient recognized as essential for plants, plays a significant role in the catalytic action of several enzymes. It acts as a precursor of amino acids and hormones such as auxins which enhance root growth and increase the ability of plants to access nutrients and water. Zn deficiencies can affect the absorption of water and nutrients from the soil and result in a reduction in vegetative growth and yield. Zn is also known to modulate important cellular functions that help plants defend against pathogens and stresses like drought. Research studies have shown that the addition of Zn (23 kg/ha as a foliar application) reduced wheat yield loss due to drought under a rainfed system with adequate N and P supply from 25 to 13%. Fe plays a significant role in various metabolic processes such as DNA synthesis, respiration, photosynthesis, and symbiotic nitrogen fixation. It is an essential part of the hemoglobin molecule which is a component of the nodules in soybean plants. Its deficiency results in a lack of nodulation and chlorosis. Mn activates several important metabolic reactions and plays a direct role in photosynthesis. It binds to and activates plant enzymes involved in metabolic pathways including redox reactions, phosphorylation, decarboxylation and hydrolysis. It serves as an activator for enzymes in growth processes and assists iron in chlorophyll formation. Cu plays a vital role in photosynthesis, respiration, metabolism of carbohydrates and proteins, and cell wall formation. The presence of copper is intricately linked to vitamin A production, and it helps to ensure successful protein synthesis by converting amino acids to proteins. B helps in the differentiation of meristem cells in plants and plays a significant role in the cell wall biosynthesis that primarily influences many growth factors including root elongation, tissue differentiation, pollen germination, pollen tube growth, and cell membrane functions. It engages in the mechanisms for the synthesis and transport of carbohydrates and proteins and is also important for nodulation in legume plants because it accelerates atmospheric N fixation. Mo plays several critical important roles in N metabolism. It is required for N fixation in legumes by both symbiotic and free-living N-fixing bacteria, thus it is a component of the nitrogenase enzyme system.

Biotic and abiotic stress mitigation

Micronutrients influence the plant response to diseases either directly, through evoking a variety of physiological responses in the plant during pathogen attack, or indirectly, through affecting the pathogen’s survivability in the crop rhizosphere or the plant. Their balanced application makes plants vigorous with increased immunity to pathogen infection. Their deficiencies increase the plants’ vulnerability to pathogen infection as they play a critical role in plant metabolism, cellular structure, and stress responses. Zn plays a direct and indirect role in plant-pathogen systems. Directly, it provides a signal for the cellular activities of proteins involved in disease resistance in cereals, and indirectly, it influences pathogen growth via the stimulation of the activity of biocontrol bacteria, producing antimicrobial metabolites such as salicylic acid, siderophores, and the antibiotic 2,4-diacetyl phloroglucinol, causing pathogen membrane damage, hyphal structural damage, oxidative stress and by acidification of the environment to a pH range that might be non-optimal for pathogen growth. Application of Zn to the soil has been reported to mitigate infection by Fusarium graminearum and root rot diseases caused by Gaeumannomyces graminis in wheat. Fe‘s role in disease resistance in plants is not well studied. However, it has been reported that Fe is required for the synthesis of phytoalexin and host-attacking exoenzymes of fungi. Fe can control or reduce the severity of several diseases such as rust in wheat leaves, smut in wheat, and Colletotrichum musae in bananas. Foliar application of Fe can increase the resistance of apples and pears to Sphaeropsis malorum and cabbage to Olpidium brassicae. B has a direct function in cell wall structure, cell wall permeability, and stability and plays a significant role in the metabolism of nutrients in controlling plant diseases. It has been shown to reduce several plant pathogens, including Plasmodiophora brassicae in crucifers, Fusarium solani in beans, Verticillium albo-atrum in tomatoes and cotton, tobacco mosaic virus in beans, and tomato yellow leaf curl virus in tomatoes. Boron suppresses gall formation by suppressing elevated levels of auxin or auxin-like substances in plants. Mn is the most studied micronutrient for its effects on the development of resistance in plants to both root and foliar diseases of fungal origin. Mn engages in lowering the inoculum potential of soil-borne pathogens. Mn application suppresses several diseases by supporting the production of lignin and suberin in the plants, which provide chemical defense and resistance against fungal infection. Lignin and suberin have been found to contribute to wheat resistance against powdery mildew and take-all disease caused by Gaeumannomyces graminis. Soil applications of Mn have been found to reduce common scab of potato, Fusarium spp. infections in cotton and Sclerotinia sclerotiorum in squash. Cu has an indirect microbicidal effect as well as a direct effect on crop response. It activates some enzymes in plants that engage in lignin synthesis and several physiological processes that confer plant resistance. It has been used in agriculture to control many fungi and bacteria due to its biocidal properties and their indirect action through involvement in several plant defense mechanisms. There have been reports regarding the association of Mo with the response of plants to diseases. Application of Mo to tomato roots reduced the symptoms of Verticillium wilt. It also had a direct effect of reducing toxins produced by Myrothecium roridum and inhibiting the formation of zoosporangia by Phytophthora cinnamomi and P. drechleri. Soil application of Mo also decreases nematode populations. It is not known if Mo within the host plays any specific role in protecting plants from disease; however, it is required by the enzymes nitrogenase and nitrate reductase for the synthesis of proteins. Therefore, any effect of Mo deficiency may be indirect through an effect of N metabolism on pathogenesis.

