The Soils of The Regional Municipality of Ottawa=Carleton

C values were calculated for common crops and management
practices in the Ottawa-Carleton Region, and are presented
inTable 17 . Field crops grown inthe area include canola,
corn, forages, grain, peas, soybeans, white beans and winter
wheat . The most common tillage practice is fall moldboard
plowing (conventional tillage), with some chisel plowing and
disking (reduced tillage) done as well . Underseeding or interseeding
of the grain and wheat crops with forages are other
practices being utilized .
The lowest C values calculated for crops in the Ottawa-
Carleton area were for forages (alfalfa, C = 0.02 ; red clover C
= 0 .015) ; crops which tend to provide good cover throughout
mostofthe year, reduce runoff, promote infiltration ofrainwater
into the soil and the residuals of which can provide erosion
resistance for up to two years after the meadow is replaced with
other crops. This benefit is reflected in the lower C values for
grain corn, one and two years after a meadow crop (C = 0.20
and 0 .34, respectively), as compared to a corn crop grown after
other field crops and inexcess of twoyears after ameadow crop
(C = 0 .37) .
Grain corn (C = 0 .37), which leaves more residue on the
surface after harvest than silage corn (C = 0 .50), also has a
lower C value than the latter. The C value also indicates that
soybeans in rotation (C = 0 .46) tend to resist erosion better
than thosegrown continuously (C = 0 .54) or after similar, low
residue-producing field crops . Fall tillage tends to expose the
soil surface for a longer period of time than spring tillage.
However the heavy texture of many of the Ottawa-Carleton
soils does not make spring tillage practices feasible. The necessity
of fall tilling can be compensated for, where possible, by
reducing tillage operations so that more residue is left on the
soil . For example, the soil conserving-benefits of reduced tillage
are reflected in the C value for winter wheat after chisel
plowing or disking as the only tillage (C = 0 .22) as compared
to the same crop after moldboard plowing and disking (C =
0 .27) .
Other practices which can help to control erosion include
the inclusion of winter cover crops in a rotation, or underseeding
crops to forage crops to provide some soil cover over the
winter months (for example, C = 0 .41 for grain crops without
underseeding, 0.36 for underseeding) .
The above C values represent the potential soil loss reduction
(compared to bare field condition) during a particular
crop year. Crop rotation systems tend to have lower average
annual Cvalues than continuous crops, as relatively high C values
of some crops are offset by low values of others, when the
values for all crops in the rotation are summed and divided by
the total number ofcrop years in the system . Forexample, continuous
silage corn has a C value of 0 .50, but if corn was
rotated on a one year basis with a crop of a lesser C value (eg .
grain after fall chiselling, C = 0.30) the average C value forthe
two-year rotation would be 0.40 (0.50 + 0.30 / 2) .
The introduction of a forage crop into the rotation (eg .
alfalfa, C = 0 .02) has a beneficial effect duringthe duration of
the meadow, and for up to two years after. A rotation of three
years corn, one year fall-plowed grain would have a C value of
0.38 . Ifthree years of alfalfa (direct seeded the first year) were
added to this rotation, the average annual C value would
decrease to 0 .25, and if the alfalfa were underseeded in the
grain the amount would be reduced even more to a C value of
0 .23 .
The C factors presented in Table 17 provide an estimate of
the erosion-controlling effectiveness of each crop . In practice,
this effectiveness will vary depending on the given site condi
tions and the quality ofmanagement . These general values can
be used to estimate the effectiveness of existing, or alternative,
102
crop and management systems within the Ottawa-Carleton
Region .
(4) Soil Erosion Assessment
Regional or site-specific assessment of soil erosion potential
may be calculated using the quantitative relationships
developed in the universal soil loss equation and information
contained in this report . A graphical solution of the universal
soil loss equation, in the form of a nomograph, is available for
ease of computation . The soil loss nomograph provides a rapid
method for assessing the effect of many land uses on soil loss
potentials, for various soil and slope conditions. The following
discussion illustrates how information contained in this section,
and elsewhere in the soil report, may be used for predicting
soil loss potentials on a regional basis .
Regional SoilLoss Pbtential
The soil maps of the Ottawa-Carleton Region provide delineations
of soil, slope, and drainage conditions, at a scale of
1 :50,000 . Information contained in this section, with respect
to soil erosion, can be used with information contained on the
soil maps and legends to assess the relative erosion potential of
most map unit delineations . Figure 36 illustrates part of a soil
map and legend for the Ottawa-Carleton Region .
Using Figure 36 as an example, the following discussion
illustrates how interpretations of regional soil loss potential
from erosion can bemade.
1 . In Figure 36, locate areas on the map for which the erosion
potential is desired, and note the map unit symbol . For
example, select soil map unit Gl/S1 .4, which represents 2
areas ofwelldrained soils of the Grenville soil association in
Figure 36 .
2 . Go to the map legend and find the appropriate soil association
- Grenville (G), and the appropriate soil landscape
unit, ie. G 1 . Stoniness class (S 1) need not be directly considered
. Slope Class (4) is noted .
3 . Determine the erodibility ofthe G 1 soil landscape unit from
Table 13 . (Gl = 0 .31)
4 . Determine the topographic factors (L-S values) for Slope
Class 4 (5-9%) from Table 16 . For example, use L = 100 m
and S = 6%, thus LS = 1.22 .
5 . Using the soil loss prediction nomograph (Figure 37), locate
the K and LS-values for the G 1/S 1 .4 map unit on the appropriateaxes
(K = .31, LS = 1 .22) . Draw a line between theK
and LS values to intersect the pivot line. Draw another line
form the current or proposed land use through the point
where the K-LS and the pivot lines intersect, to the soil loss
potential axis . For example, ifcorn was the existing or proposed
crop, the soil loss for the corn land use on G1/S1 .4
soil map unit would be approximately 14 t/ha, a moderately
severe amount . Note that the G1 soil landscape unit
has only one drainage component, ie. well or good . Many
soil landscape units have two drainage components or
classes . G4 soils, for example, are dominantly well drained
with significant proportions of imperfectly drained soils .
These two components have separate erodibility values,
both of which should be considered in an erosion assessment
. In addition to this, many soil map unit symbols contain
two soil landscape units from different soil
associations . In these cases, the K-value of each drainage
class of each soil landscape unit should be noted and
assessed according to its relative proportion in the soil landscape
. (See section on MappingSystem .)
6 . Repeat interpretations for other map unit symbols in interest
to gain a regional perspective of the potential soil erosion
loss .