Nitrogen (N) is the most important macronutrient for plants. What distinguishes it from other elements is that it can be absorbed both in the form of cation (NH4+) as anion (NO3–). According to Bredemeier & Mundstock (2000), in many production systems, nitrogen availability is almost always a limiting factor, influencing plant growth more than any other nutrient.
Given its importance and high mobility in the soil, nitrogen has been intensively studied in order to maximize the efficiency of its use. To this end, efforts have been made to reduce nitrogen losses in the soil, as well as to improve the absorption and metabolism of N within the plant (BREDEMEIER & MUNDSTOCK, 2000). An important characteristic of N availability is its wide fluctuation in the soil during the time that crops remain in the field (PURCINO et al., 2000).
Nitrogen is found in the soil essentially in organic form (approximately 98%). The other small part is found in the mineral forms of ammonium, nitrate and nitrite (ALFAIA, 2006). According to this author, mineralization is the biological transformation of organic nitrogen in the soil into inorganic nitrogen, carried out by heterotrophic soil microorganisms. Immobilization refers to the reverse process, that is, it is the transformation of inorganic nitrogen into organic nitrogen. Soil microorganisms assimilate the inorganic forms of nitrogen to form the organic constituents of their cells and tissues. The compounds synthesized by microorganisms can be partially mineralized and become available to plants.
All organic nitrogen is in the form of N-NHx (amines, amides, imides) and, in the mineralization process, the first product of this transformation in the soil is ammonium (NH4+), which subsequently undergoes oxidation through the action of microorganisms and becomes nitrate (NO3–). Once in this form, the N will be available for use by plants (HUTZINGER, 1982).
According to Stanford & Smith (1972), cited by Fonseca (2001), the amount of nitrogen mineralized in the soil in a given period depends on temperature, water availability, oxygen replenishment rate, pH, quantity and nature of plant residues.
Lack of nitrogen in plants
ON is the mineral that most limits growth, development, productivity and biomass production of most crops.
It is associated with several functions in plants, such as growth and development, as it is a component of proteins and is associated with all enzymatic functions; it acts directly in photosynthesis, as it is a constituent part of chlorophyll; it is responsible for the dark green color of leaves and vegetative tissues and acts directly in the full development of the root system.
Plants that grow in more balanced environments and are well nourished produce amino acids, but quickly bind them together, transforming them into proteins, which are more complex substances (OLIVEIRA et al, 2009). In the production of proteins, ammonia combines with sugars, forming amino acids and, in the absence of N, sugar accumulates in the plant, preventing full development, reducing productivity and attracting pests.
Oliveira et al (2009), studying the influence of nitrogen and potassium on the life cycle of B. tabaci, states that many chemical reactions that occur in plants must be catalyzed or accelerated by enzymes. However, these components can only act when they are activated by the presence of certain minerals. If an activating mineral is missing, the chemical process occurs very slowly, the substance accumulates, circulating in the sap without being able to be used by the plant. Azeredo et al (2004), in a study on the impacts of nutrients on the population of pests in potatoes, found that in relation to the accumulation of soluble sugar levels in tubers, nitrogen led to a reduction in the concentration of soluble sugar and consolidation of starch. This accumulation can attract insects to the plants due to the available sugar level, causing losses in productivity.
Nitrogen deficiency will be noticed first in older leaves, due to the high mobility of N in the plant. Symptoms such as yellowing (appearing mainly in V), necrosis, reduced growth and a drop in the protein content of the grains are the most observed.
It is important to remember that, even if there are characteristic symptoms, soil and leaf analyses are essential tools for the correct detection of mineral deficiencies and excesses in plants, and are therefore indispensable.
Excess nitrogen in vegetables
Despite being highly required, excess nitrogen can cause irreversible damage to plants. This damage can extend throughout the production cycles, harming not only the crop in question, but also subsequent crops.
Because it is directly related to protein synthesis and growth, plants show abnormal growth, with accumulation of green mass and probable etiolation. On the one hand, accelerated and abnormal vegetative growth, on the other, slow and minimized reproductive development. Several studies suggest that excess N can cause delayed flowering and reduced “setting” of flower buds, reduced fruiting, delayed ripening and fruit coloration may also be affected.
Furthermore, when present in excess, nitrogen can hinder the absorption of other nutrients, such as magnesium (Mg), calcium (Ca) and potassium (K), by creating a type of competitive inhibition. This antagonistic effect occurs due to competition for binding sites in the roots, where the element that is available in greater quantity ends up being absorbed more quickly.
Excessive use or immediate release of nitrogen in large quantities is also associated with reduced resistance to attacks by pathogens and pests. Elements such as K and Ca act to stiffen the cell wall, making it more resistant. However, as mentioned previously, nitrogen tends to compete for binding sites with these elements and, when in excess, makes these plant tissues more susceptible. According to Marschner (1995), “the damage caused by excess nitrogen can be justified by the reduction in the synthesis of phenolic compounds such as phytoalexins and lignin, which makes the plant more tender and less resistant to fungal infections”.
