Uranium is a redox active contaminant of concern to both human health and ecological preservation. In soils subject to anaerobic conditions, the more mobile, oxidized form of uranium (UO22+) may be reduced by dissimilatory metal reducing bacteria. Despite rapid reduction in controlled systems, various factors within soils may limit biological reduction of the uranyl ion, inclusive of competing electron acceptors such as nitrate, and complexation reactions. Here we elucidate the impact of uranyl speciation on the extent and rate of reduction with a focus on the formation of the ternary calcium-uranyl-carbonato species. The presence of aqueous calcium greatly decreases the rate of microbial uranyl reduction by limiting the accessibility of U(VI), both by rendering the reduction reaction less energetically favorable and by changing the conformation of the electron accepting moiety. The impact of calcium concentration is compared between systems containing ferrihydrite, an amorphous structured iron (hydr)oxide, and a more crystalline iron (hydr)oxide structure represented by goethite and hematite. A linear correlation exists between calcium concentration and decreased uranium reduction for all three iron (hydr)oxides.