All organisms consist of atoms of chemical elements. The final reservoirs of these elements on earth are the rocks, soil, the oceans and the atmosphere.
Only the green plants and certain microorganisms can primarily extract simple inorganic compounds and ions from the environment without being dependent on other living orgnisms. For this reason these organisms are called autotrophic or self-feeding. All inorganic plant requirements except carbon, hydrogen and oxygen, are obtained directly or indirectly from soil.
In 1804, de Saussure demonstrated that the inorganic elements contained in plant ash are obtained from the soil via the root system. These elements are essential to the growth and development of the plant.
As the sources of these inorganic requirements are minerals, the elements are known as mineral nutrients and the nutrition is called mineral nutrition. Scientific approach to determine the mineral content in plants started by Sachs and Knops in 1830 by culture of plants in acqueous nutrient solution, they prepared a list of elements essential to plant life.
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Criteria of essentiality:
An essential element has following criteria (i) It is indispensable for the growth of the plant. The plant cannot complete its life cycle without it.
(ii) The action of element can must be specific, no element be replaced by other elements.
(iii) The element must be directly involved in the nutrition of the plant, i.e. to be a necessary component of an essential metabolite.
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(iv) The absence or deficiency of the element produces disorders. Out of 92 natural elements nearly 40 elements are found in the living cells, of which sixteen elements have been found to be essential.
They are carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, calcium, magnesium, iron, manganese, copper, zinc, boron, molybdenum, chlorine. The other elements are called non-essential elements or nutrients.
On the basis of the quantities required, the various nutrient elements needed for plant growth have been divided into two categories, macronutrients and micronutrients.
Macronutrients
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The mineral elements needed by plants in greater amounts or are found in detectable quantities in plants are termed macronutrients. Of the sixteen elements listed above, carbon, Hydrogen, Oxygen, Nitrogen, Sulpher, Phosphorous, Potassium, Magnesium and Calcium are known as acronutrients.
Micronutrients
The mineral elements needed by plants in small amounts or traces are called micronutrients. They are copper, Boron, Managanese, Zinc, Molybdncum, Iron and Chlorine.