Improving crop yield and nutritional quality

Micronutrients promote the strong and steady growth of crops that produce higher yields, increase harvest quality, and maximize the genetic potential of plants. Their deficiency results in stunted growth, low yields, and poor-quality harvest. There is evidence that the crops grown in soils with low micronutrients produce yields of lower nutritional value to humans or livestock. Some important human and livestock health implications from diets deficient in micronutrients include; Fe – reduced blood circulation, weakened immune system, anemia and chronic fatigue; Zn – compromised immune system function, DNA damage and impaired wound healing; Mo – toxic sulfite accumulation, central nervous system dysfunction and difficulty mobilizing iron from the liver; Cu – various neurological disturbances and increased risk of cardiovascular disease; Mn – abnormal glucose tolerance, cognitive and mood disturbances and skeletal deformities and Cl – low blood sodium levels, muscle weakness or spasms, poor nutrient absorption in the digestive tract and electrolyte imbalance.

It has been observed in many studies that the addition of specific micronutrients could positively modulate the uptake of other micronutrients, to improve the overall nutritional quality of the crop. However, in other instances, upon the addition of a specific micronutrient, the levels of other micronutrients are reduced, suggesting a negative uptake interaction between such micronutrients. The negative outcome in the overall micronutrient levels of plants upon the addition of a specific micronutrient could be due to competition for uptake among micronutrients. Therefore, agronomic fortification with micronutrients should be an intervention in situations where specific micronutrients have been determined to be limiting crop productivity, as well as for the health and well-being of animals and humans whose primary source of food includes such crops. Although the agronomic fortification of crops with certain micronutrients (Fe and Zn) may be successful, their subsequent bioavailability to humans upon consumption of such crops may be impeded by the presence of anti-nutritional factors, such as phytates, which form insoluble complexes with these micronutrients.

Micronutrient use not only enhances yield but, in many instances, also improves the quality of the produce. It has been reported that the use of boron fertilizer in rice not only enhanced paddy yields but also increased kernel milling recovery and head rice recovery as well as improved cooking quality traits.

B. Formulation Technologies

Micronutrient deficiencies are often corrected by the application of fertilizers coated with various micronutrients and/or micronutrient mixtures packaged together in different formulations using various technologies as per their use for effective delivery to soil, seed, and plant. These are available from various sources which vary in their physical state, chemical reactivity, and availability, and are broadly classified as inorganic products, synthetic chelates, natural organic complexes, and fritted glass products. Most of the micronutrients have been formulated as suspension concentrates and oil dispersions to make them readily available to plants. They have also been formulated as soluble powders, water-dispersible granules, wettable powders, soluble (liquid) concentrates, slow-release fertilizers and nanoparticles. The type of formulation to be used on a crop depends on many factors such as micro-nutrient, crop stage, growing conditions, application method and objective.

Soluble powders, granules, and liquid concentrates deliver micronutrients as soluble ions that are readily bioavailable to plants. These are formulated from the water-soluble inorganic salts of the micronutrients such as sulfates, chlorides, and nitrates; and water-soluble chelates such as EDTA, and water-soluble complexes such as lignin sulphonates or citrates. Micronutrients are often formulated as aqueous suspensions. While these micronutrients may be water soluble, a liquid flowable product may be obtained with insufficient water to fully dissolve the micronutrient. Addition of a suspending agent can produce a formulation of liquid suspension of a water-soluble micronutrient. This practice allows production of a product with the advantages of a liquid with a higher concentration of the micronutrient than would be possible with a water solution. Chelated compounds, although present in soluble form in these formulations have longer availability as they first become fully solubilized in the soil solution at slow rates prior to uptake by the plants.

Aqueous suspension concentrates of micronutrient fertilizers are also based on insoluble products which offer the advantages of providing high nutrient content in an easy-to-use aqueous form.

Water-dispersible granules and wettable powders are mostly formulated from water-insoluble inorganic compounds; however, after application to the foliage, the slightly acidic conditions of the leaf surface facilitate the release of the micronutrients leading to their absorption by the plant.

Liquid micronutrient fertilizers have been formulated using flavanol polymer technology for controlled release. This technology improves plant utilization and reduces the concentration of micronutrients required. By this technology deficiencies of copper, iron, manganese, and zinc can be corrected without affecting the soil nutrient balance.

