Toxicity of Micronutrients
(i) Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered toxic. Such critical concentrations vary widely among different micronutrients.
(ii) The toxicity symptoms are difficult to identify. Toxicity levels for any element also vary for different plants. Many a times, excess of an element may inhibit the uptake of another element.
(iii) For example, the prominent symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins. It is important to know that manganese competes with iron and magnesium for uptake and with magnesium for binding with enzymes. Manganese also inhibit calcium translocation in shoot apex. Therefore, excess of manganese may, in fact, induce deficiencies of iron, magnesium and calcium. Thus, what appears as symptoms of manganese toxicity may actually be the deficiency symptoms of iron, magnesium and calcium.

Nitrogen Cycle
(i) Plants compete with microbes for the limited nitrogen that is available in soil. Thus, nitrogen is a limiting nutrient for both natural and agricultural ecosystems.
(ii) In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O).
(iii) Industrial combustions, forest fires, automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides.
(iv) Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification. Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria in the following steps:
2NH₃+3O₂⟶2NO₂+2H⁺+2H₂O
2NH₃+ 3O₂ ⟶ 2NO₂

Macro- and Micro-nutrients and their roles
1. Nitrogen. This is the mineral element required by plants in the greatest amount. It is absorbed mainly as NO₃^- though some are also taken up as NO₂ or NH₄⁺. Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells. Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins and hormones.
2. Phosphorus. Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either as H₂POor HPO). Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.
3. Potassium. It is absorbed as potassium ion (K⁺).In plants, this is required in more abundant quantities in the meristematic tissues, buds, leaves and root tips. Potassium helps to maintain an anion-cation balance in cells and is involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.
4. Calcium. Plant absorbs calcium from the soil in the form of calcium ions (Ca²⁺). Calcium is required by meristematic and differentiating tissues. During cell division it is used in the synthesis of cell wall, particularly as calcium pectate in the middle lamella. It is also needed during the formation of mitotic spindle. It accumulates in older leaves. It is involved in the normal functioning of the cell membranes. It activates certain enzymes and plays an important role in regulating metabolic activities.
5. Magnesium. It is absorbed by plants in the form of divalent Mg²⁺.It activates the enzymes of respiration, photosynthesis and are involved in the synthesis of DNAand RNA.Magnesium is a constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure.
6. Sulphur. Plants obtain sulphur in the form of sulphate (SO) . Sulphur is present in two amino acids - cysteine and methionine and is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.
7. Iron. Plants obtain iron in the form of ferric ions (Fe³⁺).It is required in larger amounts in comparison to other micronutrients. It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes. It is reversibly oxidised from Fe2+to Fe3+during electron transfer. It activates catalase enzyme, and is essential for the formation of chlorophyll.
8. Manganese. It is absorbed in the form of manganous ions (Mn²⁺).It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism. The best defined function of manganese is in the splitting of water to liberate oxygen during photosynthesis.
9. Zinc. Plants obtain zinc as Zn²⁺ions. It activates various enzymes, especially carboxylases. It is also needed in the synthesis of auxin.
10. Copper. It is absorbed as cupric ions (Cu²⁺).It is essential for the overall metabolism in plants. Like iron, it is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu' to Cu2+.
11. Boron. It is absorbed as BO or B₄O. Boron is required for uptake and utilisation of Ca²⁺, membrane functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.
12. Molybdenum. Plants obtain it in the form of molybdate ions (MoO). It is a component of several enzymes, including nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.
13. Chlorine. It is absorbed in the form of chloride anion (Cl]. Along with Na⁺ and K⁺,it helps in determining the solute concentration and the anioncation balance in cells. It is essential for the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.

