It’s common to the point of mootness, but: when describing the planet, Earth is a proper noun and should be capitalized. Otherwise you are referring to soil.
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My dictionary says it’s lowercase. I’ve only ever seen it in lowercase, unless you’re taking about it in the astronomical sense (e.g. “the planet Earth”).
See the first entry here. “On earth” is an idiom, and “earth” starts with a lowercase E.