Why is the nitrogen cycle important for animals plants and humans?

All living things need nitrogen - it is a key part of both plant and animal tissue, and plants wouldn't be able to grow without it. We get our nitrogen by eating protein, but most plants have to get it from the soil. Garden plants use up a lot of nitrogen, so it needs to be replaced with fertilizer or you won't be able to keep growing plants on the same plot of land.

In nature, some plants are able to pull nitrogen out of the air which helps replenish the soil, and when old plants die and rot the nitrogen returns to the ground. This doesn't happen in a garden if the plants are eaten for food or thrown away when they die.