Soil formation is a complex process that varies with climate, relief, parent material, organisms, and time, as well as in response to anthropogenic inputs such as acidic deposition. Time is often the one factor that is not well studied. Our objective was to understand how forest soil chemistry of two sites in central Illinois changed on a long-term basis in response to decreased acidic deposition (upland oak-hickory and bottomland silver maple soils) and by periodic sediment inputs (bottomland soils). These sites were sampled in 1986 and 2009 using the same protocols. During an 8-year period in the 1990s sediment deposition to the floodplain was measured using concrete blocks placed flush with the soil surface. At both sites, soils at some depths had increased exchangeable Ca, Mg, and K, increased pH, and decreased exchangeable Al and H (upland soil only). But the patterns appeared random. Exchangeable Ca was the dominant cation; it increased 71 to 169% at various depths in the upland soil, but this was thought to be due to inherent variability in the upland profiles that we sampled. Soil organic matter was unchanged at both sites, with greater concentrations of C, N, and S found at the bottomland site due to deposition of sediments from watershed agricultural soils. Flooding was estimated to add 37 Mg ha^-1 yr^-1 of organic rich sediment to the floodplain soils each year, and maintained the high organic matter content of the profile. Changes in exchangeable cations were mostly not due to decreasing acidic deposition, but were likely due to variability in soil chemistry at both sites.