Genesis of some soils in the central western Cascades of Oregon

Soils representative of several landscape units in the H. J.Andrews Experimental Forest, Western Cascade Range, were sam-pled, analyzed, and tentatively classified. Genetic inferences weredrawn relating soils to landscape position and other factors of soilformation. Descriptive information and nutrient capital data wereprovided to support ecosystem modelling efforts by the ConiferousForest Biome study group of the U. S./International Biological Pro-gram (IBP).
To meet the "nutrient capital" requirements of IBP, and togain insight particularly into the effects of coarse fragments on soilgenesis, a volumetric approach was used. Soil organic matter,total N, extractable P, exchangeable cations, free Fe oxides, andcation exchange capacity were expressed in terms of weight or equiva-lents per unit volume of "whole soil," defined as organic and
mineral fine earth components plus pore space plus coarse frag-ments. The various entities, in grams or equivalents per liter ofwhole soil, were observed as to their variation with depth. Addi-tional calculations showed levels of the various entities per surfacemeter3 of whole soil.
Soil temperature data from several sites within the AndrewsForest showed the mesic-frigid soil temperature regime boundaryto fall at about the 600 m (2, 000 ft) elevation on south slopes and atabout the 450 m (1, 500 ft) elevation on north slopes. The frigid-cryic boundary apparently was above the 1, 500 m (4,900 ft) elevationin the Andrews Forest.
A sequence of three fluvial and two colluvial soils ranging inelevation from 440 to 460 m was studied in conjunction with concur-rent IBP investigations into the geomorphic history of the area Thesoil on a floodplain adjacent to Lookout Creek, in the sandy-skeletal,mixed, mesic family of Fluventic Hapludolls, was between 500 and7, 000 yrs in age. The adjacent stream terrace soil, in the loamy-skeletal, mixed, mesic family of Fluventic Dystrochrepts, was> 7, 000 yrs old as evidenced by Mazarna pumice deposits on or nearthe surface of the terrace. Volumetric analysis suggested that thefloodplain soil had a mollic epipedon largely by virtue of its highcontent of coarse fragments. The coarse fragments caused a concen-tration of soil organic matter and recycled cations into a smaller
volume of fine earth as compared with the terrace soil, which waslower in coarse fragments. An alluvial-colluvial fan emanated froman adjacent slope and lapped onto the terrace. The soil in this fanwas 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 mater-ial here had experienced only shallow weathering and minimal leach-ing.
Across Lookout Creek from these landscape units was a rem-nant of a high colluvial terrace emanating from a northwest-facingwatershed. At the crest of this fan remnant the soil was a memberof the loamy-skeletal, mixed, mesic family of Fluventic Dystro-chrepts with a distinct layer of Mazama pumice at the 75 to 85 cmdepth. This terrace is cut by the watershed stream, which hasdeposited a comparatively well sorted fan. Soils are in the coarse-loamy, mixed, mesic family of Fluventic Dystrochrepts.
Eight landscape units in longitudinal and transverse cross-sections of upper McRae Creek valley, ranging in elevation from
800 to 1, 200 m, were chosen to study upland soil genesis. Proceed-ing up the valley, stage of profile development appeared to decrease,indicating a series of depositional events. Soils varied from EutricGlossoboralfs, fine, mixed on the lowermost surface to FluventicDystrochrepts, fine-loamy, mixed, frigid on the next higher surface,
to Fluventic Dystrochrepts, loamy-skeletal, mixed, frigid on thenext higher surface, to Typic Haplurnbrepts, loamy-skeletal, mixed,frigid on the backslope at the valley headwall.. The two lowermostsoils contrasted markedly with the two uppermost soils, being lowerin content of organic matter and N, and higher in base status and claycontent. The upper two soils, typical of upper valley bottom andsideslope soils in the region, were extremely low in exchangeablebases and base saturation as measured at pH 7. Compared with thetwo lower soils, however, these upper soils had relatively high soil:water pH values and relatively small drops in pH from soil:waterto soil:KC1 measurement. This may be an indication that the uppersoils were higher in amorphous content. Greater pH-dependent-CECwould have caused the upper soils to exhibit unrealistically highCEC's--and thus low base saturations--when measured at pH 7.
A topoclimosequence of soils on north, east (saddle), andsouth-facing landscape units with a single parent rock lithology wasstudied in the transverse valley transect. All three soils wereplaced tentatively in the Andic Dystrochrepts. The north-facing
soil was in a medial-skeletal, frigid family, was the deepest to bed-rock ( >1 1/Z m), and had the freshest coarse fragments of the threesoils. The saddle and south-facing soils were in medial-skeletal,frigid and medial, frigid families, respectively. They were shallow( in the regolith, demonstrating shallower, but apparently more intenseweathering on the more exposed sites. These more exposed soilswere darker in color than the north-facing soil. Soil organic matterlevels were not strikingly different among the three soils. Soil Nlevels were significantly higher in the south-facing soil than in theeast and north-facing soils. Levels of exchangeable bases, while
low, were not as low in these three soils as in the upper valley bottomand backslope soils. Saprolite horizons had higher base saturationsthan overlying horizons.