Criteria for Essentiality
(a) The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.
(b) The requirement of the element must be specific and not replaceable by another element. In other words, deficiency of anyone element cannot be met by supplying some other element.
(c) The element must be directly involved in the metabolism of the plant. Based upon the above criteria only a few elements have been found to be absolutely essential for plant growth and metabolism. These elements are further divided into two broad categories based on their quantitative requirements.
(i) Macronutrients, and
(ii) Micronutrients
Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole kg^-1 of dry matter). The macro nutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium. Of these, carbon, hydrogen and oxygen are mainly obtained from CO₂and H₂O, while the others are absorbed from the soil as mineral nutrition. Micronutrients or trace elements, are needed in very small amounts (less than 10 mmole kg^-1 of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Steps in the development of root nodules:
(a) Rhizobium bacteria contact a susceptible root hair, divide near it,
(b) Upon successful infection of the root hair cause it to curl,
(c) Infected thread carries the bacteria to the inner cortex. The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation,
(d) A mature nodule is complete with vascular tissues continuous with those of the root.

Plants uptake essential elements from the soil through their roots and from the air through their leaves. Nutrient uptake in the soil is achieved by cation exchange, wherein root hairs pump hydrogen ions (H+)into the soil through proton pumps. These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root. In the leaves, stomata open to take in carbon dioxide and expel oxygen. The carbon dioxide molecules are used as the carbon source in photosynthesis.
Though nitrogen is plentiful in the earth's atmosphere, relatively few plants engage in nitrogen fixation (conversion of atmospheric nitrogen to a biologically useful form). Most plants therefore require nitrogen compounds to be present in the soil in which they grow.

Iron Deficiency
Iron (Fe) deficiency is a plant disorder also known as "lime-induced chlorosis". It can be confused with manganese deficiency. A deficiency in the soil is rare but iron can be unavailable for absorption if soil pH is not between about 5 and 6.5. A common problem is when the soil is too alkaline (the pH is above 6.5). Also, iron deficiency can develop if the soil is too waterlogged or has been overfertilised. Elements like calcium, zinc, manganese, phosphorus, or copper can tie up iron if they are present in high amounts.
Iron is needed to produce chlorophyll, hence its deficiency causes chlorosis. For example, iron is used in the active site of glutamyl-tRNA reductase, an enzyme needed for the formation of 5-Aminolevulinic acid which is a precursor of heme and chlorophyll.
Symptoms. Symptoms include leaves turning yellow or brown in the margins between the veins which may remain green, while young leaves may appear to be bleached. Fruit would be of poor quality and quantity. Any plant may be affected, but raspberries and pears are particularly susceptible, as well as most acid-loving plants such as azaleas and camellias.
Treatment. Iron deficiency can be avoided by choosing appropriate soil for the growing conditions (e.g., avoid growing acid loving plants on lime soils), or by adding well-rotted manure or compost.
Potassium Deficiency
Plants require potassium ions (K+)for protein synthesis and for the opening and closing of stomata, which is regulated by proton pumps to make surrounding guard cells either turgid or flaccid. A deficiency of potassium ions can impair a plant's ability to maintain these processes.
Symptoms. The deficiency most commonly affects fruits and vegetables, notably potatoes, tomatoes, apples, currants, and gooseberries, and typical symptoms are brown scorching and curling of leaf tips, and yellowing of leaf veins. Purple spots may also appear on the leaf undersides.
Deficient plants may be more prone to frost damage and disease, and their symptoms can often be confused with wind scorch or drought.
Prevention and Cure. Prevention and cure can be achieved in the shorter term by feeding with home-made comfrey liquid, adding seaweed meal, composted bracken or other organic potassium-rich fertilisers. In the longer term the soil structure should be improved by adding plenty of well rotted compost or manure. Wood ash has high potassium content, but should be composted first as it is in a highly soluble form.
Calcium Deficiency
Calcium (Ca) deficiency is a plant disorder that can be caused by insufficient calcium in the growing medium, but is more frequently a product of a compromised nutrient mobility system in the plant. This may be due to water shortages, which slow the transportation of calcium to the plant, or can be caused by excessive usage of potassium or nitrogen fertilizers.
Symptoms
Calcium deficiency symptoms appear initially as generally stunted plant growth, necrotic leaf margins on young leaves or curling of the leaves, and eventual death of terminal buds and root tips. Generally the new growth of the plant is affected first. The mature leaves may be affected if the problem persists.
Treatment
Calcium deficiency can be rectified by adding Agricultural lime to acid soils, aiming at a pH of 6.5, unless the plant in question specifically prefers acidic soil. Organic matter should be added to the soil in order to improve its moistureretaining capacity.
Plant damage is difficult to reverse, so take corrective action immediately. Make supplemental applications of calcium nitrate at 200 ppm nitrogen. Test and correct the pH if needed because calcium deficiency is often associated with low pH.
Nitrogen Deficiency 
Nitrogen (N) deficiency in plants can occur when woody material such as sawdust is added to the soil. Soil organisms will utilise any nitrogen in order to break this down, thus making it temporarily unavailable to growing plants. 'Nitrogen robbery' is more likely on light soils and those low in organic matter content, although all soils are susceptible. Cold weather, especially early in the season, can also cause a temporary shortage.
Symptoms. All vegetables apart from nitrogen fixing legumes are prone to this disorder. Symptoms include poor plant growth, leaves are pale green or yellow in the case of brassicas. Lower leaves show symptoms first. Leaves in this state are said to be etiolated with reduced chlorophyll. Floweringand fruiting may be delayed.
Prevention and Control
Prevention and control of nitrogen deficiency can be achieved in the short term by using grass mowings as a mulch, or foliar feeding with manure, and in the longer term by building up levels of organic matter in the soil. Sowing green manure crops such as grazing rye to cover soil over the winter will help to prevent nitrogen leaching, while leguminous green manures such as winter tares will fix additional nitrogen from the atmosphere.
Manganese Deficiency
Manganese (Mn)deficiency is a plant disorder that is often confused with, and occurs with, iron deficiency. Most common in poorly drained soils, also where organic matter levels are high. Manganese may be unavailable to plants where pH is high.
Symptoms
Affected plants include onion, apple, peas, French beans, cherry and raspberry, and symptoms include yellowing of leaves with smallest leaf veins remaining green to produce a 'chequered' effect. The plant may seem to grow away from the problem so that younger leaves may appear to be unaffected. Brown spots may appear on leaf surfaces, and severely affected leaves turn brown and wither.
Prevention. Prevention can be achieved by improving soil structure. Do not over-lime.

