Mobile charges and lattice polarization interact in ferroelectric materials because of the Coulomb interaction between the mobile free charges and the fixed lattice dipoles. Using SHG microscopy, we have investigated this mutual screening in KTiOPO4, a ferroelectric/superionic single crystal in which the mobile charges are K+ ions. The ionic accumulation close to the crystal surfaces leads to orders of magnitude increase of SHG. This ionic space charge model is supported by the absence of such an effect in non-ionic conductor but ferroelectric BaTiO3, by its temperature dependence in KTiOPO4 and by its broad depletion at domain walls.