• Plastic role.
• Constituent of chlorophyll (the green pigment in leaves).
• It enters the constitution of all proteins.
• Promotes cell proliferation.
• It determines the yield of crops.
• The driving element of the growth process.
• Essential constituent of cytoplasm.

• 2-4% din s.u.
• Absorbed by plants through atmospheric fixation symbiotically by legumes or non-symbiotically, and from the soil is absorbed in the form of nitrate ion (NO3-) and ammonium ion (NH4+).
• Absorbed N is transported through the xylem (in the stem) to the leaf in the form of nitrate ion, or it can be reduced in the root zone and then transported in the organic form, of amino acids or amides.
• N is mobile in the plant so it can be translocated from old to young leaves to be stored in seeds or fruits.
• The organic forms of N in phloem sap are represented by amides, amino acids and ureides.

• Effect on growth rate.
• The plants remain small.
• Their constitution is frail.
• Branching is poor.
• The surface area of the leaves is small.
• Causes yellowing or chlorosis of leaves.
• Yellowing usually occurs on the base leaves while the top leaves remain green due to the fact that they
• receive N by translocation from older leaves.
• The discoloration starts at the tip of the limbus and progresses in a V shape.
• In case of severe deficiency, the leaves turn brown and die.
• The yield and protein content are reduced.

• It is externalized by the luxuriant growth of the leaves, which take on a dark green metallic blue coloration.
• The vegetation period of the plants is extended and the maturity of the harvest is delayed.
• High concentrations of NH4+ can be toxic to plant growth, especially when the soil solution is alkaline.
• The excess of N-NO3- is manifested by the etiolation of the leaves, the loss of toughness, burns and their marginal necrosis.

• It is essential for plant growth.
• In cell division.
• In the development of the root system.
• In fruiting and seed formation.
• In early ripening.
• It is a constituent in various compounds such as oils and amino acids.
• It is responsible for storing energy and transporting it in the cell, being a component of adenosine diphosphate (ADP) and adenosine triphosphate (ATP).
• It is part of phospholipids, it has a role in carbohydrate metabolism.

• It is found in smaller amounts compared to N and K, in a concentration ratio of 1:5 to 1:10 relative to plant N content relative to s.u.
• It is absorbed as o-phosphate ion, either as H2PO4- or as HPO42-, depending on the soil pH.
• If soil pH increases, the proportion in the H2PO4- form decreases and the HPO42- form increases.
• P is very mobile in the plant (not like in the soil) it circulates through both xylem and phloem.
• When the plant is deficient in P, it is very easily translocated from mature leaves to young tissues.

• Plant growth is affected by P deficiency.
• By retarding growth, branching is stunted, the root system does not develop, and ripening is delayed.
• Plants affected by phosphorus deficiency show a purple-red (anthocyanin) pigmentation of the leaves and stems.
• Deficiency symptoms usually appear on older leaves.
• A green-bluish-purple to reddish color appears that can lead to bronze and red shades.
• Phosphate deficiency in chloroplasts reduces the photosynthesis process.
• Because ribonucleic acid (RNA) synthesis is reduced, protein synthesis is also reduced.

• High amounts of P in the plant can produce symptoms of toxicity, manifested by watery edges of the leaf tissues,
• which eventually become necrotic.
• In severe cases of P toxicity, the plant dies.
• Excess P induces secondary Zn deficiencies.

• It plays an important role in regulating the water regime in the cell, with a role in stomatal opening activity.
• Role in the synthesis and deposition of carbohydrates.
• K is responsible for the activation of more than 60 enzymes, involved in the process of photosynthesis and in the transport and storage of substances in reserve organs (seeds, tubers, roots and fruits) and confers resistance to diseases, pests and storage.

• Potassium is the second most abundant nutrient after N, it is 4-6 times more abundant than macronutrients such as P, Ca, Mg and S.
• K is absorbed as the monovalent cation K+ and moves through the phloem into the plant.

• General symptoms of K deficiency are chlorosis along the leaf margins, followed by curling and browning of the tip.
• K deficiency is localized to the basal, older leaves due to the high mobility of K in the plant.
• Affected plants are stunted, with short internodes; the stem is frail and prone to falling; low resistance to diseases and pests; poor harvest, low quality.
• Poor plant development also leads to a high rate of respiration, which means high water consumption per unit of s.u. produced.

• It is a rare phenomenon, it manifests itself mainly through imbalances determined in Ca and Mg nutrition.

• As it is part of the architecture of cell walls and membranes.
• Calcium is present in the pectic membrane, gives resistance to tissues, involved in fruit ripening.
• It is involved in cell division, in root growth, in the activation or inhibition of some enzymes.
• It maintains the acid-base balance in the cell, by neutralizing excess organic acids, as evidenced by the amounts of calcium oxalate accumulated in mature cells.
• It has a very important role in the detoxification of the plant's body from other ions and radicals that arise in metabolic processes.

