4.2: What is a soil survey?
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A soil survey is a detailed inventory of the soil resources within an area. It consists of a set of soil maps and a book that describes, classifies, and interpret soil data for various land uses. It is developed by professional soil scientists who examine the soil in detail to a depth of two meters, or to rock if shallower. The soil is then classified according to a national system of soil taxonomy. The location of each kind of soil is plotted on aerial photographs. Each soil is evaluated for its suitability for various uses and management.
Soil surveys are made in the following manner: air photo reconnaissance flights are made of the area to obtain stereoscopic map photos. These photos permit one to see artificial and natural land features. In preparing a soil survey, the scientists familiarize themselves with the various soils in the area by digging many holes to expose and carefully examine the soil profile in its natural state. As they travel around the survey area, they observe steepness, the length and shape of slopes, the size and speed of streams, the kinds of trees, plants and crops, the kinds of rock exposed, and many facts about the soils.
Soil scientists compare the soil profiles in the survey area with soils in nearby counties and states. They correlate, classify and name the different soils according to the National Cooperative Soil Survey as set forth in the United States Department of Agriculture (USDA) Agricultural Handbook No. 18, Soil Survey Manual (August, 1951). Details of the classification are presented in the book Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys written by the Soil Survey Staff of the Soil Conservation Service (now the Natural Resource Conservation Service). It was published in December of 1975 as USDA Agricultural Handbook No. 436.
Once the soils have been named and their distinguishing properties identified, the soil scientists return to the field to examine all soils in the survey area. They delineate areas of similar soils directly on the aerial photographs. While the field work is in progress, other soil samples are collected and analyzed to determine chemical, physical, and mineralogical properties. The mass of detailed information about the soils needs to be organized and interpreted in a readily useful way to different groups of readers.
For many years, farmers and ranchers have used soil surveys to determine the capability of the soil to support certain kinds of crops, grasses, and trees. Also, soil surveys have served as a basis for applying needed soil and water conservation practices.
After studying the soils in an area, it is possible to make a general map showing several main patterns of soils (called soil associations). As a rule, each soil association contains a few major soils and several minor soils in a characteristic, although not strictly uniform, pattern. The soil maps are assembled and examined for continuity of soil boundaries from one photo to another and the other information is compiled into a written report.
The written report and the aerial photographs with the soil boundaries are sent to the U.S. Government Printing Office for publication. This entire process of obtaining aerial photos, field mapping, correlation and interpretation of the data, writing of the report, and subsequent publication takes several years for completion.
Soil surveys produced in this manner cost taxpayers $500 to $750 per square mile. Large expanses of land that are of little direct value for agricultural or forest uses often are too poor to justify this high cost. These low value areas are mapped on a small scale and the resulting study is called a reconnaissance survey. The reconnaissance survey identifies areas and regions dominated by soil associations in contrast to the detailed map where specific soils are delineated.