You capitalize "Earth" when it is used as the proper noun referring to our planet. You do not capitalize it when referring to the concept of soil or ground.

Here are some examples:

Capitalize:

* Earth is the third planet from the Sun.

* We live on Earth.

* Humans are the only known intelligent life on Earth.

Do not capitalize:

* The farmer tilled the earth.

* The worms burrowed deep in the earth.

* The earthquake caused the earth to tremble.