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Several soils of western Nebraska have exhibited unusually high water content at 15 bars tension (an estimate of the water content where plants begin to wilt) relative to the percent measured clay. These same soils are known to form in volcanically derived sediments. The objective of the study was to identify a mineralogical cause for the discrepancy. Representative samples from the Busher, Epping, Keith, Mitchell, Otero, and Tripp soils were collected and characterized to determine the relationship between soil moisture and measured clay contents. Calculated ratios of soil water content at 15 bars tension to the percent measured clay equaled or exceeded 1.0 or samples from all soils and suggested the presence of volcanic glass and associated minerals in the soils. Detailed mineralogical analysis of the Mitchell and Tripp soils indicated the presence of unmeasured clay coatings on sand and silt sized grains of volcanic glass and other soil minerals. The unmeasured clay contributed to the total surface area of the soils and therefore contributed to the high contents of soil water at 15 bars tension.