Granular formulation of micronutrients is more compatible with application of macronutrients in granular form and has specific advantages according to crop production practices. The homogeneous nature of the granular formulation ensures that each granule contains all the secondary and micronutrient elements for maximum consistency when mixed properly. Granular formulations are available as sulfates, oxides, and oxysulfates. Sulfates are 100% water soluble and available to the crop in the year of application. They quickly provide nutrients to the plants and soil. Oxides are insoluble, making them unavailable to the crop in the year of application. These are used to build soil nutrient levels on a long-term basis but not to correct nutrient deficiencies in the year of application. Oxysulfates are a combination of oxides and sulfates within the same granule. The availability of micronutrients depends on the balance of sulfate and oxide nutrients. A sulfate or oxysulfate granular micronutrient formulation should be used to meet the crop nutrition requirement for maximum yield potential where soil tests show a deficiency for the micronutrients. Granules of the soluble sulfate for copper, manganese and zinc should be placed in the seed row or near the seed row as these nutrients only move through diffusion. All three forms of granular micronutrients described above have a place for specific circumstances in a cropping system. The most important thing is to use the correct form at the correct time.

Chelated micronutrients are widely used in agriculture and strongly promoted by the fertilizer industry. In fertilizer technology, chelated nutrient refers to inorganic nutrients that are enclosed by an organic molecule (chelating agent) such as amino acid, organic acid or peptide. Chelation produces a complex that the plant can more easily absorb. Chelates are useful for micronutrients to be applied in alkaline soils and applied as foliar application. In alkaline soils, Fe, Mn, Zn, and Cu react with the ions found at high pH and form insoluble substances and as a result, the nutrients become unavailable to plants. The organic complex in a chelate prevents these reactions from happening in the soil. The plant roots take up the chelated nutrients, and the chelates release the nutrients within the plant. Similarly, the chelated nutrients penetrate more readily through the leaf surface. Wax on the leaf surface repels water and prevents inorganic nutrients from penetrating the leaf. Among the micronutrients, Fe, Zn, Cu, Mn, Ca and Mg can be chelated, while the other micronutrients cannot. Several organic substances such as Ethylene diamine-tetra-acetic acid (EDTA), Diethylene-triamine penta-acetic acid (DPTA), Hydroxyethylethylenediaminetriacetic acid (HEDTA), and ethylenediaminedi(o-hydroxyphenylacetic acid) (EDDHA) are used to produce chelates. EDTA is the most common synthetic chelating agent which is used for both soil and the foliar applied nutrients. DTPA is used for chelates applied to alkaline soils. It is more effective than EDTA but is usually more expensive. Iron chelates made with HEDTA and EDDHA are the most effective iron fertilizers on high pH soils but are also the most expensive. There are also several natural products such as phenols, organic acids and lignosulfonates, produced as by-products from the wood pulp industry, used to produce chelates.

Innovative technologies in the past and currently as described above, micronutrients are presented to plants as salts, chelated compounds, or bulk oxide particles. Micronutrients are delivered as soluble ions, which are both readily plant-bioavailable and rapidly lost from the soil. Whereas the chelated compounds, though present in soluble form, have longer availability time in the soil, bulk particulate micronutrients must first become solubilized in the soil solution, often at slow rates, prior to uptake by the plant. The availability of solubilized micronutrients to the crops is dictated by soil factors that often restrict their uptake, leading to low uptake efficiency. The bulk of micronutrient formulations described above have been used all over the world and are water soluble and their water solubility results in leaching and run-off, nutrient fixation by soil and high quantity requirements. All this led to the development of recent technologies to formulate micronutrients in such a way that their usability by plants is enhanced and simultaneously they become less harmful to the soil, are safer for the environment with minimal losses and are less expensive.

Slow-release formulations of fertilizers have low water solubility, have been considered as the solution to the above problems and this functionality has been achieved by using several formulation technologies such as encapsulation of water-soluble materials within a membrane and conversion to polymers of the insoluble oxides and phosphate glasses. Metaphosphates and glassy phosphates, also known as Frits, have been developed using this technology and recommended for use by growers. These products dissolve by slow hydrolysis, which is highly dependent on the soil and crop to release micronutrients into the soil. These types of insoluble compound fertilizers are effective only if rates of release of nutrients match the plant’s requirements. Fritted micronutrients are used only in sandy soils in regions of high rainfall where leaching occurs.

Slow-release combined cationic micronutrient fertilizer with Zn, Fe, Mn, and Cu in a single compound has also been formulated using a different mechanism of nutrient release where plant roots digest certain insoluble compounds by ion-exchange with the root hairs or by extracellular organic acid secretions that extract nutrients by chelation. The advantage of such a product is that a single fertilizer provides all the cationic micronutrients and is beneficial to both the crop and the farmer. In various research studies, slow-release fertilizers produced significantly higher yields than conventional fertilizers in rice and potatoes.