• Calcium together with Mg, P and S is part of the group of macronutrients with the same weight in the plant.
• It is absorbed by the roots of the plant in the form of the divalent cation Ca²⁺.
• Unlike other ions, calcium is less mobile in the plant or even immobile in the phloem.
• Ca deficiency problems are most often due to the inability of Ca to be transported through the phloem.
• Fruits are supplied with Ca+ mainly through the transpiration process, which translocates Ca2+ directly from the soil solution.
• If the sap circulating through the xylem has a low Ca2+ content or the intensity of transpiration is low, favorable conditions are created for the inadequate supply of Ca to the fruits.

• Ca deficiency manifests itself very differently.
• The leaves become small, distorted, cup-shaped, wrinkled and dark green.
• Marginal chlorosis of the leaves, blackening, wrinkling and necrosis of the apical leaves appear (cabbage, lettuce).
• Marbled appearance of the leaves (tomatoes).
• Abortion of flowers.
• The bitterness of bitter pit apples.
• Incomplete seed development.
• Apical tip rot (tomatoes, peppers).
• Blackening of the heart (cauliflower).
• Cylindering of terminal leaves (wheat, barley).
• Growth stops, plants twist and in case of severe deficiency die.

• Excess calcium triggers K and Mg deficiencies in crops.
• High contents of active CaCO3 in the soil can prevent the assimilation of Fe with the appearance in the plant of ferric or ferrocalcic chlorosis frequently reported in peach and grapevine.

• Mg occupies the center of the chlorophyll molecule, so it is vital in the process of photosynthesis.
• It is associated with the activation of some enzymes, favoring the absorption and translocation of phosphorus.
• It is involved in energy transfer, being involved in several phosphorylation reactions, maintains the cell's acid-base balance, favors protein production, carbohydrate metabolism.

• Mg belongs to the same group as Ca, P, and S if we refer to its abundance in the plant.
• Plants take up Mg in the form of Mg²⁺.
• Mg is a mobile nutrient in the plant.
• Because Mg is readily translocated from old to young plant organs, deficiency symptoms appear in old plant organs.

• A typical symptom of Mg deficiency is chlorosis between the veins of old leaves, longitudinal internervular chlorosis, the veins remain green and between them the mesophyll turns yellow or may have a marbled appearance.
• If the deficiency is severe, the leaf tissues acquire a uniform yellow color, then brown and necrose.
• The leaves are small and break easily, they are fragile.
• The branches break easily and the leaves fall early.
• The variety of symptom manifestation differs by species.

• It is rarely found on saline soils with a high content in the salts of this element, in this context it can be considered as toxic as Na and much more toxic than Ca, due to the Ca/Mg cationic antagonism phenomenon.
• Magnesium toxicity can be prevented by applying calcium supplements.

• It is part of the essential amino acids cysteine, cystine, methionine.
• It is essential in protein synthesis.
• S is involved in the formation of chlorophyll and in the activation of some enzymes.
• It is part of vitamins such as biotin and thiamine (B1) and is necessary in the formation of some oils in mustard plants, some hydrogen sulphide bonds existing in onions and in various oils.

• Sulfur is needed by plants in quantities comparable to those of P.
• The normal total amount in plant tissue is 0.12-0.35% and the ratio of N_total/S_total is around 15.
• Plant roots absorb S in the form of sulfate ion (SO₄^2-)
• The plant absorbs S from the atmosphere as SO₂ in very low concentrations.
• S circulates in the plant as sulfate anion (SO₄^2-), the mobility of S is low and it cannot be translocated when it is present in structural compounds.
• There is mobility and translocation of S to young leaves only when it is found as sulfate.

• In many cases the manifestations of deficiency symptoms in S resemble those in N.
• Unlike the symptoms of N deficiency, in S the manifestation appears on the young leaves of the upper part of the plant, reached maturity and remain present even after the application of N fertilizers.
• The color of the limb and veins becomes yellowish-green, the yellow color is not as pronounced as in the case of N deficiency.
• The veins of the leaves, especially in the upper part, sometimes acquire a lighter color than the neighboring tissues.
• S-deficient plants are small and spindly with short and brittle stems.
• Growth is retarded and grain maturity is delayed.
• Sulfur deficiency affects the accessibility of molybdenum, an essential element in biological nitrogen fixation.
• The number of nodules in legumes is low and implicitly atmospheric N fixation is reduced.
• The fruits do not reach maturity and remain light green.
• Oil formation in seeds is reduced in S deficiency and production decreases.
• Sulfur deficiency leads to a decrease in grain amino acids.

