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There are two broad categories of essential elements required by plants based on their quantitative requirements:
Macronutrients: Present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter).
- Includes Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorous, Sulphur, Potassium, Calcium, Magnesium.
- Carbon, hydrogen and oxygen are mainly obtained from CO2 and H2O, while the others are absorbed from the soil as mineral nutrition.
Micronutrients: Also known as trace elements.
- Needed in very small amounts (less than 10 mmole Kg –1 of dry matter).
- Includes Iron, Manganese, Copper, Molybdenum, Zinc, Boron, Chlorine, Nickel.
In addition to the 17 essential elements, other beneficial elements are Sodium, Silicon, Cobalt and Selenium. These are required by higher plants.
Given below is the role of macro- and micro-nutrients in plant growth and their metabolism and deficiency symptoms of these elements:
| Element | Functions | Deficiency Symptoms | Form absorbed by Plants |
| MACRONUTRIENTS: OBTAINED FROM WATER OR AIR ( H2O or CO2) | |||
| Oxygen | Electron acceptor in cellular respiration; Major component of plant’s organic compounds | Usually affects roots; cells suffocate, rotting of roots and wilting |
O2 , H2O |
| Carbon | Substrate for photosynthesis; Major component of plant’s organic compounds | Slow and poor growth (starvation) | CO2 |
| Hydrogen | Major component of plant’s organic compounds; Functions in electrical balance and establishment of electrochemical gradients |
Wilting, slow growth due to cell death (desiccation) | H2O |
| MACRONUTRIENTS: OBTAINED FROM SOIL | |||
| Nitrogen | Component of proteins, nucleic acids, ATP, chlorophyll, hormones, and coenzymes | Chlorosis at tips of older leaves (yellowing of older leaves) | NO3− (nitrate) NH4+ (ammonium ion) |
| Potassium | Necessary for osmotic adjustment in cells; Major solute functioning in water balance and operation of stomata;Required for synthesis of organic molecules; Cofactor for some enzyme |
Chlorosis at margins of leaves; Mottling of older leaves; weak stems; short internodes; roots poorly developed | K+ |
| Calcium | Important component of middle lamella and cell walls; Stabilizes membrane functions; Involved in signal transduction; Enzyme cofactor | Necrosis in meristems ; Deformation or Crinkling of young leaves; Stunted, highly branched root system; Death of terminal buds | Ca2+ |
| Magnesium | Chlorophyll component; Activates many enzymes | Chlorosis between leaf veins found in older leaves; Premature leaf drop |
Mg2+ |
| Phosphorus | Component of ATP, nucleic acids, phospholipids, and several coenzymes | Healthy appearance but stunted growth in young plants; dark green leaves with necrosis; thin stems; purpling of veins; poor flowering and fruiting | H2PO4− (dihydrogen phosphate ion) HPO42− (hydrogen phosphate ion) |
| Sulfur | Component of proteins containing methionine and cysteine; Electron transport proteins and coenzymes | Stunted growth; chlorosis in young leaves | SO42− (sulfate ion) |
| MICRONUTRIENTS: OBTAINED FROM SOIL | |||
| Chlorine
|
Needed for water-splitting step of photosynthesis; Functions in water balance and electrical balance | Wilting at leaf tips; Stubby roots; general chlorosis and necrosis of leaves or development of bronze color | Cl− (chloride ion) |
| Iron
|
Necessary for chlorophyll synthesis; Component of cytochromes and ferredoxin so needed for respiration, N2 fixation, photosynthesis; Enzyme cofactor |
Chlorosis between veins of young leaves | Fe3− (ferric ion) Fe2− (ferrous ion) |
| Manganese |
Involved in photosynthetic O2 evolution (water-splitting step) ; Enzyme activator; Important in electron transfer; Active in formation of amino acids; | Chlorosis between leaf veins of young leaves and small necrotic spots | Mn2+ |
| Zinc
|
Involved in synthesis of the plant hormone auxin ; Maintenance of ribosome structure, needed for DNA transcription; Active in formation of chlorophyll; Cofactor of some enzymes | Small internodes; stunted and distorted (“puckered”) leaves or crinkled leaves | Zn2+ |
| Boron
|
Strengthens cell walls; Required for pollen tube growth and normal membrane function; Cofactor in chlorophyll synthesis | Black necrosis in young leaves and buds; Death of meristems; Thick, leathery, and discolored leaves | H2BO3− (borate ion) |
| Copper
|
Cofactor of some enzymes; Present in lignin of xylem | Light-green leaves with necrotic spots; twisted and malformed leaves with drying leaf tips; roots stunted and excessively branched | Cu+ (cuprous ion) Cu2+ (cupric ion) |
| Nickel
|
Cofactor for enzyme functioning in nitrogen metabolism | Necrosis/ death of leaf tips | Ni2+ |
| Molybdenum
|
Cofactor in nitrogen reduction; Essential for nitrogen fixation | Chlorosis of older leaves; Death of root and shoot tips | MoO42− (molybdate ion) |
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