Soil Report - Agriculture et Agroalimentaire Canada

Table 14 . The relative effectiveness of common field crops
in reducing water erosion in Ontario
Cover type C-factor value'
Fallow land (bare soil) 1.00
Beans .47
Cucumber .45
Tomato .43
Continuous corn
- fall plowed .45
- spring plowed .39
- chisel plowed .27
- zero tillage .08
Corn in rotation .30
Mixed grains .27
Winter wheat .25
Rye .25
Tobacco .30
Hay-pasture (rotation) .06
Permanent pasture .03
'Cropping-managementfactor, Wischmeierand Smith (28).
vegetation cover has the potential of reducing water erosion
to less than 10% of that from fallow or bare land .
(3) Measurement ofFactors Affecting Water Erosion
In order to make meaningful recommendations, with
respect to soil conservation practices, one must be able to
recognize the significance of a soil erosion problem, and
provide appropriate cost-effective erosion control alternatives
when a problem is encountered . Although qualitative
approaches can be useful in many circumstances, the
temporal nature of soil erosion, as well as the difficulty in
witnessing sheet erosion losses in the field, make a quantitative
approach to erosion assessment and control recommendations
more practical .
The quantification of the factors affecting agricultural
erosion, i.e ., rainfall, soil erodibility, slope, vegetation and
conservation measures, is based on widespread erosion
research compiled from nearly 10 000 plot-years of field
data, and from rainfall records obtained from about 2 000
weather stations in North America . The resulting soil
erosion formula is in extensive use by the Soil Conservation
Service of the United States Department of Agriculture, for
applying and planning conservation measures that reduce
soil erosion to acceptable amounts (28) . It is only recently
that the erosion factors have been quantified for use in
Ontario (29, 30) .
The water erosion formula, A=RKLSCP, used to predict
average annual soil loss through sheet and rill erosion, is
called the universal soil loss equation (28), where
A - is the computed soil loss in tons per acre per year .
R - the rainfall factor, is the number of erosion-index units
in a normal year's rain .
K - the soil erodibility factor is the erosion rate, per unit of
erosion index for a specific soil, in cultivated, continuous
fallow . This unit is expressed in tons per acre.
L - the slope length factor is the ratio of soilloss from a field
slope length, to soil loss from a 72 .6 foot plot .
S - the slope gradient factor is the ratio of soil loss from the
field slope gradient, to soil loss from a 9% plot slope .
C - the cropping-management factor is the ratio of soil loss
from a field with specific vegetation or cover and
management, to soil loss from the standard, bare or
fallow condition . This factor measures the combined
effect of all the interrelated cover and management
variables, plus the growth stage and vegetal cover,
during rainfall episodes .
P - the erosion control practice factor is the ratio of soil loss
using a particular management practice, to soil loss from
a field not using that practice .
When the numerical values for each variable are multiplied
together, the product is the average annual soil loss, in
t/ac/a (conversion to metric equivalents, t/ha/a, requires
multiplication by 2 .24) . It should be emphasized that the
formula estimates sheet and rill erosion but does not
consider soil losses caused by gully erosion or stream
channel erosion . Since the erosion formula does not contain
a transport or delivery factor it does not predict sediment
load of streams . A brief description of each factor in the soil
erosion formula now follows.
(a) Rainfall Factor (R)
The R-value reflects the erosivity of rains, and is generally
related to climatic factors . R-values for Ontario range from
25 to 100 . The Haldimand-Norfolk Region has an approximate
R-value of 80 . The distribution of R-values, for
southern Ontario, is shown in Figure 44 of the erosion
appendix, and discussed elsewhere by Wall et. al. (31) .
(b) Soil Erodibility Factor (K)
The K-value reflects the inherent erodibility of a soil due
to the erosive activity of water .
Table 15 illustrates the K-values and K-ranges for the soils
of the Haldimand-Norfolk Region . The K-values were
computed by the method outlined by Wischmeier and Smith
(28) . These K-values range from a mean low of .05 for the
Brady soils, due in part to the high proportion of medium
and fine sands characteristic of these soils, to a mean high of
.41 for the Brant soils, which contain a large proportion of
easily erodible very fine sand and silt . In terms of relative
erodibility, the Brant soils are about 8 times more erodible
than the Brady soils (.41/.05), when all other soil loss factors
are held cpnstant .
Table 15 also categorizes the various soils into five soil
erodibility classes . (negligible to very severe) based upon
their respective K-values . The guidelines used for establishing
these soil erodibility classes are given in Table 16 . The
soils in the Haldimand-Norfolk Region range from negligible,
e .g . Brady, to severe, e .g. Brant, in terms of their erodibility
classes.
The particle size distribution (% sand, silt and clay) and
organic matter content are important factors in determining
soil erodibility . Table 17 provides an alternative method of
approximating K-values, if only soil texture and organic
matter content are known. The K-values reported in Table
17 were computed from data gathered from almost 1000 .
surface horizons, sampled during the soil mapping of the
Haldimand-Norfolk Region . If the textural class is known,
but not the organic matter content, a K-value can be
estimated from the mean values of the last column of Table
17.
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