Chapter 2⎯Soil Systematics SOIL SURVEY MANUAL 14
organized. This mainly gives us a perspective of the results of soil genesis. We only know what
is present by the techniques of measurement that are used, although we may speculate about
what has been removed, changed, added, or translocated. Profile features are combined into
models of soil formation and the processes and events of geomorphology that have influenced
and helped shape the hypothesized features. These mental processes of model building permit us
to shift readily from considering an ion in solution to the arrangement of horizons in profiles and
their stratigraphic relationships across landscapes.
Purposive sampling of soil map units depends on whether the answers or relationships we
desire are related in a meaningful way with the features of the soil map units. The actual clues
are not necessarily soil properties at all but are features of identification that we associate with
the unseen soil models. Mapping in most surveys involves delineating segments of the landscape,
cutting out geographic areas, and putting the boundaries on base maps. Tonal shades and patterns
on aerial photographs are used to indicate possible changes of vegetation, drainage conditions,
materials, and so forth. The patterns of the gray tones are used to delineate areas on maps. As we
look at the existing vegetation, we see differences of tones and composition of the species
makeup, and we verify or modify the boundary locations of the units accordingly. Configurations
of the visible surface of the land, stones, and other features are used as evidence of changes
important enough to be recognized as separate areas. Finally, the soils are examined at a few
locations to verify the models being used in the mapping process.
Soil surveys are conducted so that all the clues, features, and pieces of evidence that support
the delineations that are called soil map units are in fact surrogates for the models that have been
established. The measure of models of landscape evolution and soil formation relative to
observable landscape areas is provided by the constant testing that goes on in the soil survey.
The outdoors is a laboratory in which variability is subject to some level of systematic portrayal.
Thus the small items that are used to assist in locating, verifying, modifying, and developing soil
models are similar to the criteria used to identify the basis of differentiation in the classification
of soils.
Predictions of properties that exist in soil map units and the predictions made about the
qualities and suitabilities and responses of areas of land are all based on the relationships that
exist between the desired or expected result and the actuality that is represented by the models
used in mapping.
Many schemes have been proposed and tested for determining the composition of map units.
The same can be said for the distribution of properties that exist, or are thought to exist, in areas
of the landscape that can be delineated consistently on base maps. It is fairly well accepted that
certain features of soils and of landscapes are not in accord with existing models of distributions
in systematic and predictable ways. The frequency of random events can readily be predicted and
tested; however, the location of the occurrences associated with events is, and likely will remain,
a probabilistic phenomenon. Such aberrant features are what gives rise to most of the inclusions
in map units because their occurrence cannot be predicted and mapped with models even at
larger scales. It is the nonsystematic features that make all models approximations of what
actually takes place. The composition of map units can never be known. It can only be
approximated from samples of them.
It is common to employ transects to estimate the composition of map units. The first aspect
of composition is to identify the taxonomic components because they are the things that we have
learned to identify and recognize. These can be translated or interpreted as responses or
properties or whatever has an acceptable relationship. If results are not satisfactory or favorable,