1. Based on all of the entries that you make, TurboTax will determine if any portion of a Roth IRA contribution that you enter is an excess contribution. TurboTax does not make that determination until you enter a Roth IRA contribution.
2. No (or at least not any substantial amount), unless your spouse has compensation to support your IRA contribution and you file jointly. Your compensation from self-employment (farming) that will support your IRA contribution is net earnings. Net earnings are net profit minus the deductible portion of self-employment taxes. With no net profit you have no net earnings and no compensation to support an IRA contribution.
