This work documents a sharp adsorption cutoff for a cationic polymer (poly(dimethylamino ethyl methacrylate)) adsorbing onto a negative surface (silica) from buffered aqueous solution, as a function of ionic strength. In this system, electrostatic attractions drive adsorption, without contribution from chemical or hydrophobic interactions between the polymer and the surface. In the limit of dense surface charge and sparse polymer charge, a small increase in monovalent salt concentration (less than a factor of 2) completely eliminates polymer adsorption. At conditions where the polymer charge is more dense and the surface charge less dense, added salt still diminishes adsorption, albeit more gradually. The sharpness of the salt-induced polymer adsorption cutoff can be tuned through the polymer charge density. Comparisons with existing theory agree in the limit of the low-charge sharp-cut behavior. The more gradual transition at high polymer charge density is not accurately predicted.