Which of the following elements has not been proved to be essential in plants?

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Hint: The 17 essential elements of the plants are divided into two major categories called micronutrients and macronutrients. The element which is not so essential consists of an atomic number 16.

Complete answer:
These plant elements are in the form of gas or water. There are 4 elements that are helpful for proposing plant growth, but are not taken into consideration necessary to fulfill the life cycle of plants. These are silicon, sodium, cobalt, and selenium. These elements stay under the soil as salts, so plants consume these elements in the form of ions. Macronutrients are utilized in larger amounts; hydrogen, oxygen, nitrogen, and carbon responsible for over 95% of the total biomass of a plant relative to the weight of dry matter. Micronutrients are available in plant tissues in sums estimated in parts per million, going from 0.1 to 200 ppm, or under 0.02% by weight

Additional information:
The total number of essential plant nutrients are the seventeen different elements: carbon, oxygen, and hydrogen that are used up from the air, other nutrients such as nitrogen, are normally taken from the ground. Plants must get the accompanying mineral supplements from their developing medium: the macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), oxygen (O), hydrogen (H). The micronutrients (or minor elements): iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni)

So, the correct answer is ‘Sodium’.

Note: The study of the chemical elements and compounds essential for plant growth is the plant minerals and these plant minerals are also responsible for the plant metabolism and their external supply. In its absence, the plant is not able to fulfill the complete cycle of its life, or the element is part of crucial components or metabolites of the plant. This conforms to Justus von Liebig's Law of the minimum.