Final answer:
Potassium is a highly reactive metal of low density that is commonly found in fertilizer.
Step-by-step explanation:
The highly reactive metal of low density that is one of three main elements found in fertilizer is potassium. Potassium is a member of Group 1, the alkali metals, which are known for their high reactivity and low density. It reacts rapidly with oxygen to produce potassium oxide, and it is commonly used in fertilizer due to its essential role in plant growth.