The supply and absorption of specific chemical compounds needed for normal growth and metabolism of plants is defined as nutrition while the chemical compounds that functions as raw material for synthesis of different structural and functional substance of plants are termed nutrients . The inorganic material obtained from soil which are used as raw material by plants are called mineral nutrients. Absorption utilization and assimilation of inorganic compound or minerals by plants for synthesis of essential material for their growth, development, structure and physiology is called mineral nutrition.
Essential elements The mineral nutritional elements are available to the plant as ion present in soil water absorbed through root. Nutrient elements may occur in the soil (a) in the aqueous solution, (b) adsorbed on organic or inorganic soil colloids, (c) in the form of an insoluble inorganic compound, and (d) as a constituent of organic compounds, either as a residue of plants or animals or in living organisms. The uptake of nutrients by the plant roots is closely related to the form in which the elements occur. Many factors influence nutrient uptake for plants. Ions can be readily available to roots or could be "tied up" by other elements or the soil itself. Soil too high in pH (alkaline) or too low (acid) makes minerals unavailable to plants.
Liebig (1840) proposed the law of minimum which states that productivity of a soil is dependent upon the proportionate occurrence of deficient mineral.
Julius von Sachs( 1860), German botanist, demonstrated, for the first time, that plants could be grown from seedling to maturity in a defined nutrient solution in complete absence of soil. This technique of growing plants in a nutrient solution is known as hydroponics.
Arnon and Stout (1939) physiologists have given three criteria for plant nutrient essentiality :-
- Some elements are essential for supporting normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.
- The requirement of the element must be specific and the element cannot be replaced by the presence of another element. In other words, deficiency of any one element cannot be met by supplying some other element.
- The element must be directly involved in the metabolism of the plant.
Two criteria by Epstein, 2005:-
- The element is part of a molecule that is an intrinsic component of the structure or metabolism of a plant.
- The plant can be so severely deprived of element that it exhibits abnormalities in its growth, development or reproduction - that is, its “performance” - in comparison with plants not so deprived.
For example, Magnesium is a constituent of the chlorophyll molecule and is essential for photosynthesis, so it cannot be replaced by any other element for the same function . It is also required as a cofactor by many enzymes involved in cellular respiration and metabolic path ways.
Sixteen chemical elements are important in which carbon, hydrogen, and oxygen are supplied by air and water, which comprise about 98% of the fresh weight of the plant, at least 13 other chemical elements, called the essential inorganic nutrients, are needed for plant's growth and survival. The sixteen chemical elements are divided into two main groups:-
- Non-Mineral Nutrients :-The Non-Mineral Nutrients are hydrogen (H), oxygen (O), and carbon (C). In a process called photosynthesis, plants use energy from the sun to change carbon dioxide (CO2 - carbon and oxygen) and water (H2O- hydrogen and oxygen) into starches and sugars.
- Mineral 13 other chemical elements,which come from the soil, are dissolved in water and absorbed through a plant's roots called the essential inorganic nutrients, are needed for plant growth.
On the basis of average concentration in plants , Hoagland (1944) divided essential elements into two categories:-
- Macronutrients: -Macronutrients are generally present in plant tissues in large concentrations of 1.0-10.0 mg per gram of dry matter. The macronutrients include Carbon (C) , hydrogen (H), oxygen(O), nitrogen,(N) phosphorous(P), sulphur(S), potassium (K), Calcium (Ca)and magnesium(Mg).
- Nitrogen, Potassium, and Phosphorous are obtained from the soil and are the primary macronutrients. Calcium, Magnesium, and Sulphur are the secondary macronutrients needed in lesser quantity.
- Micronutrients or trace elements:-Micronutrients are present in very small amounts in plant body in concentration of equal or less than 0.1 mg per gram of dry matter. These include iron (Fe), manganese (Mn) , Copper (Cu), molybdenum (Mo), zinc (Zn), boron, (B) chlorine (Cl) .Many of them are components or cofactors of enzymes. Some are essential for electron transfer.
There are some beneficial elements such as sodium, silicon, cobalt and selenium, in addition to the 16 essential elements named above They are required by higher plants.
Essential elements can also be grouped into following types on the basis of their diverse functions :-
- Framework elements:- Carbon (C ), Hydrogen (H), Oxygen (O) synthesize the cellulose and other components of cell walls. Calcium (Ca) is component of middle lamella.
- Protoplasmic elements :- Carbon (C) , hydrogen (H), oxygen(O), nitrogen,(N) phosphorous(P), sulphur(S), iron (Fe)and magnesium(Mg) give rise to protoplasmic constituents like proteins, nucleic acids, hormones, vitamins, phospholipids,, photosynthetic pigments, etc.
- Osmotic potential :- Some essential elements, e. g., nitrate, potassium, sulphate etc. can alter the osmotic potential of a cell. Potassium plays an important role in the opening and closing of stomata and turgor movements in various plants.
- Energy transducers :-Magnesium occurs in chlorophyll and phosphorous in ATP are essential elements that are components of energy-related chemical compounds in plants, converting one form of energy into another.
- Enzymatic effects:- Some elements that activate or inhibit enzymes, e.g. K,Ca,Mg,Zn,Mn,Cl,Ni,Cu, etc. Mg2+ is an activator for both ribulose bisphosphate carboxylaseoxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation, Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.
- Balancing elements :- Calcium , magnesium potassium counteract the toxic effect of other minerals by ion balancing.
- Fe, Mn, Cu, Zn and Mo are function as co- enzymes