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Soils with Bx Horizons in the Upper Coastal Plains of South Carolina¹

ABSTRACT

Soils with firm, dense Bx horizons are extensive in the Upper Coastal Plains including the Sandhills of Georgia, North Carolina, and South Carolina. The Bx horizons are slightly brittle to brittle, but prisms with bleached, vertical faces are often weakly developed or absent. Three pedons of a series representative of these soils were studied in detail to characterize the soils, to determine if they have fragipans, and to determine if they are properly classified. Particle size distribution data show that the Bx horizons also meet the criteria of argillic horizons, as do many fragipans. Bulk densities are high in the Bx horizons. Free Fe oxide contents are highest in the Bt or Bx horizons, just as are clay contents. Clay mineralogy in the surface horizons is predominately hydroxy-Al interlayered vermiculite (HIV) and kaolinite with substantial amounts of gibbsite. HIV and gibbsite decrease and kaolinite increases with depth. Micromorphological examinations reveal dense packing of quartz skeleton grains with a mixture of clay minerals and Fe oxyhydroxides and very few voids in the Bt, Bx, and C horizons. Yellow illuviation argillans and red illuviation ferriargillans are fairly common in the Bt and Bx horizons. Some segregation of Fe oxyhydroxides has occurred in the soils. Brittleness of the Bx horizons is restricted to the reddish-colored material. This indicates that weak cementation by Fe oxyhydroxides in the presence of clay minerals is at least partly responsible for the brittleness. The Bx horizons are brittle in approximately 45 to 50% of the volume and have many of the properties of fragipans. The establishment of a Fragic subgroup of Hapludults is proposed for these and similar soils.

¹ Technical Contribution no. 2482 of the South Carolina Agric. Exp. Stn., Clemson Univ.

² Associate Professor and former Graduate Assistant, Dep. of Agron. and Soils, Clemson Univ., Clemson, SC 29634-0359. Junior author is presently an environmental science teacher in the City of Savannah School District.

Received for publication November 4, 1985.