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Mineral Nutrition Of Plants

A. Essential mineral elements.

A variety of mineral elements is present in the soil but all of them are not essential for plants growth. Besides, a particular element may be needed for the growth of one plant and may not be required at all by other plants. An essential element is defined as 'one without which the plant cannot complete its life cycle, or one that has a clear physiological role'. Therefore, in 1939 Arnon and Stout proposed the following characters for judging the criteria of essentiality of an element in the plant :

The element must be essential for normal growth and reproduction, which cannot proceed without it.

The requirement of the element must be specific and cannot be replaced by another element.

 The requirement must be direct that is, not the result of any indirect effecte.g. for relieving toxicity caused by some other substance.      

Essential elements are divided into two broad categories, based on the quantity in which they are required by plants. Macro-elements and micro-elements. Their ionic forms are respectively called macronutrients and micronutrients. Cations may be absorbed on the surface of negatively charged clay particles. Anions (e.g., nitrate, phosphate, chloride, sulphate, borate) are held to soil particles to a lesser extent. Mineral salts dissolved in soil solution are constantly passing downwards along with percolating (gravitational) water. The phenomenon is called leaching. Leaching is more in case of anions.

(1) Macronutrients (Macroelements or major elements) : Which are required by plants in larger amounts (Generally present in the plant tissues in concentrations of 1 to 10mg per gram of dry matter). The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium, magnesium.

(2) Micronutrients (Microelements or minor elements or trace elements) : Which are required by plants in very small amounts,i.e., in traces (equal to or less than 0.1mg per gram dry matter). These include iron, maganese, copper, molybdenum, zinc, boron and chlorine. Recent research has shown that some elements, such as cobalt, vanadium and nickel, may be essential for certain plants.

The usual concentration of essential elements in higher plants according to D.W. Rains (1976) based on the data of Stout are as follows :

    Element

% of dry weight

    Carbon

45

    Oxygen

45

    Hydrogen

 6

    Nitrogen

1.5

    Potassium

1.0

    Calcium

0.5

    Magnesium

0.2

    Phosphorus

0.2

    Sulphur

0.1

    Chlorine

0.01

    Iron

0.01

    Manganese

0.005

    Boron

0.002

    Zinc

0.002

    Copper

0.0001

    Molybdenum

0.0001

B. Plant analysis

(1) Ash analysis : This is the simplest method. The plant tissue is subjected to a very high temperature    (550-600°C) in an electric muffle furnace and is reduced to ash. The organic matter of the plant is completely oxidised. All carbon, hydrogen and oxygen molecules in the tissue are converted into carbon dioxide and water, both of which escape into the atmosphere as vapours. Besides some nitrogen is also lost as nitrogen gas and ammonia. The plant ash left behind forms a very small proportion of plants dry weight ranging from 2 to 10% only. Analysis of plant ash shows that about 92 mineral elements are present in different plants. Out of these, 30 elements are present in each and every plants and rest are in one or other plants. Out of these 30 elements, 16 elements are necessary for plants and are called essential elements. The ash is chemically analysed to determine these elements.

(2) Solution culture (Hydroponics) : In this method plants are grown in nutrient solutions containing only desired elements. To determine the essentiality of an element for a particular plant, it is grown in a nutrient medium that lacks or is deficient in this element.

If the plant grows normally, it indicates that the element is not essential. However, if the plant shows deficiency symptoms then it indicates that the element is essential for that particular plant.

The growing of plants with their roots in dilute solutions of mineral salts instead of soil led to increased understanding of plant nutrition. This cultivation of plants by placing the roots in nutrient solution is called hydroponics. Probably the first recorded use of soilless culture was by Woodward in 1699. In early nineteenth century, plants were grown with their roots immersed in water solutions with inorganic salts alone, without the addition of soil or organic matter. By 1860, the culture solution technique was modernized by Sachs and he showed the essentiality of nitrogen for plant growth. Another significant worker for studying the essentiality of elements was Knop (1865). The method of growing plants in aqueous nutrient solutions as employed by Sachs and Knop is used experimentally and commercially today and known as hydroponic culture. The nutrient solution composition proposed by Knop (1865) and Arnon and Hoagland's (1940) are commonly used. Arnon and Hoagland's nutrient medium has the advantage, that it contains micro-nutrients also. Iron was added in the form of ferrous sulphate which often precipitated out. Now a days a chelating agentNa2-EDTA (Disodium salt of ethylene diamine tetra acetic acid. EDTA is a buffer which is used in tissue cultures) is added.

