The definition and inclusion for essential element are not the same in plant and animal systems.

Because the trace elements essential for plants are those elements which cannot be substituted by others in their specific biochemical roles and that have a direct influence on the organism so that it can neither grow nor complete some metabolic cycle.

In general, B, Co, Cu, Fe, Mn, Mo, Si and Zn are known to be essential for plants, whilst Al, As, Br, F, I, Li, Ni, Rb, Se, Sr, Ti and V are known to be essential for some groups or species. In human and animal systems, trace elements are defined as being essential if depletion consistently results in a deficiency syndrome and repletion specifically reverses the abnormalities.

The trace elements fulfilling these requirements are As, Co, Cr, Cu, F, Fe, I, Mn, Mo, Ni, Se, Si, Sn, V and Zn.