Holdrege soils are identified by horizontal profile layers created by the prairie environment. They cover 1.8 million acres in central Nebraska on a landscape shaped by geologic processes over tens of thousands of years. Formed under mixed grasses in a climate where moisture stress is common, they also experienced enough water movement through the profile to carry clays and lime downward. This produced thick, dark-colored topsoil, clay-enriched subsoil, and a substratum containing free lime. Holdrege soils were first identified during the soil survey of Phelps County in 1917. Their natural fertility, desirable tilth, and the landscape they occupy, together with irrigation and skillful farm management, make them a valuable agricultural resource. In 1979, the legislature designated Holdrege Silt Loam as the Nebraska State Soil, a soil Nebraskans are urged to conserve to sustain agriculture for generations to come.