Genesis of some soils in the central western Cascades of Oregon Public Deposited


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  • Soils representative of several landscape units in the H. J. Andrews Experimental Forest, Western Cascade Range, were sampled, analyzed, and tentatively classified. Genetic inferences were drawn relating soils to landscape position and other factors of soil formation. Descriptive information and nutrient capital data were provided to support ecosystem modelling efforts by the Coniferous Forest Biome study group of the U. S./International Biological Program (IBP). To meet the "nutrient capital" requirements of IBP, and to gain insight particularly into the effects of coarse fragments on soil genesis, a volumetric approach was used. Soil organic matter, total N, extractable P, exchangeable cations, free Fe oxides, and cation exchange capacity were expressed in terms of weight or equivalents per unit volume of "whole soil," defined as organic and mineral fine earth components plus pore space plus coarse fragments. The various entities, in grams or equivalents per liter of whole soil, were observed as to their variation with depth. Additional calculations showed levels of the various entities per surface meter ³ of whole soil. Soil temperature data from several sites within the Andrews Forest showed the mesic-frigid soil temperature regime boundary to fall at about the 600 m (2, 000 ft) elevation on south slopes and at about the 450 m (1, 500 ft) elevation on north slopes. The frigidcryic boundary apparently was above the 1, 500 m (4,900 ft) elevation in the Andrews Forest. A sequence of three fluvial and two colluvial soils ranging in elevation from 440 to 460 m was studied in conjunction with concurrent IBP investigations into the geomorphic history of the area The soil on a floodplain adjacent to Lookout Creek, in the sandy-skeletal, mixed, mesic family of Fluventic Hapludolls, was between 500 and 7, 000 yrs in age. The adjacent stream terrace soil, in the loamyskeletal, mixed, mesic family of Fluventic Dystrochrepts, was > 7, 000 yrs old as evidenced Mazama pumice erposi tE-; on or near the surface of the terrace. Volumetric analysis suggested that the floodplain soil had a mollic epipedon largely by virtue of its high content of coarse fragments. The coarse fragments caused a concentration of soil organic matter and recycled cations into a smaller volume of fine earth as compared with the terrace soil, which was lower in coarse fragments. An alluvial-colluvial fan emanated from an adjacent slope and lapped onto the terrace. The soil in this fan was a member of the Fluventic Eutrochrepts, loamy-skeletal, mixed, mesic. It was high in base status and moderately high in clay content, apparently because the southeast-facing source area for parent material here had experienced only shallow weathering and minimal leaching. Across Lookout Creek from these landscape units was a remnant of a high colluvial terrace emanating from a northwest-facing watershed. At the crest of this fan remnant the soil was a member of the loamy-skeletal, mixed, mesic family of Fluventic Dystrochrepts with a distinct layer of Mazama pumice at the 75 to 85 cm depth. This terrace is cut by the watershed stream, which has deposited a comparatively well sorted fan. Soils are in the coarseloamy, mixed, mesic family of Fluventic Dystrochrepts, Eight landscape units in longitudinal and transverse crosssections of upper McRae Creek valley, ranging in elevation from 800 to 1, 200 m, were chosen to study upland soil genesis. Proceeding up the valley, stage of profile development appeared to decrease, indicating a series of depositional events. Soils varied from Eutric Glossoboralfs, fine, mixed on the lowermost surface to Fluventic Dystrochrepts, fine-loamy, mixed, frigid on the next higher surface, to Fluventic Dystrochrepts, loamy-skeletal, mixed, frigid on the next higher surface, to Typic Haplumbrepts, loamy-skeletal, mixed, frigid on the backslope at the valley headwall. The two lowermost soils contrasted markedly with the two uppermost soils, being lower in content of organic matter and N, and higher in base status and clay content. The upper two soils, typical of upper valley bottom and sideslope soils in the region, were extremely low in exchangeable bases and base saturation as measured at pH 7. Compared with the two lower soils, however, these upper soils had relatively high soil: water pH values and relatively small drops in pH from soil:water to soil :KCI measurement. This may be an indication that the upper soils were higher in amorphous content. Greater pH-dependent-CEC would have caused the upper soils to exhibit unrealistically high CEO s--and thus low base saturations--when measured at pH 7. A topoclimosequence of soils on north, east (saddle), and south-facing landscape units with a single parent rock lithology was studied in the transverse valley transect. All three soils were placed tentatively in the Andic Dystrochrepts. The north-facing soil was in a medial - skeletal, frigid family, was the deepest to bedrock ( > 1 1/2 m), aria had the freshest coarse fragments of the three soils. The saddle and south-facing soils were in medial-skeletal, frigid and medial, frigid families, respectively. They were shallow ( <1 m) to saprolite bedrock, with well weathered coarse fragments in the regolith, demonstrating shallower, but apparently more intense weathering on the more exposed sites. These more exposed soils were darker in color than the north-facing soil. Soil organic matter levels were not strikingly different among the three soils. Soil N levels were significantly higher in the south-facing soil than in the east and north-facing soils. Levels of exchangeable bases, while low, were not as low in these three soils as in the upper valley bottom and backslope soils. Saprolite horizons had higher base saturations than overlying horizons.
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