Modeling Spodosol Distribution along Environmental Gradients in Northern Idaho.

Soil surveys of the Kaniksu National Forest (NF) in northern Idaho, Montana, and Washington do not currently describe the occurrence of podzolized soils (Spodosols). However, subsequent inventories and studies conducted within the Kaniksu NF and adjacent areas indicate that Spodosols are potentially a major component of mid- to high-elevation landscapes. To make sound land-use decisions it is critical that the extent and distribution of these soils in the area are known, as well as the environmental factors that are responsible for their formation. Previous studies within the Kaniksu NF have developed a conceptual model that describes Spodosol distribution as a function of mainly elevation and aspect, although vegetation is also a component. Spodosols were found to occur above 1500 meters on south facing slopes and above 1100 meters on north facing slopes. However, the spatial area examined by these studies is extremely limited. In this study, the spatial extent of the study area was significantly expanded to increase the accuracy and explanatory power of the model. A stratified random sampling scheme was developed using elevation and solar insolation to select roughly 100 sample locations. Field investigations supported the findings of previous work, but found that latitude may be an important factor in soil formation. The influence of strongly contrasting types of bedrock which underlie the glacial till within the study area is also a suspected factor. Soil attributes were analyzed with respect to their locations, further refining the existing model. This improved model will result in a predictive map of Spodosol distribution and ultimately improve the existing soil survey database.