Micronutrients, which include B, Cl, Cu, Fe, Mn, Mo, Ni and Zn, are required by plants in very low concentrations for adequate growth and reproduction. However, despite their low concentrations within plant tissues and organs, micronutrients are as important as macronutrients for plant nutrition. At these low concentrations, micronutrients are essential for plant growth and development, acting as constituents of cell walls (B) and cell membranes (B, Zn), as constituents of enzymes (Fe, Mn, Cu, Ni), as enzyme activators (Mn, Zn) and in photosynthesis (Fe, Cu, Mn, Cl) (KIRKBY; RÖMHELD, 2007).
Inadequate micronutrient content in crops, which limits their growth and can go unnoticed, not only has a direct effect on crop development but also reduces the efficiency of use of fertilizers containing macronutrients. In addition, micronutrients (Cu, Mn, Zn, B) are particularly involved in the reproductive phase of plant growth and, consequently, in determining the productivity and quality of the harvested crop (KIRKBY; RÖMHELD, 2007). Micronutrients can be applied directly to the soil, through fertigation, foliar fertilization or seed treatment (GONÇALVES et al., 2019).
Due to the difficulty in uniformly distributing micronutrients through fertilization, given the small amounts required, seed treatment via pelletization represents a viable alternative for fertilization (Ohse et al., 2001). In addition to its low costs and smaller amount of fertilizer required, the application presents greater uniformity of distribution (Parducci et al., 1989) and good utilization by the plant, proving to be an effective and easy-to-perform practice. The application via seed treatment is based on the translocation of the applied micronutrients to the future plant (Cheng, 1985), especially when environmental conditions restrict root growth. However, it is important that the producer follows the recommendations for use and dosage during treatment, since high concentrations of salts near the seed can harm seedling emergence (Pessoa, 2000).
In situations where the micronutrient has low mobility in the plant, as in the case of Zn, B and Cu, it can be applied via foliar application, ensuring a greater supply of demand and allocation to the vegetative parts (Lawson et al., 2015).
For technical and economic reasons, foliar applications or seed treatment can be performed together with applications of fungicides, insecticides, herbicides and beneficial microorganisms, without compromising the quality and performance of the plants. According to Bays et al. (2007), in a study carried out with soybean crops, the joint application of fungicides, micronutrients (Co, Mo and B) and polymer ensured better uniformity in the treatments and did not compromise the quality and performance of the seeds up to the limit of 2 mL of micronutrients per kilogram of seeds, proving to be highly efficient.
An alternative is soil application, which can be done by broadcasting (with or without incorporation), applied in the hole or in the seeding furrow, aiming to increase their concentration in the soil solution. However, it is necessary that the sources of micronutrients used are dissolved in the soil at a speed compatible with the absorption rate by the roots and, applied in regions close to them, due to the low mobility of some micronutrients, mainly in clayey soils (GONÇALVES et al., 2019).
Fertigation can also be used, combining two main factors essential for plant growth and development: water and nutrients. Its advantages include reduced labor requirements, reduced soil compaction, reduced machine traffic within the area, good uniformity of fertilizer distribution, application at the time the plant needs it, ease of division, reduced soil erosion and reduced physical damage to the crop (Bernardo et al., 2008).
The lower concentrations of micronutrients are reflected in their function as constituents of prosthetic groups in metalloproteins and as activators of enzymatic reactions. Their presence in prosthetic groups allows them to catalyze redox processes by electron transfer (mainly the transition elements Fe, Mn, Cu and Mo). Micronutrients also form enzyme complexes by binding the enzyme to the substrate (e.g. Fe and Zn). It is now also known that several micronutrients (Mn, Zn, Cu) are present in superoxide dismutase (SD) isoenzymes, which act as scavenging systems to eradicate toxic oxygen radicals in order to protect biomembranes, DNA, chlorophyll and proteins. For the nonmetals B and Cl there are no well-defined enzymes or other essential organic compounds that contain these micronutrient elements. However, it is now established that B is an essential constituent of cell walls (KIRKBY; RÖMHELD, 2007).