Hydroponics or soilless culture helps in knowing

(i) The essentiality of mineral element.

(ii) The deficiency symptoms developed due to non-availability of particular nutrient.

(iii) Toxicity to plant when element is present in excess.

(iv) Possible interaction among different elements present in plant.

(v) The role of essential element in the metabolism of plant.

(3) Solid medium culture : In this method either sand or crushed quartz is used as a rooting medium and nutrient solution is added to it. The nutrient medium is provided by one of the following methods :

(i) Drip culture : It is done by dripping over the surface.

(ii) Slop culture : It is done by having the medium over the surface.

(iii) Sub-irrigation : Here the solution is forced up from the bottom of the container.

C. Major role of nutrients.

Various elements perform the following major role in the plants :

(1) Construction of the plant body :

The elements particularly C, H and O construct the plant body by entering into the constitution of cell wall and protoplasm. They are, therefore, referred to as frame work elements. Besides, these (C, H and O) N, P and S also enter in the constitution of protoplasm. They are described as protoplasmic elements.

(2) Maintenance of osmotic pressure : Various minerals present in the cell sap in organic or inorganic form maintain the osmotic pressure of the cell.

(3) Maintenance of permeability of cytomembranes : The minerals, particularlyCa++,K+ andNa+ maintain the permeability of cytomembranes.

(4) Influence the pH of the cell sap : Different cations and anions influence on thepH of the cell sap.

(5) Catalysis of biochemical reaction : Several elements particularlyFe, Ca, Mg, Mn, Zn, Cu, Cl act as metallic catalyst in biochemical reactions.

(6) Toxic effects : Minerals likeCu, As, etc. impart toxic effect on the protoplasm under specific conditions.

(7) Balancing function : Some minerals or their salts act against the harmful effect of the other nutrients, thus balancing each other.

D. Specific role of macronutrients.

The role of different elements is described below :

(1) Carbon, hydrogen and oxygen :

These three elements, though can not be categorised as mineral elements, are indispensible for plant growth. These are also called 'framework elements'. Carbon, hydrogen and oxygen together constitute about 94% of the total dry weight of the plant. Carbon is obtained from the carbon dioxide present in the atmosphere. It is essential for carbohydrate and fat synthesis. Hydrogen and oxygen would be obtained from water which is absorbed by the plants from the soil. Some amount of oxygen is also absorbed from the atmosphere.

(2) Nitrogen

(i) Source : The chief source of nitrogen for green plants is the soil. It is absorbed mainly in the form of nitrate ions . The major sources of nitrate for the plants are sodium nitrate, potassium nitrate, ammonium nitrate and calcium nitrate. Under suitable conditions, ammonium ions may substitute for nitrate ions, being easily absorbed by plants. Ordinary green plants cannot utilize elemental nitrogen which constitutes about 79% of the air. It is also trapped by nitrogen fixing bacteria which live symbiotically in root nodules of the plants.

(ii) Functions : Nitrogen is an essential constituent of proteins, nucleic acids, vitamins and many other organic molecules as chlorophyll. Nitrogen is also present in various hormones, coenzymes and ATP etc. It plays an important role in protein synthesis, respiration, growth and in almost all metabolic reactions.

(iii) Deficiency symptoms : The symptoms of nitrogen deficiency are as follows :

(a) Impaired growth

(b) Yellowing of leaves due to loss of chlorophyll,i.e., chlorosis.

(c) Development of anthocyanin pigmentation in veins, sometimes in petioles and stems.

(d) Delayed or complete suppression of flowering and fruiting.

Excessive supply of nitrogen produces following symptoms :

(a) Increased formation of dark green leaves.

(b) Poor development of root system.

(c) Delayed flowering and seed formation.

(3) Phosphorus

(i) Source : Phosphorus is present in the soil in two general forms, organic and inorganic. Plants do not absorb organic phosphorus, either from the solid or solution phase of soil. However, organic compounds are decomposed and phosphorus is made available to plants in inorganic form. Soil solution contains phosphorus in inorganic forms as the phosphate ions and . WhenpH is low phosphate ions are present in the form of . WhenpH is high, phosphate ions are represented in .

(ii) Functions

(a) Phosphorous is present abundantly in the growing and storage organs such as fruits and seeds. It promotes healthy root growth and fruit ripening by helping translocation of carbohydrates.

