The soil survey program in the United States began in the late 1890s. The first soil survey in Iowa was of the Dubuque County area. Fieldwork was completed in 1902, and the report was published in 1903. The “life” of a soil survey is estimated to be about 30 years, so all 99 Iowa counties have had multiple surveys.
Beginning in the mid-1960s, Iowa began an accelerated effort to map all counties in a short time. The agencies involved were the USDA Soil Conservation Service (now USDA Natural Resources Conservation Service), Iowa Department of Soil Conservation (now Division of Soil Conservation, Iowa Department of Agriculture and Land Stewardship) Iowa State University through the Experiment Station and Cooperative Extension Service and the counties. Costs were shared equally among federal, state, and county.
Most counties were mapped at a scale of 1:15,840 (4 inches = 1 mile) on an aerial photo base. Approximately 12 person-years were required to map a 16-township county.
Soil surveys have gone through many phases over time. Since the initial surveys, our knowledge of soils has increased, methods of collecting and presenting data have improved, and the present surveys are more detailed and accurate. However, soil surveys have always followed the same procedures of mapping, classification, correlation, interpretation, and publication.
Mapping is the delineation of soil boundaries on a base map which at the present time is an orthophotograph. A soil map of a tract of land is shown in Figure 1. Each polygon on the map is called a delineation and contains a number to identify the soil, a letter to define the slope group, and, if needed, a number that identifies the erosion phase. The number 2 is used for moderately eroded, which is interpreted as 3 to 7 inches of topsoil present. The number is 3 for severely eroded, indicating that there is less than 3 inches of topsoil present and mixing of subsoil material has occurred because of tillage. If no number is present, the soil erosion phase is none or slight, indicating that more than 7 inches of topsoil is present. All delineations containing the same set of symbols are called a map unit, for example, 138C2. The number 138 identifies the Clarion soil series. In Iowa, a statewide legend is used to identify soil series, each with a unique number. The letter C identifies the slope group as 5 to 9 percent, and the number 2 identifies the area as moderately eroded. Other symbols on the map show drainageways and contrasting soil areas that affect soil use but are too small (generally less than 2 acres) to show as a separate delineation. For example, the small circle with a plus sign indicates a wet depression with restricted permeability is present in a 107 (Webster) delineation in the upper middle of Figure 1.
Classification is the systematic arrangement of soils into groups or categories. The present system of soil classification used throughout the United States and many other countries is Soil Taxonomy. The lowest category in the system is soil series which in the above example is Clarion. The highest category is order. Soil Taxonomy includes 12 soil orders. Six of the 12 orders are mapped in Iowa. These are Alfisols, Entisols, Histosols, Inceptisols, Mollisols, and Vertisols. Mollisols and Alfisols are the dominant orders occurring in Iowa.
Correlation is a nationwide process to ensure that soil series names are defined and used consistently. For example, a soil named Clarion has the same set of soil properties as a result of the impact of a particular set of soil-forming factors wherever the Clarion name is used.
Interpretation describes the prediction of soil behavior for specific uses or management based on inferences from soil properties. They may be either qualitative or quantitative estimates of soil behavior.
Publication consists of the compilation of soil information of a survey area to include descriptions, properties, classifications, interpretations, and maps. Publications were available in hard copy and digital format. Starting in 2007, scanned copies of county soil surveys are available here.
In addition, county soil survey reports are available on DVDs or CDs that can be obtained at soil and water conservation district offices or the Iowa State University Extension and Outreach Online Store.
Additional soil and land use information, plus a state-wide soil database named the Iowa Soil Properties and Interpretations Database (ISPAID), are available.
A great advance in contributing to the improvement of soil surveys was the use of aerial photographs. They came into common usage in the late 1930s. Their use greatly increased the precision with which soil boundaries could be delineated on maps. Another important variable is the scale at which the soil map is made. Early surveys were made at a scale of one inch per mile. Beginning in the late 1950s and continuing until 1990, the most common mapping scale was 4 inches per mile. Since 1990, the scale used is 1:12,000, which is 5.28 inches per mile.
Understanding soil, its development, and its properties are based on a knowledge of the five classic factors of soil formation: climate, organisms, topography, parent material, and time. Because of the intensive use of the soil, human activity is considered by many to be a sixth soil forming factor. The need for updates of soil surveys will continue as our knowledge of the interactions of the above factors continues to increase with the introduction of new technologies, including improved remote sensing techniques, Light Detection And Ranging (LiDAR), smartphones, and many other innovations.
Figure 1. A soil map showing soil map symbols, drainage, and spot symbols.
Prepared by Thomas E. Fenton and Gerald A. Miller, Emeritus Professors of Agronomy, Iowa State University. This article is a modified version of the original article published in the Iowa Water Center 2011 report: Fenton, Thomas E. 2011. The Soil Survey in Iowa. pp. 24-25 in Getting Into Soil and Water. Iowa Water Center, Iowa State University, Ames. 40 pages.