Table 1 – Main functions of plant micronutrients.
Basic description of micronutrients:
- Boron (B) – It is particularly important in cell multiplication. It is of extraordinary importance in the germination of pollen grains, in the formation of flowers, fruits and roots, in the movement of sap and in the absorption of cations. Boron is an element that has low mobility in the plant, and it is assumed that it is transported only in the xylem, since it is practically immobile in the phloem.
- Chlorine (Cl) – Its function is related to photosynthesis, participating in the photolysis of water.
- Copper (Cu) – It activates several enzymes within the plant. It is essential for plants in oxidation and reduction processes.
- Iron (Fe) – It is essential for the formation of chlorophyll, nitrogen absorption and several enzymatic processes.
- Manganese (Mn) – Like iron, it is also necessary for the formation of chlorophyll, for the reduction of nitrates and for respiration. In some metabolic processes, it acts as a catalyst. It participates in the formation of ascorbic acid (Vitamin C).
- Molybdenum (Mo) – Participates in the biochemistry of absorption and in the transport and fixation of nitrogen.
- Zinc (Zn) – Acts on plant growth through its participation in the formation of indoleacetic acid (IAA).
Micronutrient adsorption is a process of adhesion of micronutrients to sufficiently strong soil colloids, and is considered important in controlling their abundance and passage in the soil solution and, consequently, their availability to plants (CAMARGO, 2006). Under field conditions, the supply of micronutrients to plants occurs mainly from absorption from the water-soluble pool of the rhizosphere, unless there is a supply to the leaves through foliar application. The rhizosphere pool is replenished by nutrient fluxes through mass flow and diffusion (Figure 1), with the latter process providing the greatest contribution to most micronutrients. However, for most arable soils (but not, for example, soils contaminated with heavy metals), the supply of soluble inorganic micronutrient species from this pool, including replenishment by mass flow, is much lower than that necessary to meet the plant's needs for optimal growth (KIRKBY; RÖMHELD, 2007).
In order to increase crop productivity and quality, it is necessary to improve the nutritional status of micronutrients in crops through appropriate management of these. To achieve this objective, it is necessary to use a range of strategies, including the supply of fertilizers with micronutrients, genetic improvement to obtain cultivars improved in terms of micronutrient absorption efficiency, and crop rotation to increase resistance against numerous pathogens and other biotic and abiotic stresses (KIRKBY; RÖMHELD, 2007).
Importance of micronutrients in the main Brazilian crops
For soybean crops, the influence of Mo is notable, which, in addition to being part of the nitrogenase enzyme, also acts on nitrate reductase, responsible for reducing NO.3- , to be assimilated by the plant (Pessoa et al., 1999). In floodplain soils, N fixation may be lower compared to dryland soils, making it feasible to add Co and Mo in seed treatment, aiming to increase BNF (Scholles and Vargas, 2004).
For corn, seed treatment with Zn ensures maximization of the crop's productive efficiency (Fancelli, 2001). According to Neto (2010), corn seed treatment with Zn and Mo ensures greater dry mass of the aerial part and vigor during seedling development, providing greater effectiveness in its establishment.
Ni participates in the synthesis of phytoalexins, which increase the resistance of corn plants to diseases (Reis et al, 2014). The nutrient also exerts a synergistic effect when applied with the micronutrients Cu and Fe, favoring the development and productivity of corn plants (Torres et al., 2016).
Ohse et al. (2001) working with irrigated rice (cultivar BR-IRGA 410), using treatment with Zn (ZnSO4.7H2At a dose of 0.67 g.kg-1 seeds), Cu (CuOCl at a dose of 0.135 g.kg-1 seeds) and B (H3BO3 at a dose of 0.065g.kg-1 seeds), obtained significant results on seedling vigor. However, it was observed that the use of micronutrient combinations Zn+B and B+Cu should be avoided, as they caused reductions in vigor.
