the first is better if you are giving the definition of the "soil', and the second is more appropriate if you are telling what is the upper most layer of the earth....
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vsureshSoil is the uppermost layer of the earth.This is a definition of "soil". If you need to define "soil" in the context of what you're writing, use this.
vsureshThe upper most layer of the earth is soil.If this is a sentence within a group of several that explain the composition of