• Excess S can cause large amounts of hydrogen sulfide (H₂S) under strongly reducing conditions.
• Plants are sensitive to high concentrations of SO₂ in the atmosphere, thus normal SO₂ concentrations are considered, values between 0.1-0.2 mg SO₂/m³, toxic effects occur at values exceeding the concentration of 0.6 mg SO₂/m³.
• Symptoms of toxicity in S are manifested by necrotic spots on the leaves, which then spread over the entire surface of the leaf blade.

• It has a role in the synthesis of chlorophyll, carbohydrates, cell respiration, chemical reduction of nitrate and sulfate, and N assimilation.
• Fe has a role in the synthesis of auxins, which in case of iron deficiency are no longer formed, which has the effect of slowing down or stopping root growth.

• It is absorbed by plant roots in the form of Fe2+ and very little in the form of Fe chelates.
• Absorbed Fe is immobile in the phloem.
• Among the micronutrients, Fe is found in the highest amount, about 100 ppm relative to s.u.

• Appears on young leaves.
• Often the symptoms resemble those of Mn, both Mn and Fe deficiency decrease chlorophyll production.
• Yellowing of the surface between the veins of the leaf caused by a lack of iron is called ferric chlorosis.
• In case of severe deficiency the leaves turn whitish due to loss of chlorophyll.
• Chlorosis is not always caused by a lack of Fe2+, but it can be a deficiency induced by the excess presence of CaCO₃ in the soil, by an imbalance caused by excess N and P.

• The leaves are initially covered with brown spots that over time acquire a uniform brown color.
• Iron excess manifests itself on acid soils and excess moisture where the soluble Fe content can increase from 0.1 to 50-100 ppm in just a few weeks.

• It is known that it is an activator of some enzymes and has the function of catalyst in some reactions.
• It is essential in breaking the water molecule in the process of photosynthesis, involved in the synthesis of proteins and lipids.
• It is important in N metabolism and CO₂ assimilation.

• It is absorbed by the plant in the form of the bivalent cation Mn²⁺.
• Like Fe it is immobilized in the phloem.

• The symptoms of Mn deficiency are similar to those of Fe and Mg in that they lead to chlorosis on the surface between the veins of the leaves, with the difference that in the case of Mn the symptoms are visible on young leaves while in the case of Mg deficiency they appear on the leaves old woman.
• In vegetables, it manifests itself on young leaves by spots between the veins of the leaf similar to Mg deficiency.

• It is manifested by dark brown spots (MnO₂) especially on old leaves.
• Symptoms caused by Mn toxicity are: necrosis of potatoes, necrosis of the bark of fruit trees, especially apple.

• Zn is necessary, directly or indirectly, for the activation of several enzyme systems, it has a protective role for auxins (it activates the synthesis of tryptophan, an intermediate product in obtaining auxin).
• It is involved in protein synthesis, seed formation and ripening.
• Zn is promoter in RNA synthesis.
• Zn participates in ATP formation, and in case of Zn deficiency, inorganic phosphorus accumulates, with poor ATP formation.
• Zn is involved in the reduction of nitrates, in case of Zn deficiency, ribonuclease activity decreases, nitrates, amides, organic acids accumulate in the plant, which cannot be oxidized.

• Zinc is taken up by the plant in the form of a bivalent cation Zn²⁺, absorption considered passive has recently been shown to be active, energy dependent.
• The mobility of Zn is low, it occurs mainly in young plant tissues, however Zn does not bind as stable ligands in the xylem liquid as it did with Cu and Fe.

• Stopping the growth of the plant.
• Discoloration to light green.
• Yellowing followed by bleaching.
• In the case of fruit trees, branching is compromised, small leaves being another symptom of Zn deficiency
• Internodes are short.
• Flowering, fruiting and ripening may be delayed.
• Branching being hampered, the leaves fall prematurely.
• Deficiencies appear as chlorosis in the intervening areas of new leaves producing a banded appearance.
• The edges of the leaves are often distorted and wrinkled.
• Zn is blocked by a high pH.
• It can also produce "rosetting", the bushy appearance of the plants, the stem has a low growth rate so that the terminal leaves appear piled up.

• Excess Zn is highly toxic and will cause rapid death.
• It is manifested by slowing down the growth of the root system.
• It is considered toxic when a content of 200 ppm Zn is exceeded in the tissue.

• It has a role in plant metabolism.
• It is involved in the formation of chlorophyll.
• 70% of total copper in leaves is located in complex proteins in chloroplasts, showing that it is required in photosynthesis.
• It is a component of some metalloproteins.
• Constituent of some enzymes such as cytochromoxidase.
• It has a role in oxidation-reduction processes.
• It participates in the formation of lignin, in metabolism.
• It is necessary in the symbiotic fixation of N.

• Copper is absorbed by the plant in the form of Cu²⁺.
• The mobility of Cu in the plant is low and highly depends on the form in which it is found in the plant.

