Abstract

Several soils ofwestern Nebraska have exhibited unusually high watercontentat 15 bars tension (an estimate ofthe water contentwhereplants begin to wilt) relative to thepercentmeasuredclay. These samesoilsare known to form in volcanically derived sediments. The objective ofthe study was to identifY a mineralogical cause for the discrepancy. Representative samples from the Busher, Epping, Keith, Mitchell, Otero, andTripp soils were collected and characterized to determine the relationship between soil moisture and measured clay contents. Calculated ratios ofsoil water content at 15 bars tension to thepercentmeasuredclay equaledorexceeded 1.0for samplesfrom allsoils andsuggestedthepresence ofvolcanic glass and associated minerals in the soils. Detailed mineralogical analysis ofthe Mitchell and Tripp soils indicatedthepresenceofunmeasuredclaycoatingson sandandsiltsizedgrains ofvolcanicglass andothersoil minerals. The unmeasured clay contributed to the total surface area ofthe soils andtherefore contributedto the high contents ofsoil water at 15 bars tension.