Partitioning of Clay‐Sized Minerals in Coastal Plain Soils with Sandy/Loamy Boundaries

Abstract

AbstractFour soils with sandy/loamy boundaries occurring at various depths were studied with respect to distribution of clay‐sized minerals within the soil matrix. Mineralogy of coarse and fine clay was determined by horizon using x‐ray diffraction. Also, mineralogy of clay from ped interiors vs. exteriors (coatings) was determined for the Bt horizon of one soil. Mineralogical differences in coarse‐clay distribution were observed with respect to location above or below the sandy/loamy boundary. Clay above the boundary was consistently high in hydroxy‐interlayered vermiculite (HIV) and quartz, regardless of depth to the boundary. One exception was the A horizon of a Spodosol, which consisted essentially of quartz; however, the Bh horizon of this soil (above the boundary) did contain appreciable HIV. Below the boundary, the upper Bt horizon exhibited an increase in kaolinite and a sharp decrease in quartz relative to overlying E or Bh horizons. A marked decrease in HIV below the upper Bt horizon was also evident. Fine clay contained more kaolinite and less HIV and quartz than did coarse clay, and exhibited less intra‐pedon variation. Possibly HIV comprises a greater proportion of the clay residue in eluvial horizons because of the greater size of its precursor, which could be mica, and subsequent resistance to translocation. The greater abundance of HIV in ped coatings relative to ped interiors suggests that some contemporary translocation of coarser clay from HIV‐dominated eluvial horizons has occurred along major voids between peds. The d001 peak shift with heat treatment for HIV was similar for all samples, suggesting that depth and landscape position have had little influence on degree of interlayer polymerization in these soils.