• They are visible by discoloration, twisting and bleaching of the leaf tips.
• In fruit trees, the so-called exanthema disease is known, manifested by rough bark, bleached leaves and poor fruiting.
• In peas, yellowing of the surface between the veins of the leaf occurs.
• In citrus, the leaves get stained, affecting the young branches.

• Cu excess induces Fe deficiency and implicitly ferric chlorosis as a manifestation of Fe deficiency.

• The role of B is to achieve the integrity of the cell membrane and the development of cell walls, which affect permeability, cell division.
• B is one of the vital micronutrients in the formation and development of fruits and seeds.
• Certain functions of boron in plants are similar to those of N, P, K, Ca and Zn. B has a role in N assimilation.
• Boron influences calcium absorption and its nutritional balance in plants.
• It is vital for areas in plants with intensive growth, such as root tips, newly emerging leaves and in bud development.
• It improves the transport of sugars from leaves to fruits and tubers.
• It is essential in providing the sugars necessary for root growth and for the normal development of legume nodules.
• It helps increase flower production, elongation and germination of pollen tubes.
• It is known for its antifungal properties, with studies showing that boron-treated sugar beets are much less susceptible to Sclerotium rolfsii infection than untreated plants.

• Boron is absorbed by plants in the form of undissociated boric acid (H₃BO₃) or as the borate ion H₂BO₃^-.
• The largest amount of B is taken up with water by the roots.
• B is absorbed very quickly and if it is in excess it accumulates in terminal buds and young growing parts.
• Drought limits boron availability by reducing soil water transport, the primary means by which boron is transported to the roots.
• B deficiency usually occurs in young roots, shoots and young leaves.
• In some species, boron deficiency levels can be 3-4 times higher in young leaves than in old ones.

• They appear late in the vegetation, when they can no longer be corrected, the consequence being the compromise of a good part of the harvest.
• It occurs when the boron level in the leaves is below 20 ppm.
• Boron deficiency causes the destruction of the terminal apex of the main stem, which leads to the development of secondary shoots.
• In sugar beet the deficiency symptoms appear late in the season, at the base of the bunch, the petiole of the leaves in the rosette turns brown, the browning advances in the root, attacking the tissue and producing the so-called heart rot of the sugar beet. These roots are low in sucrose.
• It causes gray rot in cauliflower.
• Deficiencies occur mainly in fruit trees during fruiting (brown spotting of apricots), but can also affect young branches that dry out.

• They occur against the background of excessive applications of B fertilizers, in arid or semi-arid areas, where the irrigation water has a high B content (greater than 1-2 ppmB).
• Yellowing of the leaf tip, followed by gradual necrosis of the tips and margins extending to the main vein.
• The leaves dry out and may fall early.

• Mo is involved in some enzyme systems, particularly in the activation of nitrate reductase, where it is required for nitrate reduction and nitrogenase involved in biological N fixation.
• Mo is directly involved in protein synthesis and N fixation by legumes.

• Molybdenum is absorbed in the form of anion MoO₄^2-.
• Mo mobility is considered to be moderate, further suggested by the relatively high seed content and the appearance of deficiency symptoms on mature and old leaves.

• In vegetables it can be associated with N deficiency due to its role in N fixation.
• May cause burns and twisting or rolling of leaf edges.
• Yellowing and stunting of plants.
• The appearance of yellow spots on citrus fruits.
• Chlorosis and then bleaching of the leaf edges in young cauliflower plants, the so-called whiptail, manifested especially on acid soils with a pH below 5.5.

• A content greater than 5 ppm Mo in s.u. in feed it is considered toxic for animal feed being associated with the production of the disease called molybdenum poisoning.

• It is essential for N fixation by organisms.
• It is part of vitamin B₁₂.

• Co is absorbed as the divalent cation Co²⁺

• Deficiency prevents symbiotic and non-symbiotic nitrogen fixation.
• Contents between 20 and 40 ppb are considered for vegetables to be the limit of occurrence of deficiency symptoms in Co.
• Contents below 5 ppm in feed cause acobaltosis in animals.

• In case of excess, brown spots and necrosis appear on the leaves.

• Improving the drought resistance of crops.

Silicon is widely recognized as a beneficial element for plants. Silicon deposition in plant tissues has been linked to increased resistance to bacterial and fungal infections as well as increased tolerance to abiotic stresses.

Silicon is not yet considered an essential plant element, but a high concentration of Si in leaf tissues can have beneficial effects:

• better resistance to abiotic and biotic stresses;
• improving the ability of plants to intercept light;
• mitigation of leaf evaporation/transpiration losses.

Key mechanisms involved in silicon-triggered drought stress in crops include:

• activation of antioxidant systems;
• stimulation of gene expression and defense responses;
• adjusting osmotic processes and maintaining homeostasis;
• increases in nutrient absorption;
• regulation of photosynthesis and gas exchange;
• improving plant growth and water absorption.