Another type of slow-release fertilizer which belongs to the bio-release types has been produced in which the micronutrient ions are in a chemical form wherein they are insoluble and are also plant-available. The technologies for producing phosphate-based Zn and Cu fertilizers and methods for assessing the limits of polymerization have been described in many patents.

Hydrophilic organic gel-forming polymers such as polyacrylamides with inexpensive soluble iron sulfate have been known to significantly improve the efficiency of Fe source fertilizer materials for iron-sensitive plants growing on iron-deficient soils. Hydrogels restrict contact of soluble Fe fertilizers with the soil, thereby minimizing the extent of chemical reactions with the soil to reduce the availability of the applied micronutrients to plant roots.

This technology provides consistent, uniform micronutrient distribution on fertilizer surfaces, enhanced color and appearance, and dust suppression characteristics. This technology has been used successfully over several years, particularly in the Brazilian market, to correct severe micronutrient deficiencies in the soils of the vast and increasingly agriculturally important region of the country.

Nanotechnology is another technology that has been used to formulate micronutrients in the nano-size range. One advantage of nanotechnology is the initial quick release of plant-usable ions immediately following application and then a more sustained slow release over time. In recent years, several studies have compared the efficacy of micronutrients formulated as nanotechnology products with currently used formulations in enhancement of plant growth and crop yield, nutritional quality and disease resistance. For example, black-eyed peas foliar fertilized with Nanotechnology P-Fe resulted in an increase of 4, 46, and 10 percent yield, Fe, and chlorophyll contents respectively, compared to FeSO4. In other studies, improved biomass production, seed yield, nutrient content, and/or disease control in different plants treated with Fe, Cu, Mn, and Zn-based nanoparticles have been reported relative to their respective salt or bulk counterparts.

Hybrid nano (HN) fertilizer produced with the use of nanotechnology saves resources and reduces the outflow of fertilizers. HN fertilizer can significantly reduce the leaching problems often associated with commercial fertilizers. Additionally, HN is effective at reducing nutritional deficiencies in plants, providing nutrient-rich crops, and lowering environmental pollution. HN fertilizers facilitate the initial fast release of plant-usable ions at application and then a more sustained slow release over time. In several recent studies, micronutrients such as Cu, Fe, and Zn packaged in a single nano product have been found to improve crop growth and yield compared to standard formulations for example, urea-modified hydroxyapatite that is incorporated with nanoparticles of Cu, Fe and Zn increase plant availability and nutrient efficiency.

There are numerous micronutrient products, formulated using different technologies, available in the market. For producers, there is always a challenge in determining which micronutrient product works best in their specific nutrient program. A number of factors affect micronutrient bioavailability in plants. Often, product labels make claims about how available the product is, but the true test is how well the product works in the field. There are differences in plant availability of various products. Efficiency is also related to application methods.

The farmer should select and apply the products that contain the essential micronutrients as indicated by soil or tissue samples. Additionally, selecting products that are readily available for plants and applying them at the right time. In some cases, chelated products are needed due to specific soil or environmental conditions. An example of this is the chelated Fe products used to mitigate soybean Iron Deficiency Chlorosis in high pH soils. The application method is also a critical component for overall fertility success. In general, banding fertilizers in the seed row is more efficient than broadcast and broadcasting of some micronutrients is not recommended because the use rates are so low. However, broadcasting may be the only alternative for some other micronutrients. This may be especially important in no-till systems.

Summary and Conclusion

Though required in minor amounts, in the absence of any one of the seven essential micronutrients, plants will not survive. These essential micronutrients are important in many critical plant physiological processes, and management of their availability is essential to plant production. In addition to the impact on crop yield, micronutrient availability impacts nutritional quality, size, color, taste and earliness. Essential micronutrients are also important in the health of humans and livestock that consume plants. Deficiencies in micronutrients thus negatively impact the food value of crops. Application of micronutrients in various forms is often necessary for optimum plant growth and crop production. Important considerations in application to correct deficiencies of micronutrients include; formulation, crop, soil organic matter, soil pH, nutrient levels in the soil and availability of micronutrients present in the soil.

Proper management of micronutrients can have a profound impact on sustainability. Many micronutrients play a role in mitigating plant disease through a direct response on the plant’s survivability or impact on pathogen survival in the rhizosphere. Many formulation options can impact solubility and mobility of micronutrients in the soil and reduce the opportunity for leaching or loss in runoff water. Pollution of ground or surface water can be reduced. New and developing formulation options can also increase availability or absorption of micronutrients, which may allow for a reduction in applied rates.