For an element to be regarded as essential, all of the following criteria must be met, except:
no other element can perform the function.
the element is directly involved in plant nutrition.
the element is inorganic.
the plant cannot complete its lifecycle without the element.

The element is inorganic.

Explanation:

Inorganic elements are all substances formed by atoms or molecules of at least two different elements, and which do not contain in their structure carbon atoms and hydrogen bonded atoms. In some situations carbon may appear in the structure of the compound, but due to the way it is bonded, it is considered inorganic, as is the case with CO2 and Na2CO3.

Although inorganic elements may have some importance in maintaining plant life, they are not essential elements. Essential elements have specific functions, are directly involved in plant nutrition and the plant cannot complete its life cycle without them.