Penistaja Soil Series

Penistaja soil formed over thousands of years from sandstone and shale — the compressed remains of an inland sea that covered the Southwest during the Jurassic and Cretaceous periods. Wind and water eroded that ancient rock and deposited the resulting material on the Colorado Plateau's mesas and slopes.

The defining feature of Penistaja is the argillic horizon: a clay-rich layer a few inches below the surface. That clay holds moisture from summer monsoon thunderstorms, giving grasses and dryland crops enough water to survive in a climate that sees only 10 to 14 inches of rain per year.

Soil scientists at the USDA Natural Resources Conservation Service selected the Penistaja series to represent New Mexico because it captures the state's most distinctive geological landscape: the sandstone and shale mesas of the Colorado Plateau in the northwest.

The series is named after Penistaja Wash, a dry creek bed in San Juan County. The type location — the official reference site — was established there, where the soil was first formally described and classified.

The Penistaja series is recognized by the USDA as New Mexico's state soil, chosen to represent the Colorado Plateau landscapes that define the state's northwest corner.

Dig into Penistaja soil and you start in a pale reddish-brown sandy loam — loose and dry at the surface, bleached by high desert sun. A few inches down, the soil tightens into a sticky clay loam as the argillic horizon begins. Deeper still, the subsoil turns pale tan streaked with white calcium carbonate. At 30 to 40 inches, you hit shale or weathered sandstone.

Penistaja soil is found on the Colorado Plateau in northwestern New Mexico — on the flat tops and gentle slopes of sandstone mesas, on cuesta backslopes, and along ridgelines carved from ancient sedimentary rock. The terrain is open and windswept, at elevations of 5,500 to 7,500 feet.

The soil is concentrated in San Juan and McKinley counties, the heart of New Mexico's Colorado Plateau country. It also appears in Rio Arriba, Sandoval, and Cibola counties to the south and east.

Penistaja soil supports piñon pine and one-seed juniper woodland — the signature forest of New Mexico's mesa country. Blue grama, galleta grass, and big sagebrush fill the open ground between trees, forming the rangeland that has supported Navajo and Pueblo cattle and sheep for over a century.

Where rainfall is slightly higher, dryland farmers grow winter wheat and pinto beans in Penistaja soil. The argillic clay layer holds enough monsoon moisture for small grain crops to reach maturity without irrigation.

At the highest elevations near 7,500 feet, ponderosa pine grows in the cooler Penistaja sites. These areas are used for limited timber harvest and provide habitat for mule deer, pronghorn, and wild turkey.