Variation in the mode and magnitude of development of argillic horizons in Celina and Morley soils of western Ohio was studied by physical, chemical, mineralogical, and morphological means to explain the difference in Bt/C clay ratios between these Hapludalfs. Although Celina may have formed on a somewhat older land surface than Morley, parent material must be considered the chief factor responsible for differences in genesis of sola since other soil-forming factors are quite similar. Both soils are derived from Wisconsin-age till deposits but, in this study, till associated with Celina soils has a mean clay content of near 23% and a calcium carbonate equivalent of near 40% whereas till underlying Morley soils has near 33 and 25%, respectively. Mean maximum clay content in Bt horizons is near 41% for Celina and near 50%; for Morley which yields a Bt/C clay ratio of 1.8 and 1.5, respectively.
Carbonate particle-size distribution in C horizons indicates very coarse sand, coarse sand, and coarse silt generally have the highest calcium carbonate equivalents whereas coarse clay contains <10% and fine clay <1% carbonates. Reconstruction of gains and losses of clay in Bt horizons were calculated both on a weight and volume basis. These results indicate that most of the difference in Bt/C clay ratios is due to carbonate dilution of clay in C horizons and that the remaining difference can be attributed to a larger clay gain in Celina Bt horizons.
Other differences in genesis of argillic horizons in these soils are indicated by mineralogical and micromorphological data. Clays with a 17.7A d-spacing when glycolated which is indicative of montmorillonite were found in Celina argillic horizons but these clays were not apparent in Morley argillic horizons. Thin section studies suggest that upper Bt horizons of Celina soils are actively undergoing degradation and the meager quantity of fine clay gain is due to montmorillonite and vermiculite being diagenetically transformed from illite in situ. Illuviation of clay accounts for the majority of the large clay gain in the lower Bt horizons of Celina soils. The clay gain in Bt horizons of the Morley soils is accounted for by illuviation of clay from overlying horizons and in situ formation of vermiculite. Both processes account for similar quantities of clay gain but together do not form as large a clay increase in the Bt horizons of Morley as Celina soils.