nly has this
1y parts for
arge browse
the rougher
se are very
ive by their
in order to
tive growth
ction rarely
ly support
are difficult
the various
rom photo-
waste areas
iceous vege-
olor photo-
'e a charac-
lany grasses
nany decid-
listinguished
nd interpre-
‚takem at a
'ences, more
amount of
th lenses in
at various
interpret.
d problems,
, “The Use
on of each,
must be di-
ork in soils
ly from the
ong narrow
in soils has
purpose of
1e field and
on basis for
n the other
ely limited,
(667)
not so much because of the lack of information, but because it has been develop-
ed chiefly by corporations in a competitive field and, therefore, the information
that they develop is in the class of “trade secrets.”
Consequently, there is a great deal of information on soil inventories and a
relatively small amount on mineral inventories, and this is confined to the ob-
vious quantitative features that are common knowledge to experienced inter-
preters.
Almost universally, soils are mapped on the basis of soil associations or
land forms, the two being the same, differing only slightly in concept and ter-
minology. Although the applications are widely distributed throughout the soils
engineering field, soil conservation and pedological soil mapping, the funda-
mentals differ only slightly. Everyone meets on common ground in using topog-
raphy and slope (land form) as an important feature. Second in importance is
the type and amount of drainage that develops on the area under study. Third-
ly, tone of the soil, where it is feasible to observe it, and the type of vegetation
is significant. Soil erosion characteristics and land use are also used. These all
point to the determination of physical properties of the soil mass. This, in gen-
eral, describes the texture of the soil. When this degree of identification has been
established, any refinements beyond this are necessarily based upon skill and
local experience of the interpreters and their knowledge of soil science.
In several organizations, very sensitive shades of refinement are being used
to identify such additional characteristics as topsoil, texture, degree of stoniness,
parent material characteristics, and chemical properties based upon inference
and known relationships with other ground-tested soils. It is the consensus of
all that aerial photography cannot completely replace ground reconnaissance;
especially in the early stages of training of an interpreter, a large amount of
time in the field is necessary, working with the photographs and exploration
instruments. The fact that a large percentage of the people who use aerial photo-
graphs make only casual use of the stereoscopic coverage, is a continuing source
of amazement.
The refinements in interpretation are often related to local or regional
conditions. These are greatly influenced by the origin of the soil materials and
the climatic influence. For example, in some of the areas reported, it is possible
to segregate the areas of soil in which the alkali content varies from 1% to 3%
to 5%.
Interpreters are predicting physical conditions at various depths beneath
the surface and in somewhat varying terms. An analysis of methods applied
throughout North America shows that the ability to predict physical properties
at depth is a function of experience as well as ability of the interpreter. But
perhaps more important is his concept of the history of the land form in which
he is working. For example, predictions of texture below the surface in some
alluvial deposits is extremely limited, while in other types of formations having
deep uniform materials in them, the predictions of texture at considerable depths
are feasible.
The accuracy of predictions is also an important variable that moves on a
sliding scale. In the morainic deposits of the glaciated regions, even a loose
description of texture is subject to considerable inaccuracy, whereas the iden-
tification of texture of dune sand, loess, lacustrine deposits and numerous others
can be made with a high degree of precision. In engineering, this is also true and
