BULK-DENSITY AND FRAGIPAN IDENTIFICATION IN LOESS SOILS OF THE LOWER MISSISSIPPI RIVER VALLEY

High bulk density (rho(b)) 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 rho(b). This study was conducted to evaluate the fragipan-rho(b) relationship in several major loessial soils of the lower Mississippi River Valley. Average rho(b) (field-moist, saran-coated clods) obtained from five Memphis-Loring-Grenada catenas extending from northwest Tennessee to southeast Louisiana show that rho(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 (xBAR = 1.56). A wider range of 1.38 to 1.86 Mg m-3 (xBAR = 1.55) occurred at similar depths in the Memphis (fine-silty, mixed, thermic Typic Hapludalf) pedons. Within most Loring and Grenada pedons, the rho(b) 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 (r2 = 0.44) between total sand content and rho(b), suggesting that higher rho(b) 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 rho(b) than those developed entirely in loess, suggesting that the high rho(b) are partially inherited. These results indicate that Pb 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 rho(b).