Essential plant nutrients with the definition, objectives, classification, and sources of plant nutrients elements
Plant nutrition
Nutrition may be defined as the supply and absorption of chemical compound required for plant life, growth and metabolism. It is the process of absorption and utilization of essential elements for plant growth and reproduction. Arnon (1954) has defined following criteria/objectivesfor essentiality of any nutrient:
i.The plant cannot grow or complete its life cycle in absence of the element;
ii.The element is very specific and cannot be replaced by another element;
iii.The element plays a direct role in plant metabolism.
Thus, an essential plant nutrient may be defined as “an element so essential for a specific metabolic activity that in its absence the plants cannot continue to grow or complete their life cycle and it cannot be substituted by another element.
Classification of plant nutrient
Basedon the relative requirement by plants the essential plant nutrients are classified as Major or macro nutrients.
1.Major or macro nutrient:
Those nutrientwhich are required by plants in concentrations exceeding 1000 ppm (0.1%) are termed as major or micro nutrients.
The term ‘macro’ refers to the amount used (usually 50mg/ kg or more in the plant body) and to the essentiality.
a. primary nutrients: C,H,O,N,Kare the primary elements which are essential from seed germination and for plant growth
C, H, O are found abundantly in water and atmosphere.
N, P, K are either obtained from soil or supplied through chemical fertilizers.
b. Secondary nutrients: They are secondary in the sense that they are needed only they have started growing (secondary growth) they are Ca, Mg and S.
2. Minor or micro nutrients:
The elements which are required by plants in concentration less than 100 ppm are termed as minor or macro nutrients. They are also called as “trace elements”.
The term ‘micro’ refers to the amount used (usually less than 50mg/ kg in the plant body) rather than the essentiality.
Beneficial and trace elements
Beneficial elements:They are beneficial for some specific plants not for all. e.g. Na, Si (for rice).
Trace elements: They are some micronutrients and other non-essential elements (but not macro-nutrients) present in soil or in plant body in extremely small amounts. e.g. Cd, Pb. As, V, Se.
Sources of plant nutrients (in general)
Plants obtain nutrients in the following ways:
i.From the soil solution. i.e. the free water in soil containing dissolved salts,
ii.From exchangeable ions on the surface of extremely small particles of soils and of organic matter called “humus” particles respectively
iii.From readily decomposable minerals and;
iv.Through the very small openings on the surface of leaves, called “stomata’
*Chief sources of nutrients to plants are clay and humus of soil.
*Nutrients such as NH4+, Ca++ k+, Mg+ , etc. are held on to clay particles in an exchangeable
and available form for use by plants.
*Soil organic matter serves as the principal storehouse for supply of anions such as H2PO4–or
SO4— to the plants.
Plants absorb nutrients through the following organs
i.Root hair: All higher plants uptake nutrients in ionic forms are from soil solution through the root hairs.
ii.Stomata of leaves: O2 and CO2 are uptaken in gaseous forms through stomata. Besides, foliar applied nutrients are also uptaken.
iii.Cuticle:Woody perennials and shrubs can absorb foliar applied nutrients through cuticle of bark.
iv.Special organ: Haustaria: in case of dodder.
Velamen tissue: in case of orchid.