The technique of growing plants in a nutrient solution is known as hydroponics.
This method requires purified water and mineral nutrient salts. After a series of experiments, in which the roots of the plants were immersed in nutrient solutions and an element was added/ removed or given in varied concentrations, a mineral solution suitable for the plant growth was obtained.
By this method, essential elements were identified and their deficiency symptoms discovered. Hydroponics has been successfully employed as a technique for the commercial production of vegetables such as tomato, seedless cucumber and lettuce.
Yet, most of the crops are still grown on land because the nutrient solutions must be adequately aerated to obtain the optimum growth in hydroponis. Moreover, the minerals must be continuously added in the solution. No, such activity is required in the soil.

(i) Macronutrients These are generally present in plant tissues in large amount(in excess 10m mole Kg^-1 of dry matter).The macronutrients include carbon,hydrogen, oxygen, nitrogen,phosphorous,sulphur,potassium,calcium and magnesium.
(ii) Micronutrients Micronutrients or trace elements, are needed in very small amount (less than 10m mole g^-1 of dry matter).These include iron,manganese,copper,molybdenum,zinc,boron,chlorine and nickel.
(iii) Beneficial nutrients The elements which are not essential for plants, but their presence are beneficial for the growth and development. Such, elements are called beneficial elements.
Toxic elements Any mineral ion concentration in tissues, that reduces the dry weight of issues by about 10% is considered toxic. e.g., Mn inhibits the absorption of other elements.
(iv) Essential elements The macronutrients including carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium, which are required directly for the growth and metabolism of the plants and whose deficiency produces certain symptoms in the plants are known as essential elements.

The kind of deficiency symptoms shown in plants, include chlorosis, necrosis, stunted plant growth, premature fall of leaves and buds, and inhibition of cell division. Chlorosis is the loss of chlorophyll leading to yellowing in leaves, and necrosis is the death of tissue, particularly leaf tissue.