A soil auger for creating smooth-walled cylindrical holes in the earth is provided for use with a drill. The auger has a central rotational axis with a tubular body that is open at a bottom cutting end, and enclosed at a top end by a cap assembly. The cap assembly has a top wall and a bottom wall. The top and bottom walls have apertures aligned with the central rotational axis for receiving a shaft in alignment therewith, and for attachment to a drill. The distance between the top wall and bottom wall compared to the overall length of the assembled auger determines the minimum height of the cap assembly to minimize the rotational imbalance of the auger during use for a given body length. A method of manufacturing the auger is provided using precision cutting tools to create apertures in the body and to maintain concentricity of the auger components.