Modeling Spodosol Distribution In Northern Idaho Using Topographic and Environmental Variables.

Spodosols have not been mapped in the Kaniksu National Forest (NF) of northern Idaho. However, their presence at relatively high elevations has been noted within the last several decades during studies across very limited geographic areas within the Kaniksu NF. This study was undertaken to improve existing soil survey information by developing a terrain attribute-based model to predict the distribution of Spodosols across the entire Kaniksu NF. A stratified random sampling scheme was developed using elevation and solar insolation to select sample locations, of which 72 were described and sampled. Ammonium oxalate and pyrophosphate dissolution data, as well as soil pH and NaF pH, were used to confirm the taxonomic classes of the sampled soils. Topographic factors such as elevation, solar insolation, and aspect parameters were used to model the presence or absence of Spodosols using classification trees. The resulting model indicates Spodosols are found above 1500 meters on south-facing slopes and 1100 meters on north-facing slopes. In addition, Spodosols are found locally at lower elevations on coarse-textured parent materials. Model results suggest there are approximately 265,000 ha of Spodosols and soils with spodic horizons in the Kaniksu NF. However, the majority of soils with spodic morphology fail to meet current taxonomic criteria for albic horizons and therefore do not meet Spodosol criteria.