The Dickson and Zanesville Soils of Washington County, Arkansas: II. Micromorphology of Their Fragipans

The micromorphological features of fragipan horizons of a Dickson silt loam and a Zanesville silt loam were studied by means of thin sections of known orientation with respect to the soil surface. The features observed were classified according to the system developed by Brewer and co-workers in Australia. Their abundance, particularly those classified as plasmic separations and concentrations, was found to be much greater in fragipan horizons than in overlying B subhorizons. Plasma aggregates are larger, more easily recognized, and separations more common (sepic fabric) in fragipans than in overlying horizons. In the latter, plasma aggregates are recognizable although small, and randomly distributed. The resultant fabric is basically asepic but an incipient sepic fabric exists in the subhorizon immediately overlying the fragipan where plasmic separations become more noticeable. The salient micromorphological feature of both soils is the striking increase in number and degree of development of illuviation cutans within their fragipan horizons. These cutans are associated with the void system and increase in thickness with depth. Corresponding increases in per cent clay content in the fragipans, however, are not of the magnitude expected from the observed increase in cutan development. This suggests that an important difference between fragipan and nonfragipan horizons is in the nature of clay arrangement (i.e., its plasmic structure) within the soil matrix rather than amount of clay. Conclusions are: (i) Brewer's classification provides a useful tool in this kind of study; (ii) micromorphology of the Dickson and Zanesville profiles are very similar except that the Dickson contains lithorelicts of chert whereas in the Zanesville they are siltstone or sandstone; and (iii) much of the silt in these soils can be accounted for through weathering of chert or siltstone. Further tentative conclusions are that fragipan brittleness and polygonal color pattern apparently result from the precipitation of iron oxides and clays through diffusion processes within the soil matrix associated with movements of water suspensions and solutions in the void system of the soil, and that fragipans in the soils studied have formed coincidentally with the rest of the solum and can be explained by processes of soil genesis in operation today.