EXPRESSION OF ANDIC AND SPODIC PROPERTIES IN TEPHRA-INFLUENCED SOILS OF NORTHERN IDAHO, USA

Tephra from Mount Mazama (Crater Lake) has strongly influenced genesis and morphology of forest soils in the Selkirk Mountains of northern I daho. Characteristics associated with Andisols and their intergrades a re common in soils throughout the area. In addition, tephra-influenced soils of the higher elevations exhibit properties of Spodosols as wel l. This study was initiated to examine development and relative expres sion of these properties along an elevational transect with its associ ated bioclimatic gradient. This paper presents data from three soils t hat encompass the range in expression of andic and spodic properties f ound along this gradient. Cooler, moister soils of the higher elevatio ns have formed under subalpine fir forest and are strongly influenced by volcanic ash as indicated by color. NaF-pH, glass content, and quan tities of oxalate-extractable Fe, Al. and Si. Additionally, podzolizat ion processes have created E-Bhs horizon sequences in these soils with in the last 6800 years. The clay fraction of the very strongly acid E horizon is dominated by a well-crystallized smectite mineral. Formatio n of an E horizon has effectively erased andic characteristics such as accumulation of short-range order minerals (SROMs), high NaF-pH, and high P retention. Soils of the middle elevations have formed under wes tern hemlock forest and also have significant ash influence. These soi ls have properties transitional to those of the higher-elevation Spodo sols and do not exhibit the overprint of podzolization processes. Eluv ial horizons are absent although there has been significant accumulati on of metal-organic complexes and inorganic SROMs in the upper horizon s of these Andisols. Soils of the lower elevations have developed unde r relatively warmer and drier conditions associated with the lower end of the western hemlock forest zone and have comparatively less volcan ic ash influence. The ash has been extensively mixed with coarse-textu red glacial drift, and, as a result, these lower elevation soils lack the morphological or chemical properties associated with Spodosol deve lopment but do exhibit weak andic characteristics.