Forkwood Soil Series

Forkwood soil sits on the flat to gently rolling upland terraces and plains of eastern Wyoming — the semiarid grassland country between the Black Hills and the Rocky Mountain foothills. It formed in alluvium and eolian material deposited on old terrace surfaces, then slowly built its dark surface layer from centuries of shortgrass prairie root decay and organic matter accumulation.

The soil is classified as an Aridic Argiustoll — a Mollisol with an argillic clay subsoil that is dry for significant parts of the growing season. The defining feature is the mollic epipedon: a dark grayish brown surface layer at least 7 inches thick, darkened by organic carbon from decomposed grass roots. Below it, the subsoil grades into a brownish clay loam argillic horizon before reaching a calcium carbonate accumulation zone and the lighter alluvial parent material.

Forkwood soil is moderately alkaline at depth, with pH typically between 6.5 and 8.5 depending on horizon. It is well-drained, with water moving through the loamy surface and into the subsoil without pooling. Annual precipitation in Forkwood soil territory averages 12 to 16 inches — enough for native grasses and dryland wheat, but not for irrigated row crops without supplemental water.

The Forkwood series is named for Forkwood Draw, a drainage feature in eastern Wyoming where USDA soil scientists first described and mapped the series during federal soil surveys of the region. The name follows the USDA convention of naming soil series after nearby geographic features — creeks, draws, ridges, and towns — in the area where the series was first formally described.

The Soil Science Society of America recognized Forkwood as Wyoming's state soil because it represents the dominant productive soil type across the state's eastern plains — the agricultural and ranching landscape that has defined Wyoming's land use since the late nineteenth century. Forkwood and related Mollisol series underlie the dryland wheat belt and the cattle rangeland that remain central to Wyoming's economy.

Forkwood was chosen over the state's many Aridisol and Entisol series because its mollic epipedon sets it apart: the dark, grass-built surface layer is a visible record of the native shortgrass prairie that once covered eastern Wyoming from horizon to horizon. No other feature better connects Wyoming's soil to its landscape history.

Digging into Forkwood soil reveals a profile written by grass. The dark surface is thick and granular — built from thousands of years of shortgrass roots decomposing into organic carbon. Below it, a pale transition zone gives way to the brownish argillic subsoil, where clay has accumulated and iron compounds give the layers a warm color. Deeper still, white calcium carbonate accumulations mark where mineral-rich water evaporated before reaching the lighter alluvial parent material below.

Forkwood soil is concentrated in the eastern third of Wyoming — the upland terraces and plains east of the Laramie Range and north of the North Platte River. It is most extensive in Converse, Niobrara, Goshen, and Platte counties, where the terrain is flat to gently rolling at elevations between 4,500 and 6,000 feet.

The soil extends north into Campbell and Weston counties where similar terrace surfaces and grassland conditions persist. It appears on the broad interfluve ridges between stream drainages — the high, flat ground that drains slowly and has accumulated deep alluvial and eolian material over millennia.

Dryland winter wheat is the primary crop on Forkwood soil. Eastern Wyoming's 12 to 16 inches of annual precipitation sits near the minimum threshold for dryland wheat, and Forkwood's well-drained loamy surface and moderate water-holding capacity make it one of the better dryland crop soils in the region. Goshen and Platte counties produce the largest wheat volumes, with some fields on Forkwood soil rotating into fallow years to conserve moisture.

Cattle rangeland covers the majority of Forkwood soil acreage. Blue grama, buffalo grass, western wheatgrass, and needle-and-thread grass are the native species that built the mollic epipedon and still dominate Forkwood rangeland. The well-drained surface supports livestock grazing without the muddy conditions found on heavier clay soils in wetter parts of the state.

No significant tree cover grows naturally on Forkwood soil. Cottonwood and willow appear only in draws and along creek banks where soil moisture is supplemented by run-in from surrounding uplands. On dryland farms, shelterbelts of Russian olive and green ash were planted in the twentieth century to reduce wind erosion on the flat terrace surfaces.