PROPERTIES, GENESIS, AND CLASSIFICATION OF SOILS ON TWO GEOMORPHIC SURFACES IN THE NORTH PLATTE RIVER VALLEY IN WESTERN NEBRASKA

Two soils on two geomorphic surfaces (a terrace and a pediment) that occupy a large area in Scotts Bluff County in Western Nebraska were studied in relation to how these soils formed and how they are classified. The morphology of the soils was determined in the field. Physical, chemical, and mineralogical analyses needed to classify the soils were performed in the laboratory following standard procedures. The two geomorphic surfaces on which the two soils developed are of different age. Climate and vegetation are similar for both soils. The terrace soils appear to be more developed than the pediment soils. The pediment soils had only A and C horizons. The terrace soils had a cambic horizon. The two soils differ in the mineralogy of their parent materials, especially in their volcanic ash contents. The pediment soils had more volcanic ash than the terrace soils. Evidence suggests that the volcanic ash had significant effect on soil properties such as surface area, 15-bar water holding capacity, and CEC. X-ray diffraction studies revealed a similar suite of clay minerals. Diagnostic properties suggest that the pediment soils are with the Fine Silty, Mixed, Calcareous, Mesic, Typic Ustorthents (based on measured and calculated clay). However, the weighted average of measured and calculated clay in the control section differed. Calculated clay was higher. The terrace soils are with the Coarse loamy, Mixed, Mesic, Typic Haplustoll (based on measured clay) and Fine loamy, Mixed Mesic, Typic Haplustoll (based on calculated clay). The two soils studied should be classified on the basis of calculated clay. A modification of the present classification of soils studied when ash is from 15-60% is proposed. Ashy should replace particle size class names or mineralogy classes or both. It appeared that the criteria for the presence of finely divided lime in the mollic epipedon may require modification. The differences in phosphate response noted in the two soils influenced by volcanic ash can be attributed to high phosphate retention, fixation and sorption of one of the two soils