The poll tax analogy is a bad one. Section 1 of the 15th Amendment says:
The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude . . .
The poll taxes, like literacy tests, were specifically intended to, and had the effect of, keeping African-Americans from voting. Therefore, the poll taxes clearly abridged the right to vote based on race and/or previous condition of servitude in violation of the 15th Amendment.