Threebear Soil Series

The Threebear soil series is Idaho's official state soil. It sits on mountain slopes and ridges in the forests of northern Idaho, where it formed from layers of volcanic ash deposited over older loess and basalt.

The soil is very deep and well drained. Volcanic ash from Cascade eruptions gave the upper layers an unusually fine, silky texture and high water-holding capacity — properties that help support Idaho's densest forests.

Below the ash-influenced layers, the soil transitions into older loess-derived material with a denser, clay-enriched subsoil. This two-part structure, ash above and loess below, is the defining characteristic of the Threebear series.

Idaho's soil scientists and the USDA Natural Resources Conservation Service identified the Threebear series as the soil that best represents the forested landscapes of the state's northern panhandle — a region defined by steep mountain slopes, heavy snowfall, and some of the largest conifer forests in the Rocky Mountain West.

The series was established and described by NRCS soil scientists working in Clearwater and adjacent counties, where the overlap of volcanic ash deposits, loess, and basalt bedrock creates a distinctive soil unlike anything in the southern part of the state.

The name Threebear comes from Threebear Lakes, a cluster of mountain lakes in Clearwater County. Naming soil series after nearby geographic features — lakes, creeks, towns — is standard practice in USDA soil science.

The Threebear profile has two distinct parent materials stacked on top of each other. The upper portion formed in volcanic ash — fine, silky, light in weight. The lower portion formed in older loess and basalt weathering products — denser and clay-enriched. Together they create a deep, productive soil column.

Threebear soil is concentrated in the northern panhandle and north-central mountains of Idaho. It sits on forested mountain slopes and benches, typically between 2,500 and 5,500 feet in elevation, where snowfall is heavy and conifer forests are dense.

The soil is found primarily in Clearwater, Latah, Idaho, Shoshone, and Benewah counties — the core of Idaho's timber country. Clearwater County contains some of the largest concentrations, including the Clearwater National Forest where soil surveys first mapped the series.

The same volcanic ash deposits that built the Threebear series extend east across the Palouse region into Washington, connecting Idaho's forested mountains to the dryland wheat fields of the Palouse.

Timber is the primary land use on Threebear soil. The deep, moist, well-drained profile supports ponderosa pine at lower elevations and Douglas fir, grand fir, and western red cedar higher up. Western larch and Engelmann spruce also grow in the northern ranges.

Idaho County and Clearwater County contain some of the largest remaining stands of old-growth ponderosa pine in the western United States, much of it on soils like the Threebear series. These forests supported decades of commercial logging and still underpin Idaho's timber industry.

Where forest grades into the Palouse transition zone in Latah County, farmers grow dryland wheat, lentils, and peas — crops Idaho is well known for. Latah County produces more lentils than almost any other county in the country, though those fields typically sit on Palouse series soils rather than Threebear.