Thirteen elements are known to be essential for normal plant growth. Three others – silicon, sodium and cobalt – have beneficial effects on growth or health under special circumstances but are not considered important for commercial horticulture.
The essential elements fall into two groups according to their relative abundance in plant tissues. Those found to be in the highest concentrations are called the major or macro nutrients. They include: nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and Magnesium (Mg).
Those found in the lowest concentrations are called the micro or trace elements. They are: iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo) and chlorine (Cl).
Plants absorb many other elements from the soil but their presence in tissue is not necessary for the proper development and functioning of the tissues. 

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