Question
What is the essential element for healthy plant growth?

Answer
Justus von Liebig is called the 'Father of the Fertilizer Industry' for cooking up the world's first chemical Nitrogen Fertilizer.  Nitrogen is needed to make Chlorophyll and Leaves.  It is the first Macronutrient.

Phosphorus is the second Macronutrient.  It is needed for flowers and roots, among other things.

Potash is the third Macronutrient.  Plants that get enough Potash (aka Potassium) are tougher during winter; have better developed root systems; grow more vigorously; and produce bigger, better fruits and vegetables.

Macronutrients are the primary nutrients needed by Green Plants.  But Micronutrients are essential, too.

If you would like to expand on your question, I might be able to elaborate in a way that will help you more.  Thanks for writing.