Although a sole-proprietor is both employee and employer, none of the contributions to your solo 401(k) go on schedule C. Schedule C line 19 is only for reporting the contributions made on behalf of non-owner employees. Contributions to the sole-proprietor's own solo 401(k) account appears on Schedule 1 line 16 as self-employed retirement contributions. In TurboTax this is done through entries on the Keogh, SEP and SIMPLE Contribution Worksheet.
