From Latin 'solum' (ground, floor), via Anglo-Norman — unrelated to the verb 'to soil' (to dirty).
The upper layer of earth in which plants grow; ground.
From Anglo-Norman 'soil' meaning 'a piece of ground, seat,' from Latin 'solium' (seat, throne) or 'solum' (ground, floor, soil). The verb 'to soil' (to make dirty) is a different word from Old French 'souiller.' Key roots: solum (Latin: "ground, floor, base").
The noun 'soil' (earth) and verb 'to soil' (to dirty) are completely unrelated words — one from Latin 'solum' (ground), the other from French 'souiller' (to wallow in mud).