Some physical and chemical factors contributing to fragipan strength in Kentucky soils

Abstract

Sixteen pedons developed in various parent materials in Kentucky were studied in order to quantify fragipan strength and expression. The contribution to soil strength by clay, bulk density and amorphous or poorly crystalline chemical constituents in the soil fraction were also examined. The triple beam balance apparatus was successfully used to calculate strength of undisturbed, moist clods. Most argillic horizons were found to have strengths less than 40 Mg m −2, while most fragipan horizons had strengths exceeding 60 Mg m −2. Results using this simple technique corresponded favorably to those achieved by more sophisticated geotechnical equipment for unconfined compression tests. The contribution to strength by clay and bulk density decreased as peds became less plastic and more brittle. Amorphous materials were extracted by a selective sequential procedure using acid ammonium oxalate (in the dark), citrate-dithionite-bicarbonate, and alternating acid (6 N HCl)-alkali (KOH) treatments. Additional strength in fragipan horizons appeared to be related to an amorphous “binding” phase with a 0.5 Si (Al + Si) molar ratio, suggesting that the strength of fragipans is more than a “clay-bridging” or density phenomenon.