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Some elements and their compounds have an important role in the living protoplasm and are required in large amounts. These nutrients are called 'macro nutrients' or 'essential nutrients'.
Some of the macronutrients are - carbon, nitrogen, oxygen, hydrogen, potassium, calcium, magnesium, sulphur and phosphorus.Some elements are required in minute quantities and have a role in the activities of the living protoplasm. These nutrients are called 'micronutrients' or 'trace elements'.
Some of the micronutrients are - manganese, copper, zinc, boron, molybdenum, vanadium, cobalt, chlorine and sodium.Some elements (such as sodium) can act as both macro and micronutrients depending upon the species and its requirements.
As these elements are provided by the earth and are used by the organisms for their body building and metabolism, they are called 'Biogenetic nutrients' or 'Biogeochemicals' (bio - living organisms; geo - soil, rock, etc, chemicos - elements and their compounds).These materials have been used again and again in the formation of new generations of the organism. In other words, the matter is continuously recycled.
By definition,"The movement or circulation of biogenetic nutrients through the living and non-living components of the biosphere or of any ecosystem is called biogeochemical cycling".
Or
"Biogeochemical cycles are the cyclic pathways through which chemical elements move from environment to organism and back to the environment".
Since such movements of elements and inorganic compounds is essential for maintenance of life, they are also called 'nutrient cycles'.In biosphere, the biogenetic nutrients exist in two states-
a) Reservoir pool - is the pool of nutrients from which the nutrients are slowly transferred to cycling pool.b) Cycling pool - is the pool of nutrients which is repeatedly exchanged between the biotic and abiotic components of the biosphere.
