1- Briefly describe the usage of Nitrogen, Phosphorus, and Potassium as fertilizer/a fertilizer/fertilizers in plants.
After checking the Internet, I've found out that the fertilizer consists of a compound made up of nitrogen, phosphorous and potassium, so I don't think the comma before "and Potassium" is correct. It would if the three chemical components were each a fertilizer in its own right. Since it is the mixture that counts as a fertilizer, you should use "a fertilizer."
2- When did the U.S start using nuclear bombs as a weapon/weapons.
You should also use the singular here. "Weapons" would be used if different, separate weapons were mentioned. The phrase "as a weapon" means "as a type of weapon."