Agriculture Reference
In-Depth Information
Soil Survey and Mapping
The Traditional Method
The traditional method of soil survey is normally used for general-purpose soil
assessment and is carried out at varying intensities, ranging from reconnaissance
(scale 1:2,000,000 to 1:100,000) to detailed (1:50,000 to 1:1000).
3
In this case,
soil survey may start with an examination of air photographs to identify possible
soil changes from landscape features, for example, stream courses, slopes and val-
leys, and changes in land use and vegetation. Notional boundaries between soil
classes are identified and checked by free survey on the ground when samples are
taken with an auger or soil corer. Figure 1.13(A) shows examples of manually oper-
ated soil augers and corers; a more versatile hydraulic soil corer that can drill down
to 1.3 m is shown in figure 1.13(B). Once the boundaries separating classes have
been confirmed, rectangular pits are dug to expose the soil profile thought to be
representative of each class. Figure B1.1.1 is an example of a soil profile pit. The pit
allows the visual characteristics of each horizon to be described accurately; these
include sharpness of boundaries, depth, color and mottling, presence of organic
matter, stones, structure, and presence of carbonate or other salt deposits, as well as
the “feel” or texture of the soil (see “Texture Analysis and Calibration for Texture,”
chapter 2). Samples are also collected from each horizon for laboratory analysis.
The output of such a survey is a map that shows the spatial distribution of
soil classes. The degree of homogeneity of each class depends on the intensity of
soil sampling and the scale at which the class distribution is mapped. If the soil
distribution is relatively simple or the map scale is large (>1:5000), individual
classes can be displayed. However, if the distribution is complex (as it often is with
alluvium-derived soils) or the map scale small, two or three classes may be grouped
into a mapping unit. From such a conventional survey, which is usually linked to a
general-purpose soil classification, the key properties of a soil typical of each class
are identified. This typical soil is called a soil type, and the properties of each type
are supplied in a legend that accompanies the soil map. Figure 1.14 shows an exam-
ple of a typical soil map, derived from a general-purpose soil survey, which shows
the dominant soil classes in plain colors as well as the complexes of codominant
and subdominant classes in hashed colors. Because such maps are rarely available at
a scale larger than 1:5000 (and more commonly at 1:50,000 or 1:100,000), much
detail about soil variation over distances of several meters is lost.
In Australia, when a soil survey is required for a new vineyard or the redevel-
opment of an existing vineyard, a standard practice has been to locate pits on a
75
×
75 m grid, without necessarily any recourse to air photographs, and for the
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A scale of 1:1000 means that 1 cm on the map represents 10 m on the ground.
