Essential elements may be classified into three groups:
1. Major elements or Primary nutrients
- The essential elements, which are required by the plants in comparatively larger amounts, are called Major Elements or Primary Nutrients.
- C, H, O, N, P and K.
2. Secondary elements or Nutrients
- These elements are also required by the plant in larger quantity next to primary nutrients. Examples are: Ca, Mg and S.
3. Minor elements or Micronutrients or Trace elements
- The essential elements required in smaller amounts or traces by the plants are called as Minor or Trace Elements.
- They are: Fe, Mn, Cu, Zn, Mo, B and Cl.
- Apart from these elements, recently some more elements have also been shown to the essential for the normal growth of some plants such as Na for Atriplex, Si for rice and Cl for coconut and Al, Va and Co for ferns.
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Classification of Plant mineral nutrients according to their Biochemical functions
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Mineral Nutrient
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Functions
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Group 1
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Nutrients that are part of carbon compounds
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N
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Constituent of amino acids, amides, proteins, nucleic acids, nucleotides, coenzymes, hexoamines, etc.
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S
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Component of cysteine, cystine, methionine, and proteins. Constituent of lipoic acid, coenzyme A, thiamine pyrophosphate, glutathione, biotin, adenosine-5'-phosphosulfate, and 3-phosphoadenosine.
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Group 2
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Nutrients that are important in energy storage or structural integrity
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P
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Component of sugar phosphates, nucleic acids, nucleotides, coenzymes, phospholipids, phytic acid, etc. Has a key role in reactions that involve ATP.
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Si
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Deposited as amorphous silica in cell walls. Contributes to cell wall mechanical properties, including rigidity and elasticity.
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B
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Complexes with mannitol, mannan, polymannuronic acid, and other constituents of cell walls. Involved in cell elongation and nucleic acid metabolism.
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Group 3
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Nutrients that remain in ionic form
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K
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Required as a cofactor for more than 40 enzymes. Principal cation in establishing cell turgor and maintaining cell electroneutrality.
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Ca
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Constituent of the middle lamella of cell walls. Required as a cofactor by some enzymes involved in the hydrolysis of ATP and phospholipids. Acts as a second messenger in metabolic regulation.
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Mg
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Required by many enzymes involved in phosphate transfer. Constituent of the chlorophyll molecule.
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Cl
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Required for the photosynthetic reactions involved in 02 evolution.
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Mn
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Required for activity of some dehydrogenases, decarboxylases, kinases, oxidases, and peroxidases. Involved with other cation-activated enzymes and photosynthetic 02 evolution.
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Na
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Involved with the regeneration of phosphoenolpyruvate in C4 and CAM plants. Substitutes for potassium in some functions.
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Group 4
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Nutrients that are involved in redox reactions
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Fe
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Constituent of cytochromes and nonheme iron proteins involved in photosynthesis, N2 fixation, and respiration.
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Zn
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Constituent of alcohol dehydrogenase, glutamic dehydrogenase, carbonic anhydrase, etc.
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Cu
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Component of ascorbic acid oxidase, tyrosinase, monoamine oxidase, uricase, cytochrome oxidase, phenolase, laccase, and plastocyanin.
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Ni
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Constituent of urease. In N2-fixing bacteria, constituent of hydrogenases.
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Mo
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Constituent of nitrogenase, nitrate reductase, and xanthine dehydrogenase
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Source: http://xa.yimg.com/kq/groups/21666630/1921577070/name/CH--Mineral-Nutrition.pdf
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Adequate tissue levels of elements that may be required by plants
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Element
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Chemical symbol
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Concentration in dry matter
(% or ppm)
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Relative number of atoms with respect to molybdenum
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Obtained from water or carbon dioxide
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Hydrogen
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H
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6
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60,000,000
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Carbon
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C
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45
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40,000,000
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Oxygen
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0
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45
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30,000,000
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Obtained from the soil
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Macronutrients
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Nitrogen
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N
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1.5
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1,000,000
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Potassium
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K
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1.0
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250,000
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Calcium
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Ca
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0.5
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125,000
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Magnesium
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M
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0.2
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80,000
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Phosphorus
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P
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0.2
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60,000
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Sulfur
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S
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0.1
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30,000
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Silicon
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Si
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0.1
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30,000
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Micronutrients
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Chlorine
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Cl
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100
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3,000
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Iron
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Fe
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100
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2,000
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Boron
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B
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20
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2,000
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Manganese
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Mn
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50
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1,000
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Sodium
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Na
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10
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400
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Zinc
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Zn
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20
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300
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Copper
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Cu
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6
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100
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Nickel
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Ni
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0.1
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2
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Molybdenum
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Mo
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0.1
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1
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Source: http://xa.yimg.com/kq/groups/21666630/1921577070/name/CH--Mineral-Nutrition.pdf
The values for the non mineral elements (H, C, O) and the macronutrients are percentages. The values for micronutrients are expressed in parts per million.
Micronutrients are usually present in the plants in different chemical forms as:
1. Inorganic ions
2. Undissociated molecules or
3. Organic complexes as chelates
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