Studying the stresses caused by the lack or excess of nitrogen in rice plants, Nohatto et al. (2013) stated that “both the use of excessive doses of nitrogen and the limitation of this resource can cause a condition of oxidative stress in rice plants”. In a study on doses and forms of nitrogen for nutrition of Tanzania grass, Correr (2015) cites Magalhães et al (2006), and states that the inappropriate supply of nitrogen generates stress caused by the oxidation of nitric oxide (NO–) by hydrogen peroxide radicals (H2THE2), which result in the formation of the hydroxyl radical (OH–) and nitrogen dioxide (NO2) and the photochemical production of O3, and that these compounds can promote morphological changes.
Nitrogen absorption forms
Nitrogen assimilation involves the processes of reducing nitrate to ammonium and incorporating ammonium into amino acids. The rate and quantity of nitrogen assimilated by plants during their cycle depend on the activity of the enzymes involved in the nitrogen cycle and the availability of energy required for the assimilation processes (BREDEMEIER & MUNDSTOCK, 2000).
According to Andrade et al (2001), the concentration of nitrate and ammonium in the soil solution varies within relatively short periods and the plant requirement also varies depending on the species, age, physiological stage and availability of carbohydrates.
According to Brown et al. (1983a, b) and Blackmer (2000), cited by Heinrichs et al. (2006), in general, at the beginning of plant development, there is a preference for absorption of the ammoniacal form and, as the vegetative cycle progresses, absorption in the nitric form increases.
However, both forms must be offered in a balanced manner. The differential supply of nitrate and ammonium to the plant can affect the levels of enzymes involved in nitrogen metabolism, resulting in changes in plant growth and production (CARVALHO, 2012).
Once inside the cells, NO3– (nitrate) can follow four distinct routes: (1) in the roots, it is first reduced to NO2– (nitrite), after NH4+(ammonium), finally being assimilated in the form of amino acids, contributing to root growth; (2) NO3– (nitrate) absorbed in the roots is transported to the aerial part where it is reduced to NH4+ (ammonium) and assimilated as amino acids, promoting overall plant growth; (3) a significant amount of NO3– (nitrate) can be stored as reserve in the vacuoles; (4) a small part of the NO3– (nitrate) absorbed can be excreted back into the soil (PURCINO et al, 2000).
Problems with nitrogen mineral fertilization
The duration and permanence of mineral nitrogen in the soil depends on a series of factors, such as absorption by plants, mineralization of organic matter, rainfall regime in the region, losses due to volatilization, leaching and the action of denitrifying bacteria, which transform nitrogen compounds into N.2 (atmospheric nitrogen).
Many fertilizers currently on the market work with compounds that quickly release and make nitrogen available in the soil, such as urea, which, a priori seems to be something interesting. From an agronomic point of view, urea presents a serious limitation in applications to the soil surface, due to the chances of losses due to NH volatilization3 (ammonia) (FILHO, 2010). In this case, ammonia comes from the reduction of nitrite (NO2–) which, in turn, derives from the reduction of nitrate.
Furthermore, excessive use of these fertilizers can generate salinity in the soil, making it difficult to absorb water and other nutrients. The mineral can even be easily lost through leaching if the application period coincides with rainy periods, which will result in partial or total loss of the investment made in fertilization.
There are reports in corn crops in which nitrogen applied as top dressing in the form of urea and supplied at the V3 to V5 stage is absorbed by the plant quickly and in large quantities, which ends up triggering the process of synthesis and metabolism of hydrogen peroxide (H2THE2), a substance that is highly toxic to the plant. This causes it to use part of its energy to “detoxify” itself, causing losses in productivity.
Mineral fertilizers can also become an environmental problem, as some of the minerals are lost and carried away by rainwater, contaminating rivers and springs; or infiltrate the soil, contaminating groundwater.
Importance of gradual nitrogen supply
Gradual-release fertilizers are those that have technologies that allow the release of N throughout the production cycle, increasing the efficiency of the product. Several studies report that nitrogen, when released continuously, offers better yield, greater stability, reduced lodging, lower loss through leaching and favors the development of soil micro and macro fauna.
The nitrogenous organomineral produced by ILSA Brasil, the AZOSLOW, presents this characteristic, where the supply of organic nitrogen present in the fertilizer occurs gradually, as it will be mineralized through the action of microorganisms. Given this fact and in addition to countless other advantages, we can say that this type of fertilization allows a reduction in the number of applications, as the mineral will be made available naturally throughout the development of the plant, often without the need for subsequent fertilization.
Another advantage is increased resistance to diseases and pest attacks. This is due to the fact that N is not released in a single step, but rather throughout the cycle, avoiding competitive inhibition of the absorption of other nutrients responsible for the rigidity of plant tissues.
Because they feature a matrix composed of natural amino acids, ILSA Brasil fertilizers are sustainable and safe for the environment, and can even be recommended for organic crops, with Ecocert® certification (see products with certification in our website).
Remember to always keep your soil analysis up to date and if you have any questions, contact us. our team!
Authors:
- Agricultural Eng. MSc. Aline Tramontini dos Santos
- Agricultural Engineer Ana Elisa Velho
- Agricultural Eng. MSc. Thiago Stella de Freitas
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