(b) It is present in plasma membrane, nucleic acid, nucleotides, many coenzymes and organic molecules as ATP.

(c) Phosphorus plays an indispensable role in energy metabolismi.e., hydrolysis of pyrophosphate and various organic phosphate bonds being used to drive chemical reactions. Thus it is required for all phosphorylation reactions.

(iii) Deficiency symptoms

(a) Leaves become dark green or purplish.

(b) Sometimes development of anthocyanin pigmentation occurs in veins which may become necrotic (Necrosis is defined as localised death of cells).

(c) Premature fall of leaves.

(d) Decreased cambial activity resulting in poor development of vascular bundles.

(e) Root and shoot growth is checked.

(f) Prolonged dormancy.

(g) Sickle-leaf disease.

(4) Sulphur

(i) Source : Sulphur is present as sulphate  in mineral fraction of soil. It is also found inFeS andFeS2 forms, which are not available to plants. In industrialized areas, atmospheric sulphur dioxide and sulphur trioxide in low concentration) may be important sources of sulphur nutrition.

(ii) Functions

(a) Sulphur is a constituent of amino-acids like cystine, cysteine and methionine; vitamins like biotin and thiamine, and coenzyme A.

(b) It increases the nodule formation in the roots of leguminous plants. It favours soluble organic nitrogen and there is decrease in the quantity of soluble nitrogen with its increase.

(c) The characteristic smell of mustard, onion and garlic is due to the presence of sulphur in their volatile oils.

(d) Sulphur in plants is required in stem and root tips and young leaves. It is remobilised during senescence.

(iii) Deficiency symptoms

(a) Leaves remain small and turn pale greeni.e., symptoms of chlorosis. Chlorosis affects young leaves more because of immobile property of the sulphur. The young leaves develop orange, red or purple pigment.

(b) Leaf tips and margins roll downwards and inwardse.g., tobacco, tea and tomato.

(c) Premature leaf fall.

(d) Delayed flowering and fruiting.

(e) Apical growth is retarded whereas premature development of lateral buds starts.

(f) The tea yellow disease is caused in tea plants.

(g) Decrease in stroma lamellae and increase in grana stacking.

(h) Increase in starch and sucrose accumulation, and decrease in reducing sugars.

(5) Potassium

(i) Source : Source of K+ to the plants is inorganic compounds like potassium sulphate, potassium nitrate, etc. Potassium is usually present in sufficient amount in clay soils, where it is firmly bound (largely as an exchangeable base). It is prevalent cation in plants and may be involved in the maintenance of ionic balance in cells. It contains approximately 0.3 to 6.0 percent of whole plant. In seeds, it is found in less amount.

(ii) Functions

(a) It differs from all other macronutrients in not being a constituent of any metabolically important compound.

(b) It is the only monovalent cation essential for the plants.

(c) It acts as an activator of several enzymes including DNA polymerase.

(d) It is essential for the translocation of photosynthates, opening and closing of stomata, phosphorylation, synthesis of nucleic acid and chlorophyll.

It takes part in the formation of cell membrane and it is also responsible for maintenance of turgidity of cells. It is considered that whole of potassium in plant is found in soluble form and most of it is contained in cell sap and cytoplasm.

(iii) Deficiency symptoms

(a) Mottled chlorosis followed by the development of necrotic areas at the tips and margins of the leaves.

(b) K+ deficiency inhibits proteins synthesis and photosynthesis. At the same time, it increases the rate of respiration.

(c) The internodes become shorter and root system is adversely affected.

(d) The colour of leaves may turn bluish green.

(e) Widespread blackening or scorching of leaves may occur as a result of increased tyrosinase activity.

(f) Rosette or bushy habit of growth may be seen in plants.

(g) Reduction of stem growth, weakening of stem.

(h) Lowered resistance to pathogens.

Destruction of pith cells of tomato and increased differentiation of phloem elements.

(6) Calcium

(i) Source : The element is abundant in most soils and plants under natural conditions are seldom deficient in it. It is absorbed by the plants in the form of  from calcium carbonate etc. It occurs abundantly in a non-exchangeable form such as anorthite . Much of the exchangeable calcium of the soil is absorbed onto the surface of clay micelle.

(ii) Functions

(a) It is necessary for formation of middle lamella of plants where it occurs as calcium pectate.

(b) It is necessary for the growth of apical meristem and root hair formation.