Si can aid rice cultivation in soils with Al toxicity (Nhan and Hai, 2013). According to Freitas et al. (2012), there was an increase in grain yield in upland rice with increasing doses of Si and cultivation under Al and N stress, also reducing the Al content in the aerial part of the plants (Freitas et al., 2015).
For common beans, Silva et al. (2014) observed that the application of Cu in chelated form (EDTA) significantly increased the content of Cu available in the soil and in the plant, whereas when the sulfated source was applied, no increase in the nutrient was observed in the plant. As for Zn, the same authors observed that the source most absorbed by the common bean is the sulfated one.
Foliar application of micronutrients with ILSA fertilizers
As discussed in the text, increased crop productivity and quality are linked to the use of several management practices, including the correct supply of micronutrients that are essential for plant development. However, many characteristics of these nutrients, such as their mobility, end up affecting their absorption by plants. Therefore, supplying these elements in foliar form is an alternative to avoid possible deficiencies in plants, making nutrients available in a timely, local and rapidly absorbed manner.
ILSA Brasil has a complete line of foliar fertilizers, rich in micronutrients, obtained from the GELAMIN matrix® which, when combined with mineral raw materials of nutrients, is capable of forming natural complexes or chelates, thus increasing the availability and speed of absorption of these elements by plants. In addition, GELAMIN® It has 16 essential amino acids in its composition for plant metabolism, which act as nutrient carriers, facilitating their absorption and preventing possible deficiencies.
Check out the complete line of ILSA Brasil foliar application fertilizers on the website:
References:
Bays R et al. Coating of soybean seeds with micronutrients, fungicide and polymer. 2007.
BARROS, José. Soil fertility and plant nutrition. 2020.
Bernardo S et al. Irrigation manual. 2008.
CAMARGO de, OA Reactions and interactions of micronutrients in soil. 2006.
Cheng T. The effect of the seed treatment with microelements upon the germination and early growth of wheat. 1985 .
Fancelli AL. Physiology of corn plants in off-season conditions. 2001.
Freitas LB et al. Silicon and aluminum interaction in upland rice plants grown in aluminic soil. 2012.
Freitas LB et al. Silicon in mineral nutrition and aluminum accumulation in upland rice plants. 2015.
GONÇALVES, Ana Stella Freire; MACHADO, Guilherme Gonçalves. Use of micronutrients in agriculture: effects and applications. Rev Agr Bras, v. 3, p. 1-4, 2019.
KIRKBY, Ernest Arnold; RÖMHELD, Volker. Micronutrients in plant physiology: functions, absorption and mobility. Agronomic information, v. 118, no. 2, p. 1-24, 2007.
Lawson PG et al. Soil versus foliar iodine fertilization as a biofortification strategy for field-grown vegetables. 2015.
Parducci S. et al. Micronutrients. 1989.
Pessoa ACS et al. Soybean productivity in response to foliar fertilization, seed treatment with molybdenum and inoculation with Bradyrhizobium japonicum. 1999.
Pessoa ACS et al. Germination and initial development of corn plants in response to boron seed treatment. 2000.
Neto AJM et al. Effect of seed treatment with micronutrients (Zn and Mo) on the development of corn (Zea mays) seedlings. 2010.
Nhan PP; Hai NT. Improvement of aluminum toxicity on OM4900 rice seedlings by sodium silicate. 2013.
Ohse S et al. Germination and vigor of irrigated rice seeds treated with zinc, boron and copper. 2001.
Reis AR. et al. Physiological role of nickel: essentiality and toxicity in plants. 2014.
Scholles D; Vargas LK. Feasibility of inoculation of soybean with Bradyrhizobium strains in flooded soil. 2004.
Silva AA et al. Soil and foliar micronutrient contents with application of chelated and sulfated sources in beans. 2014.
Torres GN et al. Growth and micronutrient concentration in maize plants under nickel and lime applications. 2016.
Authors
Agr Eng. Dr. Angélica Schmitz Heinzen
Agricultural Eng. Msc. Carolina Custodio Pinto
Agricultural Eng. Msc. Thiago Stella de Freitas
