Soil Maps
Mapping the earth's soils is a technical and scientific challenge. Yet the results can help us understand the capability and limitation of the land to support agriculture, housing, transportation, and many other uses. The information that follows is a condensed version of a free 16-page guide entitled Wisconsin Soil Mapping, published by the SCO in 1997.
- Soil Characteristics
- Soil Classification
- Soil Surveys
- Uses of Soil Surveys
- Methods of Soil Mapping
- Digital Soil Mapping
- Further Information
Soil Characteristics
The soil hosts much of the recycling of resources critical to planetary life, including energy, water, gases, and nutrients. The soil also anchors and supplies nutrients for vegetation. Soils are composed of a wide variety of constituents. All soils originate from the same basic materials -- the solid rocks of the earth's crust (which themselves vary spatially). Rocks are weathered, transported by wind, water or glacial movement, deposited, and compacted through a series of processes which are unique to each soil.
Once in place, these building blocks of the soil, known as parent material, are changed through a series of processes that depend on living organisms, climate, topography, and time, to form the soils we observe at the earth's surface. In other words, each soil has a unique history and a unique set of characteristics.
The history of a soil is best seen in its profile, a vertical slice from the surface down into the unweathered material below. A soil profile contains several soil horizons -- layers in the soil that represent the history of biological and chemical weathering, and water movement through the different horizons.
Soil Classification
Information about soils is organized in the General Soil Classification System introduced by the U.S. Department of Agriculture (USDA) in 1960 and revised every few years. This classification system arranges the soils of the world into broad groups called orders, of which there are 11 in the United States.
Each order is subdivided into more detailed categories based on more specific characteristics. The primary classification unit is a soil series. There are over 10,500 different soil series in the United States alone, although only about 580 are mapped in Wisconsin.
Soil Surveys
Soil surveys classify, map, and describe soils as they appear across the landscape. Soil surveys can be performed at several scales, with the county-wide soil survey being the most common. The Natural Resource Conservation Service (NRCS) (formerly the Soil Conservation Service) is the division of the USDA that is in charge of the completion of modern soil surveys of every county in the United States. At this time, most counties in the U.S. have had soil surveys completed, although earlier surveys need updating.
Modern soil surveys are comprised of three basic components: text, maps, and tables. The text of the survey provides general information on the climate, geology, and topography of the area being covered, usually a county. A typical county soil survey contains two types of maps: one general overview soil map for the entire county and a series of detailed soil maps providing complete coverage of the entire county on a larger scale. Soil surveys also contain tables summarizing relevant climatological data and the approximate total area of each mapped soil unit.
Soil Surveys are classified into survey orders, on a scale from 1st to 5th. A first order survey is the most intensive (at a scale larger than 1:10,000) and a fifth order survey the most generalized, usually in areas where the soils have never been mapped (often at a scale of 1:1,000,000). In Wisconsin, the NRCS' National Cooperative Soil Survey is second order.
Recent priorities for the Wisconsin NRCS have been to complete soil surveys for all counties, and to participate in mapping the Major Land Resource Area 105, which includes 18 counties in the "loess hills" of southwest Wisconsin. (During the 1960's the Dept. of Ag. divided the U.S. into Major Land Resource Area (MLRA's), defined on the basis of climate, land use, soil distributions, and water resources.) These programs, along with a survey of the 10 northwestern-most counties represent the focus of NRCS field activities in Wisconsin in recent years. As of spring 2000, the NRCS and several state agencies have supported a major initiative to complete digital soil surveys for all Wisconsin counties by 2006.
For information on which counties have completed soil maps, contact the NRCS Wisconsin Office.
Uses of Soil Surveys
Soil surveys are valuable tools for land management and planning. The utility of a soil survey depends largely on three things: the accuracy with which soil properties are measured and mapped, the relevance of those properties to the user's purpose, and the scale (or level of detail) of mapping.
A soil survey assists agricultural consultants, engineers, federal, state, and local planning and development agencies, private investors, and others involved in decision making regarding land use and management. The accuracy of a soil survey map is dependent on the accurate field interpretation of the mapped units, the accurate delineation of map unit boundaries, and the accurate transfer of line work and map unit symbols to the base map, known as compilation.
Methods of Soil Mapping
Generally speaking, a soil survey consists of four operations:
- identification of soils to be mapped in the county
- development of map unit descriptions
- field work
- compilation
Aerial photography serves two main purposes in the soil mapping process. For one, in Wisconsin all new NRCS soil surveys are published on orthophotoquads. Secondly, aerial photointerpretation of landforms, vegetation, land use, photo tone, and relief can greatly reduce the amount of costly field work required to accurately set soil map unit boundaries. It is commonly used to draw provisional soil boundaries in the office prior to field investigations.
The field mapping stage has traditionally been the most time consuming, labor intensive, and costly component of the job. In the field, soil scientists establish relationships between soil properties and surface features and determine the soil series/phases to be mapped. They must differentiate the map unit boundaries that can be accurately transferred from the earlier photointerpretation from those that must be established through ground investigation. Map unit boundaries are then sketched directly on the aerial photographs.
The map compilation procedure consists of the accurate transfer of map unit boundaries, symbols, drainage, and cultural features from the field sheets to orthophoto base map overlays. During the final compilation of the survey, correlations with map units of adjacent counties are checked and adjusted. Once the maps and survey reports are reviewed and edited, the county soil survey is ready for publication.
Digital Soil Mapping
Digitizing modern soil survey data is an essential step toward the creation of a comprehensive land information database, the backbone of an automated GIS or LIS. Once in place, such a system can greatly increase the ease in which needed soil information can be accessed and applied to land use and resource management problems.
The NRCS is responsible for the national coordination and certification of digital soil data. As of spring 2000, twenty of Wisconsin's 72 counties already had digital soil surveys that met NRCS standards, and another 14 counties were already scheduled to by completed by 2004. A cost-share agreement between the NRCS, the Wisconsin Land Information Board (WLIB), and the Wisconsin Dept. of Administration (DOA) will cover complete soil survey mapping, conversion of existing maps to digital form, and NRCS certification for the remaining 38 counties. Statewide digital soil survey coverage is expected to be achieved by June 2006.
Visit the USDA - Natural Resource Conservation Service to find out where each county stands in this process.
Further Information
Most of the soil maps and associated data for Wisconsin may be obtained from the NRCS. The Wisconsin Geological and Natural History Survey (WGNHS) also has a collection of maps portraying Wisconsin's soils. The most recent is entitled "Soil Regions of Wisconsin"(223kb) and was published in 1993.
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