morphology (topographic features produced by
erosion).
The use of aerial photographs to show and identify
soils is based upon your ability to recognize typical
patterns formed under similar conditions. An example
might be soil profile and weathering. Principal elements
that can be identified on a photograph and that provide
a trained observer with clues to the identification of soils
are landform, slopes, drainage patterns, erosional
characteristics, soil color or “tone,” vegetation, and land
use.
The form or cofiguration of the land in different
types of deposits is definitely characteristic and can be
identified on aerial photographs; for example, in desert
areas, characteristic dune shapes indicate areas covered
by sand subject to movement by wind.
Prevailing ground slopes are clues as to the texture
of the soil. Steep slopes are characteristic of granular
materials, while relatively flat and smoothly rounded
slopes may indicate more plastic soils.
The absence of surface drainage or a very simple
drainage pattern often indicates pervious soil. A highly
integrated drainage pattern often indicates impervious
soils that are plastic and usually lose strength when wet.
Drainage patterns tend to reflect underlying rock
structure.
The pattern of erosion often provides clues as to the
character of the soil. For instance, the cross section or
shape of a gully is controlled mainly by the cohesiveness
of the soil. Each abrupt change in grade, direction, or
cross section indicates a change in the soil profile or rock
layers. Short, V-shaped gullies with steep gradients are
typical of noncohesive soils; U-shaped gullies with
steep gradients indicate deep, uniform silt deposits.
Cohesive soils generally develop round, saucer-shaped
gullies.
The color of soil is shown on aerial photographs by
shades of gray; they range from almost white to almost
black. Soft, light colors or tones generally indicate
pervious, well-drained soils. Large, flat areas of sand are
frequently indicated by uniform, light gray color tones,
a flat appearance, and a lack of conformity; this
indicates a natural surface drainage. Clays and organic
soils frequently appear as dark gray to black areas. In
general, a sharp change in color tones represents a
change in soil texture.
The character of the vegetation may reflect the
surface soil type; however, its significance is often
difficult to interpret because of the effects of climate and
other factors. To those with local experience, both
cultivated and natural vegetation cover are good
indicators of soil type.
Knowing the use to which agricultural land is put is
often helpful in soil identification. For example,
orchards require well-draining soils; therefore, the
presence of an orchard implies a sandy soil.
FIELD OBSERVATIONS
Through the use of the various types of published
information and aerial photographs, the exploration of
a general area maybe narrowed down to several smaller
areas suitable for further investigation. The extent and
method of collecting more detailed information by field
observations depends on the time available.
Rapid ground observation along the proposed
highway or airfield location may yield valuable
information when conditions do not permit you to make
a complete or deliberate soil survey. Observe the soil
profile along the natural banks of streams, eroded areas,
bomb craters, road cuts, or other places where you see
stratified areas. Such observations may indicate types of
soil and depths of layers. Scrape off loose surface soils
before you examine and make field identification.
Samples may be taken from exposed soils for testing in
a field laboratory; however, sampling and testing are
normally at a minimum in this type of soil survey.
Surface soils may be exposed by the use of pick and
shovel, particularly in areas of questionable soils or at
critical points in the location. Soils identified in the hasty
survey may be located by field sketches or on available
maps or photographs.
METHODS FOR COLLECTING
SAMPLES
A deliberate investigation is made when time and
equipment are available and when a more thorough
investigation of the subsoil is needed than can be
obtained by hasty field observations. The two most
commonly used methods of obtaining soil samples
for deliberate investigations are test pits and test
holes.
A test pit is an open excavation that is large enough
for a man to enter and study the soil in its undisturbed
condition. This method provides the most satisfactory
means for observing the natural condition of the soil and
the collection of undisturbed samples. The test pit is
usually dug by hand; however, power excavation by
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