The U.S. Soil Survey program began in the late 1890s, with Iowa’s first survey conducted in Dubuque County and published in 1903. Soil surveys were typically updated every 30 years, and all 99 Iowa counties received at least one update in the 20th century. However, the last county to receive a full update was in 2013. Some counties have not received a full update since 1968.
Efficiency for mapping soil landscape details improved in the 1930s with the introduction of aerial photography maps. This innovation supported the shift from 1 inch per mile to 4 inches per mile in soil maps. The original soil maps were limited to only representing differences in soil driven by soil parent material (geologic deposit). The aerial photographs enabled the inclusion of more differences resulting from topography.
In the mid-1960s, Iowa launched an accelerated effort to map all counties, with support from the USDA Soil Conservation Service (now the Natural Resources Conservation Service, or NRCS), the Iowa Department of Soil Conservation (now part of the Iowa Department of Agriculture and Land Stewardship, or IDALS), Iowa State University, and local counties. Costs were shared among federal, state, and county governments.
The second generation of the U.S. Soil Survey was completed using aerial photography at a scale of 1:15,840. Mapping a 16-township county required approximately 12 person-years. After the 1990s, traditional soil maps were produced at a map scale of 1:12,000 (5.28 inches per mile). However, most counties in Iowa have not undergone a complete soil survey since 1990.
At the heart of the second generation of traditional soil maps were five components:
Mapping involved delineating soil boundaries on orthophotographs. Each delineation was labeled with a map unit symbol, such as 138C2, where “138” identifies the Clarion soil series, “C” indicates a 5–9% slope, and “2” denotes moderate erosion (3–7 inches of topsoil remaining). Additional symbols mark features such as wet depressions or small, contrasting areas of soil.
Classification grouped soils using Soil Taxonomy, a national system with 12 soil orders. Iowa contains six soil orders: Alfisols, Entisols, Histosols, Inceptisols, Mollisols, and Vertisols. Mollisols and Alfisols are the most common.
Correlation ensured consistent use of soil series names across regions. For instance, Clarion soils exhibit the same defining properties regardless of their location.
Interpretation predicted how soils will behave under various uses or management practices, based on their properties.
Publication compiled all survey information, including maps, soil descriptions, classifications, and interpretations. Reports were digitized, and older hard copies were scanned by 2007. Surveys were then available as DVDs, CDs, or through the ISU Extension Online Store.
Soil surveys have evolved significantly, becoming more detailed and accurate due to advances in data collection, geospatial technologies, and advancements in soil science knowledge, which enhance interpretation. Classic soil mapping has relied on five factors of soil formation to predict variations in soil properties: climate, organisms, topography, parent material, and time. Today, human activity is often considered a sixth factor due to its influence on soil change. Updates to soil surveys are essential as new technologies, such as LiDAR and enhanced remote sensing, improve our ability to monitor, continue to deepen our understanding of soil systems.