The word "earth" can be substituted with a multitude of synonyms based on the context of its usage. In the context of soil or dirt, you can use terms like dirt, soil, ground, or land. For ecology, you can use terms like biosphere, environment, or ecosystem. When referring to the planet earth, substitutes like world, globe, or planet can be used. Additionally, substitutes like terra, clay, loam, humus, or sod can be used based on the type of soil or earth. Synonyms like terrain, topography, or geology can be used when describing the features, structure, and composition of the earth's surface.