(c) It acts as activator of several enzymes,e.g., ATPase, succinic dehydrogenase, adenylate kinase, etc.

(d) Along withNa+ andK+ it maintains the permeability of plasma membrane.

(e) It is involved in the organisation of spindle fibres during mitosis.

(f) It antagonises the toxic effects ofNa+ andMg++.

It is essential for fat metabolism, carbohydrate metabolism, nitrate assimilation and binding of nucleic acids with proteins.

(iii) Deficiency symptoms

(a) Ultimate death of meristems which are found in shoot, leaf and root tips.

(b) Chlorosis along the margins of young leaves, later on they become necrotic.

(c) Distortion in leaf shape.

(d) Roots poorly developed or may become gelatinous.

(e) Young leaves show malformation and leaf tips becomes hooked.

(f) Its deficiency checks flowering and causes the flowers to fall early.

(7) Magnesium

(i) Source : Magnesium occurs in the soil in the form of magnesite (MgCO3), dolomite (MgCO3,CaCO3), magnesium sulphate (MgSO4) and as silicates. It is absorbed from the soil in the form of (Exchangeable cation) ions (Mg++). It is easily leached and thus become deficient in sandy soils during rainy season.

(ii) Functions  

(a) It is an important constituent of chlorophyll.

(b) It is present in the middle lamella in the form of magnesium pectate.

(c) It plays an important role in the metabolism of carbohydrates, lipids and phosphorus.

(d) It acts as activator of several enzymes.

(e) It is required for binding the larger and smaller subunits of ribosomes during protein synthesis.

(f) It is readily mobile and when its deficiency occurs, it is apparently transferred from older to younger tissues, where it can be neutralised in growth processes.

(iii) Deficiency symptoms

(a) Interveinal chlorosis followed by anthocyanin pigmentation, eventually necrotic spots appear on the leaves. As magnesium is easily transported within the plant body, the deficiency symptoms first appear in the mature leaves followed by the younger leaves at a later stage.

(b) Stems become hard and woody, and turn yellowish green.

(c) Depression of internal phloem and extensive development of chlorenchyma.

E.  Specific role of micronutrients

(1) Iron

(i) Source : It is present in the form of oxides in the soil. It is absorbed by the plants in ferric as well as ferrous state but metabolically it is active in ferrous state. Its requirement is intermediate between macro and micro-nutrients. Therefore, sometimes it is also considered as a macronutrients.

(ii) Functions : (a) Iron is a structural component of ferredoxin, flavoproteins, iron prophyrin proteins (Cytochromes, peroxidases, catalases, etc.)

(b) It plays important roles in energy conversion reactions of photosynthesis (phosphorylation) and respiration.

(c) It acts as activator of nitrate reductase and aconitase.

(d) Although iron is not a component of the chlorophyll molecules, it is essential for the synthesis of chlorophyll.

(iii) Deficiency symptoms

(a) Chlorosis particularly in younger leaves, the mature leaves remain unaffected. (b) It inhibits chloroplast formation due to inhibition of protein synthesis. (c) Stalks remain short and slender. (d) Extensive interveinal white chlorosis in leaves. (e) It may develop necrosis aerobic respiration severely affected. (f) In extreme deficiency scorching of leaf margins and tips may occur.

(2) Manganese

(i) Source : Like iron, the oxide forms of manganese are common in soil. However, manganese dioxide (highly oxidised form) is not easily available to plants. It is absorbed from the soil in bivalent form (Mn++). Increased acidity leads to increase in solubility of manganese. In strong acidic soils, manganese may be present in toxic concentrations. Oxidising bacteria in soils render manganese unavailable to plants atpH ranging from 6.5 to 7.8.

(ii) Functions

(a) It acts as activator of enzymes of respiration (malic dehydrogenase and oxalosuccinic decarboxylase) and nitrogen metabolism (nitrite reductase).

(b) It is essential for the synthesis of chlorophyll.

(c) It is required in photosynthesis during photolysis of water.

(d) It decreases the solubility of iron by oxidation. Hence, abundance of manganese can lead to iron deficiency in plants.

(iii) Deficiency symptoms : (a) Chlorosis (interveinal) and necrosis of leaves. (b) Chloroplasts lose chlorophyll, turn yellow green, vacuolated and finally perish. (c) Root system is poorly developed. (d) Formation of grains is badly affected.

