Nitrogen in Plant Food
It's pretty easy to see just how much a plant needs nitrogen to grow. Just take any two plants (they could be small house plants, or plants growing outside) and sprinkle a nitrogen-based plant food over one of them, and not the other one. Follow the directions on the box. Within a week or two you will see that the plant with extra nitrogen grows much faster and looks greener than the other plant.
But maybe that plant was just naturally stronger than the other plant? Try reversing your experiment: this time, give all the plant food to the other plant for a couple of weeks. What happens?
There's nitrogen in urine (pee), so when a dog pees on or near a lawn, you'll see that the grass grows greener there. But if the dog keeps peeing there every day, or every few days, after a while the grass turns brown and dies. In urine, the nitrogen is part of a larger ammonia molecule, and too much ammonia will kill a plant.