Soil Properties of the Arizona Rangelands

Soil Parent Materials

• Arizona is located in two geographic regions: deserts and arid mountains of the Basin and Range Province of the southern Rocky Mountains and, above the escarpment at the Mogollon Rim, the Colorado Plateau. The deeply folded Basin and Range strata, or thick horizontal layers of bedrock, include a wide range of volcanic, metamorphic and sedimentary rocks, and the sediment fill in the basins was eroded from the continent, rather than settling from shallow seas. The Colorado Plateau, including Black Mesa, is thick layers of sedimentary rocks, eroded to plateaus and cut by canyons, and includes some ancient volcanic fields.

Basin and Range Soils

• Arizona Basin and Range Province soils vary as widely as their parent materials. In the mountains, granite and volcanic rocks weather to sand and fine gravel, and degraded granite breaks apart without chemical change. Moved downstream by wind or water erosion, shallow granitic soils have no profile development, or horizontal layering and weathering. Volcanic rocks are softer and can weather to fine-grained soil with well-developed subsoil horizontal layers. At higher and moister elevations, older soils are more acidic, with a higher organic content than soils at lower elevations. Soils on old sediments washed down to the base of mountains have clay subsoils and layers of calcium carbonate that can become as hard as rocks.

Colorado Plateau Soils

• In the Grand Canyon section of the Colorado Plateau, soils are mostly shallow to only moderately deep, because of ongoing collapse from the canyon walls. In the Navajo section, which includes the drainage of the Little Colorado River, soils that formed over sandstone and shale sediments in washes and canyons are generally shallow and lack soil profiles, or horizontal layering. The Painted Desert section is mostly shale badlands, with a few soils that are shallow to deep and lack horizontal layering. On the Defiance Plateau and in the Carrizo-Chuska Mountains of the Navajo section and along the New Mexico border, soils formed on sandstone can be shallow to moderately deep, without horizon development or with moderate clay horizons.

Diversity

• The diversity of Arizona rangeland prevents detailed descriptions of specific soils and sites. For example, just within the 3 million acres of Tonto National Forest, the land varies from 1,300 to 7,900 feet in elevation and from cactus desert to mountains with pine forests. The Forest Service leases 104 grazing allotments among six Ranger Districts within the forest, and allows sustainable grazing on land as diverse as deserts to open grasslands and river valley drainages.