(e) 'Grey spot disease' in oat appears due to the deficiency of manganese, which leads to total failure of crop.

(f) 'Marsh spot's in seeds of pea. (g) Deficiency symptoms develop in older leaves.

(3) Copper

(i) Source : Copper occurs in almost every type of soil in the form of complex organic compounds. A very small amount of copper is found dissolved in the soil solution. The bivalent copper cationCu2+ is available in plants in exchangeable forms. It is found in natural deposits of chalcopyrite (CuFeS2).

(ii) Functions  

(a) It activates many enzymes and is a component of phenolases, ascorbic acid oxidase, tyrosinase, cytochrome oxidase.

(b) Copper is a constituent of plastocyanin, hence plays a role in photo-phosphorylation.

(c) It also maintains carbohydrate nitrogen balance.

(iii) Deficiency symptoms

(a) Both vegetative and reproductive growth are reduced.

(b) The most common symptoms of copper deficiency include a disease of fruit trees called 'exanthema' in which trees start yielding gums on bark and 'reclamation of crop plants', found in cereals and legumes.

(c) It also causes necrosis of the tip of the young leaves (e.g., Citrus). The disease is called 'die back'.

(d) Carbon dioxide absorption is decreased in copper deficient trees.

(e) Wilting of entire plant occurs under acute shortage.

(f) Grain formation is more severely restricted than vegetative growth.

(4) Molybdenum

(i) Source : Molybdenum occurs in the soil in three forms – dissolved, exchangeable and nonexchangeable forms. It is available to the plants mostly as molybdate ions. It is required in extremely small quantities by plants. It is found relatively in higher concentration in mineral oil and coal ashes.

(ii) Functions

(a) Its most important function is in nitrogen fixation because it is an activator of nitrate reductase.

(b) It is required for the synthesis of ascorbic acid.

(c) It acts as activator of some dehydrogenases and phosphatases.

Deficiency symptoms

(a) Mottled chlorosis is caused in the older leaves as in nitrogen deficiency, but unlike nitrogen-deficient plants, the cotyledons stay healthy and green.

(b) It is also known to inhibit flowering, if they develop, they fall before fruit setting.

(c) It leads to drop in concentration of ascorbic acid.

(d) Its deficiency causes 'whiptail disease' in cauliflower and cabbage. The leaves first show an interveinal mottling and the leaf margins may become gray and flaccid and finally brown.

(5) Zinc

(i) Source : Zinc occurs in the soil in the form of ferromagnesian minerals like magnetite, biotite and hornblende. When weathering of these minerals takes place, zinc is liberated in bivalentZn2+ form. Increase in soilpH decreases the availability of zinc.

Bivalent form of zinc (Zn++) is exchangeable and is readily available in the soil. Plants require this mineral only in traces and its higher concentrations are highly toxic.

(ii) Functions : (a) It is required for the synthesis of tryptophan which is a precursor of indole acetic acid-an auxin.

(b) It is a constituent of enzymes like carbonic anhydrase, hexokinase, alcohol dehydroge-nase, lactic dehydrogenase and carboxypeptidase.

(c) It is required for metabolism of phosphorus and carbohydrates.

(d) Zinc also appears to play an important role in protein synthesis because in its absence there is substantial increase in soluble nitrogenous compounds.

(iii) Deficiency symptoms : (a) The first symptom appears in the form of interveinal chlorosis of the older leaves, starting at the tips and the margins.

(b) Growth becomes stunted due to formation of smaller leaves and shortened internodes. Reduced stem growth is due to less synthesis of auxin.

(c) The leaves become distorted and sickle shaped and get clustered to form rosettes. This effect is known as 'little leaf disease'.

(d) In maize, zinc deficiency produces 'white bud disease' which leads to greatly reduced flowering and fruiting as well as poorly differentiated root growth.

(e) Its deficiency causes khaira disease of rice and mottled leaf of apple, Citrus and walnut.

(6) Boron

(i) Source : Boron is present in the soil in very small amounts. It appears in exchangeable soluble and nonexchangeable forms in the soil or . It occurs in highly complex forms such as borosilicates, boric acids and calcium and manganese borates. It is absorbed from the soil as boric acid and tetraborate anions.  Its calcium and magnesium salts are soluble. Its availability to plant decreases with increase in pH.

(ii) Functions

(a) It facilitates the translocation of sugars.

(b) It is involved in the formation of pectin.

