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					                                                    The Variability of the Boundary Line
                                                    Between Soil Survey Mapping Units.
                                                                                                 Dustin Hancock

Why is this research important?                                      Methods                                                                                                       Results
The accuracy of a soil survey’s mapping unit properties is          Two map units were selected for study in Jefferson County, West                                                 From the properties (texture, hue, value, chroma, textural class,
vital in determining the appropriate agricultural management        Virginia. These map units were located on Tabler Farm, which is                                                 pH, and horizonal depth and thickness) analyzed, the boundary
practices, forestry techniques, or community planning
                                                                    owned by Shepherd College (Fig.1). The FbB mapping unit was                                                     line between mapping unit FbB and DgB established by the soil
strategies. Inaccuracy in a soil survey's mapping unit              composed of soil from the Frankstown taxonomic series, which                                                    survey was determined to be outside of the transect sampling area
properties may have a negative impact on the health of the          is characterized by deep and very deep well drained, moderately                                                 or could not be determined due the transition of soil properties
soil and the economic viability of the operation taking place       permeable soils formed in residual materials derived from                                                       over an area larger than the transects used in the study.
on it.                                                              siliceous limestone and interbedded limy shale. The DgB                                                         Frankstown series percent accuracy for averaged transect data of
 Introduction                                                       mapping unit was composed of soil from the Duffields
                                                                    taxonomic series, which is characterized by deep to very deep
                                                                                                                                                                                    the Ap horizon for 9 out of 10 points was 37.5% (Fig. 3). The Bt1
                                                                                                                                                                                    horizon increased in accuracy from 81.25% at 10 feet from the
Soil surveys are maps and data that give a range of                 well drained, moderately permeable soils formed in residuum                                                     boundary established by the soil survey, to 100% at 90 feet from
properties for a particular soil taxonomic unit, with the aim       from limestone bedrock Two transects were selected that ran in                                                  the boundary line in the Duffields DgB mapping unit (Fig. 4).
of determining the spatial extent of map units with similar         a southeasterly direction from the top of a slope where mapping                                                 Duffields’ series percent accuracy for averaged transect data of the
taxonomic properties within a landscape. The most                   unit FbB was located, to the bottom of the slope where mapping                                                  Ap and Bt1 horizons showed a increase from 75% at 10 feet from
meaningful soil maps constructed are those that diminish            unit DgB was located. These transects were 180 feet long with                                                   the established boundary for both the Ap and the Bt1 horizons, to
internal heterogeneity within mapping units. This is                approximately 90 feet of each transect being located within each                                                94% and 100% respectively at 90 feet from the boundary line in
accomplished through the conveying of the central                   mapping unit (Fig.2). Samples                                                                                   the Duffields’ DgB mapping unit (Fig. 3 & 4).
tendencies and variations around central tendencies of the          were taken along transects at 20
soil properties within a mapping unit. The accuracy of soil         feet intervals with a total of 10                                                                              represents the mapping unit
                                                                                                                                                                                                                                            Percent Accuracy of Frankstown and Duffields
                                                                                                                                                                                                                                            Series for Averaged Ap Horizon Transect Data
                                                                                                                                                                                                                                                                                                                 Percent Accuracy of Frankstown and Duffields
                                                                                                                                                                                                                                                                                                                 Series for Averaged Bt1 Horizon Transect Data

                                                                                                                                                                                                                                                                                           within series range
                                                                                                                                                                                                                      within series range

                                                                                                                                                                                                                                                                                            % of properties
                                                                                                                                                                                                                       % of properties
                                                                                                                                                                                                                             100                                                                  110

surveys varies greatly depending on the sampling methods                                                                                                                           boundary set by the soil                   90
                                                                                                                                                                                                                              80                                         Frankstown Ap
                                                                                                                                                                                                                                                                                                   90                                       Frankstown Bt1

                                                                    sample holes per transect. At                                                                                  survey.                                    70
                                                                                                                                                                                                                                                                         Duffields Ap
                                                                                                                                                                                                                                                                                                   60                                       Duffields Bt1

and techniques used to construct the map, along with the            each hole samples were taken                                                                                   represents samples that fit soil
                                                                                                                                                                                                                              10                                                                   10

resources available and the needs of the users in the area.         of the Ap and Bt1 horizons. Soil
                                                                                                                                                                                   survey description with a                   0
                                                                                                                                                                                                                      Top of Slope 90 70 50 30 10 10 30 50 70 90 Bottom of Slope
                                                                                                                                                                                                                      (Frankstown)                                (Duffields)
                                                                                                                                                                                                                                                                                           Top of Slope 90 70 50 30 10 10 30 50 70 90 Bottom of Slope
                                                                                                                                                                                                                                                                                           (Frankstown)                                  (Duffields)
                                                                                                                                                                                   85% accuracy.                                                Feet                                                                 Feet

Map orders, which are determined by the level of detail,            observations and samples were                                                                                                                           Figure. 3                                                        Figure 4

range from a high of 1:1000-1:15,000 to a low of
1:250,000-1:10,000,000, with second-order county soil
                                                                    taken with a soil auger. Field         Fig.1 tabler Farm,Shepherdstown. (
                                                                    observations included soil                                                                                       The fact that a boundary line between mapping units, which
surveys published by the NCSS being the most common.                structure, color, and horizonal                                                                                  correlated with the boundary established by the soil survey,
These second order surveys are georeferenced orthophotos            depth and thickness.All samples                                                                                  could not be discerned suggests that the soil landscape is highly
with a scale of 1:20,000 and a minimum size delineation of          were analyzed in the Shepherd                                                                                    variable, and only a small portion of that variability is
1.6 ha. Limitations of these surveys include presence of            College Soils Laboratory for                                                                                     represented in soil survey reports. The accuracy of mapping unit
inclusions, lack of mechanisms to quantify spatial                  color (hue, value, and chroma),                                                                                  boundaries in the Jefferson County Soil Survey may limit its
variability, and uncertainty of the accuracy of the                 pH, and textural class.                                                                                          effectiveness for technical users, but it is an invaluable tool for
taxonomic mapping unit boundaries. This study addressed             The percent accuracy for both                                                                                    non-technical users, such as farmers and homeowners. One of
the accuracy of mapping unit boundaries in order to                 the Frankstown and Duffields                                                                                     the main factors in determining the accuracy of a soil survey is
                                                                                                        Fig. 2. Transects used in the study. (Jefferson County soil Survey, 1973
determine if taxonomic classes adequately define variability        taxonomic series was                                                                                             the needs of the users in the area. When the Jefferson County
within and among soil mapping units.                                determined by the number of soil properties (texture, hue, value,                                                Soil Survey was developed in 1973, it met the needs of the large
                                                                    chroma, textural class, pH, and horizonal depth and thickness) that                                              number of farmers in county. Now with the large influx of
  Research Objectives                                               were found within the ranges established by the soil survey at each
                                                                    point along the transect. Percent accuracy for the two soil series were
                                                                                                                                                                                     development in the county, new surveys may need to be
                                                                                                                                                                                     developed that characterize map units statically to the fullest
  •To determine the variability of the boundary line between
                                                                    compared over the transects to determine where the boundary line                                                 extent possible.
  Jefferson County Soil Survey mapping units FbB and DgB.
                                                                    between the two series was located.
  •To determine if the boundary line between the soil mapping                                                                                                                        Research Conclusions
  units is within 99 feet of the boundary line established by the                                                                                                                     •The soil landscape is highly variable, and only a small portion
  soil survey.                                                                                                                                                                        of that variability is represented in soil survey reports.
                                                                                                                                                                                      •Technical users of the soil survey need more detailed soil maps.

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