Bulk Density and Fragipan Identification in Loess Soils of the Lower Mississippi River Valley D. L. Lindbo,* F. E. Rhoton, J. M. Bigham, W. H. Hudnall, F. S. Jones, N. E. Smeck, and D. D. Tyler ABSTRACT High bulk density (pb) compared with overlying soil horizons has long been considered one of the principal characteristics of fragipans, yet recent investigations have noted inconsistent relationships between fragipans and Pb. This study was conducted to evaluate the fragipan- pb relationship in several major loessial soils of the lower Mississippi River Valley. Average pb (field-moist, saran-coated clods) obtained from five Memphis-Loring-Grenada catenas extending from north- west Tennessee to southeast Louisiana show that p b in the fragipans of the Loring (fine-silty, mixed, thermic Typic Fragiudalf) and Grenada (fine-silty, mixed, thermic Glossic Fragiudalf) pedons range from 1.45 to 1.78 Mg m- 3 Gc = 1.56). A wider range of 1.38 to 1.86 Mg m" 3 Or = 1.55) occurred at similar depths in the Memphis (fine-silty, mixed, thermic Typic Hapludalf) pedons. Within most Loring and Grenada pedons, the pb of the fragipan horizon is not statistically unique (based on Duncan's new multiple-range test) and does not always represent the maximum for a given pedon. There is also a weak positive correlation (r 2 = 0.44) between total sand content and pb, suggesting that higher pb in these soils are more closely related to lithologic factors than to pedogenic densification. Specifically, pedons with a significant influence of Coastal Plain sediment have higher pb than those developed entirely in loess, suggesting that the high pb are partially inherited. These results indicate that p b should not be used as a principal test for differentiating between fragipan and nonfragipan horizons in these soils. It appears that root and water restriction typically associated with fragipans in the lower Mississippi River Valley are related to physical or chemical characteristics of the fragipans other than pb. N UMEROUS FIELD CLUES are listed by the Soil Survey Staff (1975, 1992) to assist in the recognition and delineation of fragipan horizons. Although there are no established chemical or laboratory procedures for fragipan identification, it is widely acknowledged that fragipans have a high p b , which decreases permeability and root penetration and profoundly impacts properties and land use. Consequently, high p b relative to overlying horizons has been suggested as a possible quantitative means for fragipan identification (Witty and Knox, 1989; Quandt and Glocker, 1993). The proposal presents several problems. For example, some researchers have noted that: (i) a dense horizon does not always indicate the presence of a fragipan (Buntley et al., 1977; Parfitt et al., 1984; Mclntosh and Kemp, 1991); (ii) the fragipan may not always have the highest p b in a profile (Anderson and White, 1958; Grossman et al., 1959; Norton and Franzmeier, 1978; D.L. Lindbo and F.E. Rhoton, USDA-ARS, National Sedimentation Lab., P.O. Box 1157, Oxford, MS 38655; J.M. Bigham, F.S. Jones, and N.E. Smeck, Dep. of Agronomy, Ohio State Univ., Columbus, OH 43210; W.H. Hudnall, Agronomy Dep., Louisiana Agric. Exp. Stn., Louisiana State Univ. Agricultural Center, Baton Rouge, LA 70803; and D.D. Tyler, Dep. of Plant and Soil Science, Univ. of Tennessee, Jackson, TN 38301. Contribution from the USDA-ARS National Sedimentation Lab. Received 7 Apr. 1993. "Corresponding author. Published in Soil Sci. Soc. Am. J. 58:884-891 (1994). Habecker et al., 1990); and (iii) p b may be related to degree of development of a fragipan (Smith and Daniels, 1989). Research on a variety of parent materials has also indicated that high p b may be an inherited feature rather than a pedogenic one (Miller et al., 1971; Wang et al., 1974; Smith and Callahan, 1987; Franzmeier et al., 1989; Smeck et al., 1989; Habecker et al., 1990; Lindbo and Veneman, 1993) or that the densification process may not be entirely pedogenic (Calhoun, 1980). For example, Bryant (1989) noted that high densities could occur soon after deposition of sediments due to physical ripening (desiccation) or self-weight collapse (Hand, 1973). Ray (1967, p. D221-D227) suggested that slow deposition of loess was often accompanied by greater weathering and compaction as fine silts settled into packing voids. As a result, internal drainage de- creased and pore spaces became clogged by illuvial clay and other weathering products, thereby increasing p b and decreasing permeability. Such occurrences fall in the gray area between geogenic and pedogenic processes. Data from several Grenada pedons, located in Arkan- sas, Kentucky, Louisiana, and Mississippi, showed that the fragipans were not consistently denser than overlying horizons (Romkens et al., 1986). Additionally, some Pennsylvania soils developed in either eolian or lacustrine materials did not exhibit significant differences in p b between fragipan and nonfragipan soils (Petersen et al., 1970). In one New Zealand study, field morphology (vertical gray veins, prismatic structure, brittleness, and root restriction) was the distinguishing factor, as p b was the same in both fragipan and nonfragipan pedons (Mcln- tosh and Kemp, 1991). Soils of the Memphis catena extend throughout the lower Mississippi River Valley. Several members of the catena exhibit classic fragipan characteristics (brittleness, vertical gray tongues, prismatic structure, and root re- striction), whereas the Memphis soil does not. The objec- tives of this investigation were to: (i) compare the p b of fragipan and nonfragipan soils within the Memphis ca- tena; and (ii) evaluate the effectiveness of p b in delineating the fragipan horizons in silty soils of the lower Mississippi River Valley. MATERIALS AND METHODS The sites for this investigation are within the southern Missis- sippi Valley Silty Uplands designated as Major Land Resource Area 134 by the Soil Conservation Service (1981). Five sites were selected, approximately 150 km apart, from northwestern Tennessee to southeastern Louisiana (Fig. 1). Three of the sites (Obion County, Tennessee and Tate and Yazoo counties, Mississippi) are located in deep loess (Zones B and C; loess thickness >3 m, Fig. 1), whereas the other two (Fayette County, Tennessee, and West Feliciana Parish, Louisiana) are located in shallow loess underlain by Coastal Plain sediments (Zone E; loess thickness =2 m, Fig. 1). At each site, three pedons (Memphis, Loring, and Grenada) were described and 884