(c) It is also required for flowering, fruiting, photosynthesis and nitrogen metabolism.

(d) Boron is required for uptake and utilisation of Ca2+, pollen germination, seed germination and cell differentiation.

(e) It regulates cellular differentiation and development.

(iii) Deficiency symptoms

(a) The first major symptom of boron deficiency is the death of shoot tip because boron is needed for DNA synthesis.

(b) Generally flowers are not formed and the root growth is stunted.

(c) The leaves develop a thick coppery texture, they curve and become brittle.

(d) Some of the physiological diseases caused due to boron deficiency are internal cork of apple, top rot of tobacco, cracked stem of celery, browning of cauliflower water core of turnip, hard fruit of Citrus and heart rot of sugar beets and marigold. These diseases can be cured by application of small doses of sodium tetraborate in the soil.

(e) Fruits when affected are severely deformed and useless.

(f) Its deficiency checks the cells division of cambium but continues cell elongation.

(7) Chlorine

(i) Source : It is absorbed from the soil as chloride ions. It is required in very small amounts and almost all types of soils contain enough chlorine for the plants. Hence, it is rarely supplied as fertilizer.

(ii) Functions

(a) It is required for photolysis of water during photosynthesis in photosystem-II.

(b) In tobacco, it increases water volume inside the cell and also regulates carbohydrate metabolism.

(c) With Na+ andK+, chlorine helps in determining solute concentration and anion cation balance in the cells.

(d) It is essential for oxygen evolution in photosynthesis.

(iii) Deficiency symptoms : (a) The deficiency symptoms of chlorine consist of wilted leaves which later become chlorotic and finally attain a bronze colour.

(b) Roots become stunted or thickened and club shaped and fruiting is reduced.

(c) Photosynthesis is also inhibited.

G. ACTIVE ABSORPTION

  ” Generally, the lipid-protein membrane of a cell is largely permeable to free ions.

❒  ”The energy is considered to be involved in the transport of such free ions across the membrane. The absorption of ions involving use of metabolic energy is called active absorption.

” Energy used in these mechanisms comes from metabolic activities especially respiration.

”Carrier concept: This theory of active absorption suggests involvement of definite chemical compound, called carrier, present in the membrane. The membrane does not allow the ions to pass through as it is. The ions form a complex with the carrier called carrier-ion complex, which is capable of moving across the membrane. The ions are released on the inner side of the membrane.

❒  ”The carriers are specific i.e., they combine with a particular type of ion. Carriers are probably proteinaceous in nature because addition of protein degrading substances in the system also reduces the rate of adsorption.

”Anion respiration hypothesis: This theory was proposed by Lundegardh (1954). According to this explanation only anions are absorbed actively, i.e., anion uptake requires energy, (i.e., they are absorbed actively).

”At the outer surface of the membrane, the cytochrome undergoes oxidation and loses one electron and in exchange picks up an anion.

"This is then transported to the inner side of the membrane through the cytochrome chain and on the inner surface of the membrane, the anion is released and the cytochrome gets reduced by the action of dehydrogenase involved in respiration.

”The cations move passively along the electrical gradient created by the accumulation of anions at the inner surface of the membrane.

”The evidence in favour of Lundegardh’s hypothesis is that the respiration is increased when a plant is transferred from water to salt solution. The increased respiration was calledsalt respiration oranion respiration.

H. NITROGEN NUTRITION IN PLANTS

”Nitrogen is one of the essential and important mineral elements for plants. It is found in such essential compounds, such as, proteins, nucleic acids, chlorophyll, alkaloids, growth regulators and many vitamins.

”This element alone constitutes about 5-30% of total dry weight of the plant.

” Most of the free nitrogen is found in atmosphere, yet it is energetically difficult for living organisms to obtain it directly for their use. Therefore, the molecular or elemental nitrogen has to be fixed i.e., combined with other elements such as C, H, O, etc. to form compounds.

”The process of conversion of free nitrogen into nitrogenous compounds to make it available for plant absorption is called nitrogen fixation.

”Higher plants absorb nitrogen only in the combined form from the soil. Thus plants get their nitrogen supply only from the soil.

  ”Plants utilize the following four types of nitrogenous compounds:

Nitrates (NO3−)

  Nitrites (NO2−)

Ammonium (NH4+) salts and

”Organic nitrogenous compounds (e.g., Urea).

”Out of these four compounds nitrates and ammonium ions are the most effective ones.

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