Casa Grande Soil Series

The Casa Grande soil series is Arizona's recognized state soil. It sits on the flat basin floors and ancient fan terraces of southern Arizona — nearly level ground built from river alluvium washed down from surrounding mountain ranges over thousands of years.

Casa Grande soil is a desert soil with a problem: salt. High heat, very low rainfall — about 7 inches a year — and poor drainage cause sodium and calcium salts to concentrate in the subsoil. That salt layer makes the soil naturally alkaline, sometimes reaching a pH of 9.6. Without irrigation, most crops cannot grow in it.

Soil scientists at the USDA Natural Resources Conservation Service selected the Casa Grande series to represent Arizona because it captures the state's defining landscape: the hot, flat desert basins of the Sonoran Desert where Arizona's agricultural history began.

The series was first described in Pinal County in 1936 and is named after the city of Casa Grande and the nearby Casa Grande National Monument — home of a large earthen structure built by the Hohokam people nearly 1,000 years ago.

Arizona's Legislature has not passed a formal law designating an official state soil. The Casa Grande series is recognized by the USDA as Arizona's representative state soil, chosen by Arizona soil scientists to represent the desert basins at the heart of the state's farming history.

If you dug into Casa Grande soil, you would start in pale, dry sandy loam — light in color and loose in your hand. A few inches down, the soil tightens into clay and darkens slightly. Deeper still, the subsoil is packed with salt crystals and calcium carbonate. The deeper you go, the more alkaline it gets.

Casa Grande soil is found on the flat basin floors and fan terraces of southern and central Arizona, in the heart of the Sonoran Desert. About 260,000 acres have been mapped, but there are likely several million acres more across central and southwestern Arizona that have not yet been fully surveyed.

The soil sits at elevations of 700 to 2,000 feet, on nearly level ground with slopes of 0 to 5 percent. The type location — the official reference site for the series — is on the Gila River Indian Reservation in Maricopa County.

In its natural desert state, Casa Grande soil supports saltbush, wolfberry, and scattered mesquite — plants adapted to heat, low rainfall, and high salt levels. These are the same plants that covered Arizona's basin floors before irrigation agriculture arrived.

With irrigation, Casa Grande soil becomes productive farmland. Water flushes the excess salt downward and allows crops to establish. The main uses are irrigated cotton, small grains, and vegetables — the same crops the Hohokam raised here nearly 1,000 years ago using an elaborate canal system.

Modern farmers in the Casa Grande Valley and the Salt River Valley have farmed this soil for over a century. Arizona's irrigated agriculture — one of the most productive desert farming systems in the world — is built largely on soils of this type.