Mineralogical and Geomorphological Relationships of Select Alabama Coastal Plain Soils.

Alabama (U.S.) Coastal Plain soils exhibit a variety of morphological and mineralogical properties. In many ways, these soils are transitional between Atlantic Coastal Plain soils to the east and the Western Gulf Coastal Plain. Ultisols with low activity clays consisting of kaolinite, hydroxy-interlayered vermiculite, quartz and sesquioxides with minimal smectite and weatherable minerals typify significant upland portions. Many of these soils have kandic horizons or classify in subactive CEC activity classes. These properties are due to a prevailing warm, humid climate, areas of landscape stability due to a predominant backwearing development, and recycled sedimentary parent materials. Thermodynamic modeling, improved quantification of clay minerals, and fluvial chronosequences (Holocene to Late Pliocene aged fluvial terraces) illustrate that clay mineralogy systematically varies with soil development. Plinthite, an iron-rich organic poor mixture of clay and quartz, is also commonly found throughout the southern Coastal Plain. Plinthite in Alabama and Georgia is typically 3 to 5x higher in iron than the soil matrix, has less iron than plinthite in tropical soils, and its presence is associated with both past (relict) and contemporary soil saturation. In contrast, Alabama and Mississippi Blackland Prairie soils forming from Cretaceous chalks possess appreciable smectite and carbonate. Acid and alkaline soils occur in close proximity in this region, and evidence suggests lithologic discontinuities control this distribution. Macro- and micromorphology, isotopic, and elemental (e.g. rare earth) analyses indicate appreciable pedogenic carbonate in the lower solum of these soils, which is atypical for soils of humid climates.