Slaking in Fragipan and Argillic Horizons

Abstract

Fragipan horizon identification is based on seven field clues, none of which are quantitative in nature. The ability of uncemented horizons (including fragipans) to slake or fracture when immersed in water is the basis for the last of these clues. The objectives of this study are to determine if slaking is unique to fragipan horizons and to quantify slaking in selected horizons from soil of the southern Mississippi Valley Silty Uplands, designated as Major Land Resource Area (MLRA) 134. Clods from Btxl horizons of Loring and Grenada soils (fine-silty, mixed, thermic Typic Fragiudalfs and fine-silty, mixed, thermic Glossic Fragiudalfs, respectively) and Bt horizons of Memphis soils (fine-silty, mixed, thermic Typic Hapludalfs) were used for comparisons. Slaking was quantified by weighing the air-dried clods and measuring the length of time required for the clod to completely slake after being immersed in water. All of the horizons investigated slaked in water. Although slaking rates varied from site to site, samples from the Btxl horizon of the Loring and Grenada pedons (2.3 and 1.7 g min⁻¹, respectively) were significantly lower (slower) than those from the Bt horizon of the Memphis pedons (4.6 g min⁻¹). Regression analysis suggests a weak to no relationship of slaking rates to sand content (r² = 0.24) and no relationship of slaking rates to silt or clay content, or to extractable Fe or Si. The data presented suggest that quantified slaking rates may be useful in differentiating fragipans, but the data set needs to be expanded to include other parent materials and soils from other MLRAs. Contribution from the USDA-ARS National Sedimentation Lab.