Farmer Tax Fairness Act of 2005 - Amends the Internal Revenue Code and title II (Old Age, Survivors and Disability Insurance) of the Social Security Act to revise the optional method of computing the net earnings of self-employed individuals for purposes of determining social security benefit eligibility. Replaces the numerical formula for determining benefit eligibility with an indexed calculation based on a lower limit (the amount required under the Social Security Act for a quarter of coverage) and an upper limit (150 percent of the lower limit).