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How Soil Surveys Are Made <br />Soil surveys are made to provide information about the soils and miscellaneous <br />areas in a specific area. They include a description of the soils and miscellaneous <br />areas and their location on the landscape and tables that show soil properties and <br />limitations affecting various uses. Soil scientists observed the steepness, length, <br />and shape of the slopes; the general pattern of drainage; the kinds of crops and <br />native plants; and the kinds of bedrock. They observed and described many soil <br />profiles. A soil profile is the sequence of natural layers, or horizons, in a soil. The <br />profile extends from the surface down into the unconsolidated material in which the <br />soil formed or from the surface down to bedrock. The unconsolidated material is <br />devoid of roots and other living organisms and has not been changed by other <br />biological activity. <br />Currently, soils are mapped according to the boundaries of major land resource <br />areas (MLRAs). MLRAs are geographically associated land resource units that <br />share common characteristics related to physiography, geology, climate, water <br />resources, soils, biological resources, and land uses (USDA, 2006). Soil survey <br />areas typically consist of parts of one or more MLRA. <br />The soils and miscellaneous areas in a survey area occur in an orderly pattern that <br />is related to the geology, landforms, relief, climate, and natural vegetation of the <br />area. Each kind of soil and miscellaneous area is associated with a particular kind <br />of landform or with a segment of the landform. By observing the soils and <br />miscellaneous areas in the survey area and relating their position to specific <br />segments of the landform, a soil scientist develops a concept, or model, of how they <br />were formed. Thus, during mapping, this model enables the soil scientist to predict <br />with a considerable degree of accuracy the kind of soil or miscellaneous area at a <br />specific location on the landscape. <br />Commonly, individual soils on the landscape merge into one another as their <br />characteristics gradually change. To construct an accurate soil map, however, soil <br />scientists must determine the boundaries between the soils. They can observe only <br />a limited number of soil profiles. Nevertheless, these observations, supplemented <br />by an understanding of the soil -vegetation -landscape relationship, are sufficient to <br />verify predictions of the kinds of soil in an area and to determine the boundaries. <br />Soil scientists recorded the characteristics of the soil profiles that they studied. They <br />noted soil color, texture, size and shape of soil aggregates, kind and amount of rock <br />fragments, distribution of plant roots, reaction, and other features that enable them <br />to identify soils. After describing the soils in the survey area and determining their <br />properties, the soil scientists assigned the soils to taxonomic classes (units). <br />Taxonomic classes are concepts. Each taxonomic class has a set of soil <br />characteristics with precisely defined limits. The classes are used as a basis for <br />comparison to classify soils systematically. Soil taxonomy, the system of taxonomic <br />classification used in the United States, is based mainly on the kind and character <br />of soil properties and the arrangement of horizons within the profile. After the soil <br />