The Soils of The Regional Municipality of Ottawa=Carleton

P :
R :
T:
W:
Table 8. Extremely severe, very severe, and severe subclass definitions
EXTREMELY SEVERE VERY SEVERE
Excessively stony soils (more than
50% ofthe surface) . The soils are
too stonyto permit cultivation, and
improvement is not practical .
Soils with a very restricted rooting
zone (hard rock at 20-50 cm depth)
and rock outcrops covering 10-25%
ofthe surface. Ifmore than 25% of
the surface is covered byoutcrops,
downgrading of six classes may be
applied .
Slopes greater than 30% which prevent
the use offarm machinery.
Associated with escarpments and
eroded channels .
Excessive wetness associated with
poor or very poor drainage. Excessive
moisture occurs at or near the
surface for extended periods during
the growing season, and improvement
by artificial drainage is not
feasible.
(d) Soil Capability Ratings
A capability rating has been determined for individual
land areas, each ofwhich is portrayed as a map unit on the soil
maps . Each rating consists of acapability class and all capabil
ity subclasses pertinent to the map unit . Within a map delineation
there may be one or two map units identified depending
on whether one or two distinct types of land areas are present .
The amalgamation of symbols within a delineation on the soil
maps is referred to as a map unit symbol . A simple map unit
symbol represents one map unit . A compound map unit symbol
represents two map units each of which must be evaluated
separately for its soil capability.
The capability class assigned to each capability rating has
been arrived at by assessing the degree of severity of the subclasses
or limitations which are attributed to each map unit . If
the limitations are minor, moderate, major, or severe, an additive
process is used in which the downgrading values of all limitations
are added together to arrive at the number of classes the
map unit should be downgraded from Class 1 . Additively,
these limitations can not downgrade the map unit lower than
Class 5, even if a more severe downgrading is indicated . The
reason for this class restriction is that the individual severity of
these limitations is such that each may contribute to reduced
capability down to Class 5, but individually or combined they
are not severe enough to warrant placing a soil in Class 6
because improvement practices may be feasible .
If a very severe limitation has been attributed to a map
unit, soil capabilitycan be no better than Class 5 . The presence
ofa limitation with this degree of severity automatically down
grades amap unit four classes, thus restricting capability to the
production of perennial forage crops . Other limitations of
equal or lesser magnitude may also be present, some of which
have been allowed to downgrade some map units further to
P* : Soils with enough stones (15 to
50% ofsurface) to prevent annual
cultivation ; considerable clearing
must be done.
Soilswith a restricted rooting zone
(hard rock at 50-100cm depth) and
rock outcrops covering 10-25% of
the surface, OR, soils with a very
restrictedrooting zone (hard rock
at20-50 cm depth) and rock outcrops
covering 2-10 010 ofthe surface.
Slopes of 16-30% severely restricting
machine workability and preventing
annual cultivation . Require
special protection measures against
erosion .
(e) Guidelines Used inRating Soils
SEVERE
R" : Soils with a very restrictedrooting
zone (hard rock at 20-50 cm depth) .
Class 6 . In these instances, the cumulative severity ofthe limitations
would limit capability to that of permanent pasture,
and improvement practices would not be feasible.
The presence of an extremely severe limitation in a map
unit is very restrictive because improvement of that limitation
is not practical, and soil capability can be no better than Class
6 . Additional limitations ofequalor lesser magnitude may also
be present, some of which have been allowed to downgrade
some map units further to Class 7 . In these instances, the
cumulative severity of the limitations makes any type of agricultural
use impractical .
(1) Wetness(W) andDrainage Variability (V)
Natural drainage may be uniform or variable in map
units . The defined degrees of severity for wetness given in
Tables 7 and 8 are applied when drainage is uniform . However,
when variations in drainage occur within map units, limitations
v, V, or V' may be applied, depending on the contrast in
soil moisture conditions . The minor limitation v is usually
applied along with w or W to indicate minor variations in
drainage which are not severe enough towarrant downgrading
by more than one class. In most instances, it is used when the
less extensive or significant drainage component in a map unit
is not as well drained as the most extensive or dominant component
. An example would be a poorly drainedcomponent in a
dominantly imperfectly drained unit .
Limitations V and V', however, represent landscape situations
in which the difference in soil moisture conditions is significantly
more pronounced, and the map unit must be
downgraded accordingly. They are applied to map units in
which the dominant component has a moisture deficiency
(good to rapidly drained), and the significant component has