E. Huffman et al. Agriculture, Ecosystems and Environment 81 2000 113–123 119
Table 5 Matrix depicting the salinization ‘risk factor’ of different combi-
nations of land cover types Permanent cover
of polygon
a
Summerfallow of cropland 0–10
10–20 20–30
30–40 40–50
75–100 1
2 3
4 5
50–75 2
3 5
7 8
25–50 3
5 7
8 9
0–25 4
6 8
10 10
a
Forest, hay and pasture.
into consideration in developing the SRI were the present status of the extent and severity of saline
conditions Px, topography Tp and soil drainage Dr. The dynamic factors of aridity Ar and surface
cover Sc were incorporated to give: SRI=Px× Tp×Dr×Ar×Sc. A brief outline of each factor is
presented in Table 4. The risk of salinization due to the distribution of summerfallow and permanent
cover within a polygon is presented in Table 5.
3. Results and discussion
3.1. Soil cover Compilation of the area under each cover category
at a national level Fig. 2 shows: 1 a fairly sharp increase in the amount of farmland under low cover
between 1921 and 1971, followed by a decline to 1991;
Fig. 2. Area of cropland under high, medium and low amounts of soil cover in Canada, 1901–1991.
2 an increase in medium cover between 1901 and 1941 and a fairly level trend since then; and 3 a very
minor increase in high cover between 1901 and 1981, followed by a dramatic increase between 1981 and
1991. The major proportion of farmland falls in the ‘medium cover’ category in all years, but 1991 shows
the highest percentage under high cover 31 since 1901 36, when hay, pasture and spring cereals were
the dominant crops. The percentage of farmland under low cover is higher in 1991 23 than in 1901 5,
but is lower than at any point since 1921. In the period 1981–1991, the proportion of farmland under medium
and high cover increased from 66 to 77.
In Canada, land use trends have contributed to a decline in agricultural soil cover between 1901 and
1991. This decline is due to an increase in the area of farmland, an increase in the proportion of farmland
that is cultivated, an increase in the proportion of crop- land devoted to annual rowcrops and an increase in
the amount of summerfallow. On the other hand, two trends; a decline in summerfallow since 1971 and an
increase in conservation and no-tillage systems, have positive implications for soil cover levels. These two
indications of a move to improve soil cover levels are particularly important in that they occur in the latter
part of the time period, between 1961 and 1991 and especially between 1981 and 1991. For the first time
since the mid 1920s, there is currently more high cover than low cover in Canada. However, the overall im-
pact of these two trends has been significantly negated by concurrent shifts to crops which provide less cover,
such as potatoes, corn, soybeans and canola.
120 E. Huffman et al. Agriculture, Ecosystems and Environment 81 2000 113–123
Although it is clear that the expansion of crop pro- duction in Canada in this century has exposed a great
deal of soil to environmental degradation processes Sparrow, 1984, it is difficult to make a definitive
judgment of whether that trend is continuing or not. Farmland expansion has leveled off and summerfal-
low levels have declined, while conservation tillage is expanding greatly, but the proportion of farmland
cultivated and the proportion in rowcrops is still on the rise. The analysis is hindered by the use of
‘qualitative’ measures, but it is apparent that there is a variety of forces and reactions at work and there
is a need to explore these issues in more detail. It is critical to identify parts of the country that should be
monitored, and where increased efforts to address soil degradation through conservation and crop choices
should be made.
3.2. Wind erosion For the 1981–1991 trend analysis, the total amount
of erosion in tonnes was calculated for each polygon by multiplying the estimated soil erosion rate by the
area of cultivated land in the polygon. The SLC poly- gon results were ‘rolled up’ by dominant soil texture
types to provide ecoregion summaries Table 6.
Table 6 Reduction in the risk of wind erosion in the Canadian Prairies, 1981–1991
Ecoregion typical soil type
a
Soil texture Reduction in risk, 1981–1991
Mixed Grassland Brown Chernozem, Aridic Kastanozem Sand
7.4 Loam
5.1 Clay
6.9 Mean
5.3 Moist Mixed Grassland Dark Brown Chernozem, Haplic Kastanozem
Sand 23.3
Loam 4.6
Clay 9.6
Mean 7.0
Parkland Black Chernozem, Chernozem Sand
12.4 Loam
5.8 Clay
6.1 Mean
6.8 Boreal Transition Dark Gray Chernozem, Greyzem
Sand 8.2
Loam 11.2
Clay 12.1
Mean 11.3
a
Canadian and WRB FAO systems Soil Classification Working Group, 1998.
In general, the risk of wind erosion in the prairies is greatest in the arid south and diminishes to the north
where wind speeds tend to be lower, temperatures cooler and precipitation higher. In the more north-
ern areas, less summerfallow and greater amounts of residue also contribute to lower erosion risk.
Between 1981 and 1991, wind erosion risk in the prairies declined by nearly 7 overall. One-third of
this decline can be attributed to changes in cropping systems, particularly reduced summerfallow, and the
remainder is due to increased adoption of conservation tillage. The relative importance of changes in crop-
ping versus tillage varies across the region, however, with conservation tillage being most important in the
more arid south and reduced summerfallow and in- creased forages more significant in the north. Analy-
sis at the detailed level of SLUs shows that cropping system changes to incorporate more oilseeds and spe-
cialty crops on clay soils in the south Brown and Dark Brown soil zones have marginally increased erosion
risk in some areas.
3.3. Soil salinity For application in a broad-scale analysis of
farmland in the Canadian Prairie Region Alberta,
E. Huffman et al. Agriculture, Ecosystems and Environment 81 2000 113–123 121
Saskatchewan, Manitoba, SRI values were grouped into three risk classes: 2.9=low, 3.0–8.9=moderate
and 9=high. Since the concept of risk does not result in a definitive indication of a change in salinity, this
analysis indicates only whether management prac- tices are subjecting any susceptible land to increasing
risk of salinity. For example, a moderate rating may be the result of severe salinity being managed well
or mild salinity being managed poorly. The risk class was calculated for each SLC polygon for the years
1981 and 1991, and results were compared to identify changes in the risk of salinity.
According to information in the soils database, the total extent of land in the prairie region currently af-
fected by significant salinity Ec
e
8 dSm and 1.0 of the landscape affected is 1.4 million ha 2.5 of
farmland. Incorporation of land use information in the SRI showed that in 1991, 62 of farmland was at
low risk of increased salinity, 28 was at moderate risk and 10 was at high risk. The implications are
that under 1991 land use practices, a considerable area that is not currently negatively affected by salinity is
at risk of becoming so.
Comparison of the risk under 1981 land use prac- tices with that of 1991 shows that the situation
improved somewhat over the 10 years, with approx- imately 93 of farmland showing no change in risk,
about 7 showing a lowered risk and 1 showing an increase in risk. Although the data presents two
‘snapshots’ in time and so does not conclusively show a trend, the results can be attributed to a considerable
reduction in summerfallow area i.e. more continuous cropping and an increase in conservation tillage.
4. Conclusions
