There are many synonyms for "earth" depending on the context you're using it in. Here are a few examples:

For the planet:

* Globe

* World

* Planet

* Terra (Latin)

* Gaia (Greek)

For soil:

* Ground

* Soil

* Dirt

* Land

* Mold

* Clay

For the ground:

* Floor

* Surface

* Terrain

* Landscape

For the human world:

* Humanity

* Mankind

* Civilization

For the natural world:

* Nature

* Environment

* Ecosystem

To give you the most accurate synonym, please provide me with the sentence or context where you want to use it.