The influence of the ionic adsorption on the anchoring energy of a nematic liquid crystal sample is investigated. We determine the behavior of the anchoring energy as a function of the thickness of the sample, and as a function of the adsorption energy of ions. We show that the contribution to the anchoring energy, due to ionic adsorption, can be of the same order of magnitude of the bare anchoring strength. Our analysis generalizes similar calculations previously published by incorporating the effect of adsorbed charges on the potential and field profiles in the sample.