The soil scientist, when making a soil map, determines
the boundaries between different soils based on the soil forming factors. It is
the job of the soil mapper to differentiate the various landscapes into
the landform components.
By understanding
how landforms will influence soil development the soil scientists can
separate landform positions on the landscape that will have different soils.
The goal of the soil mapper is to draw a line around all of the similar soils.
Each soil profile that has the same horizons are said to belong to the same
soil (same soil series) or they will be in the same soil mapping unit.
By inspecting various soil
pedons, the soil mapper can determine where the lines between soil mapping
units needs to be drawn. The soil pedon is the one member of the set of
polypedons that make up the soil mapping unit. 
However, drawing a line around all the same pedons is difficult because of
the scale of mapping and this is why a soils
map is not the same as a road
map.
The units the soil scientist used to delineate the different soils are called
map units and can consist of one or more
soil series (but mostly just one). The map units are identified by
map symbols. Each soil mapping unit
will contain soils that are perfect examples of the named soil, (central
pedons) and a few soils that are just like the central pedon but maybe have a
slight difference (non-central pedons).
The soil mapping unit will also contain some soils that are called inclusions, or these soils are very different from the named soils, but because of the scale of mapping or the complexity in which soils occur across the landscape they can not be separated out from the named soil. These inclusions should make up only 25% or less of the soils inside the map unit boundary.
In some cases, the soils are so complex that two soils are named in the map
unit in order to accomplish naming 75% of the soils. An example would be the
Lester-Storden -Estherville complex, 2-6% slopes. Inclusions can be similar
(have similar land use properties as the named soil) or dissimilar inclusions,
that have different land use interpretations from the named soil..
If we wanted our map to be precise we would need to use a scale of 1:1, but of course that is not practical. The larger the scale of mapping ( large scale > 1:20,000) the more difficult it is to accurately depict the soil differences on the landscape and the more inclusions contained in each mapping unit.
Below is a sample of a detailed soil map from map sheet 24, just above Lake Washington. This map is an example of a detailed soil map. Instead of general soil patterns, the detailed map shows the nearest practical approximation of the shape and aerial extent of each kind of soil. Because of base map limitations, it is not feasible to show on a detailed soil map the exact boundaries of every kind of soil that might be there, or to show soils of less than 2 acres in area. Rather, the objective is to record those soil types that will help the user make land use decisions that involve from two to several hundred acres.
A soil series consists of soils that are almost alike except for differences in slope, stones, texture of surface, texture of the underlying materials, and a few other characteristics that affect their use. On the basis of such differences, a soil series is divided into soil phases. Most of the areas shown on the detailed soil map are phases of soil series. Soil series are given a name for some feature where they were first identified, or the Lester Series is named for Lester Prairie, a small town west of the Twin Cities.
On Map Sheet 24 from the Le Sueur County Soil Survey,
the map symbol 106B was identified by the finger. From the soil legend on the
back of the index map sheet for Le Sueur County, 106B is identified as Lester
loam, 2 to 6% slope. 106 is the number for the Lester soil
series. B is the slope category; 2-6% is the range of slope for B
slopes. North on the map is up, and north of 106B are map units 106C2 and 106D.
Or the Lester soil with slopes of 6-12% for C and 12-18% for D. The area is
getting steep as you go northwest from the lake. The map unit 183 is between
the lake and map unit 106B. Map unit 183 represents the soil Dassel Loam.
To learn about slope go to Slope Classes and Slope Diagram
The soil scientist's job was to map the soil boundaries on the aerial photograph using the soil legend as a guide for the kinds of soils in the area. The map is one component of the soil survey.
The other components of the survey can be looked at using the Sibley County
Soil Survey which will be available in the lab. Be sure to investigate the: 1)
Soil Series information and 2) Tables on selected Soil Interpretations in the
SIbley County Soil Survey or go on line to the WEB Soil Survey for Sibley
County http://websoilsurvey.nrcs.usda.gov/app/
Soil Survey Chapter 3
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