Acid soils have a low pH. The H is for hydrogen. The p means it is a negative logarithm, which means the lower the number the more acid. The logarithm means that pH = 6 is 10 times more acid than pH=7.
Soils can be acid from the original geologic material or they can become acid via leaching (washing) of basic minerals. Acid rain, usually coming from industrial smokestacks, can also be a factor, but that has been much reduced in recent decades.
The current issue is all about ammonium fertilizers that can lower soil pH. The common nitrogen fertilizers in use in the Prairie provinces are listed in Table 1 (below).
Ammonium sulphate is the most acidifying per pound of nitrogen applied.
In Alberta, acid soils have been well known and studied for decades in the Peace River country and in central Alberta. Liming is the well-known solution, but sources are often not easily available.
In Saskatchewan, acid soils have not been a big issue except for the Scott and Meadow Lake areas. In fact, for calcareous (naturally limy) soils like Yorkton loam in eastern Saskatchewan, the acidifying effect of nitrogen fertilizers would be a bonus.
Manitoba was mostly considered to have almost no acid soils. In some areas with sandy soils and high nitrogen fertilizer rates for some crops, that has changed in recent years.
The real issue now is to get a good handle on the geography of the problem so solutions can be targeted to where the real problems occur.
What follows are the existing soil pH maps for each province (Figures 1, 2 and 4 below). They are not directly comparable but are useful as a “high-level” view to start with.
On the Alberta map (Figure 1 below), the dark red and red areas have many acid soils.
The Peace River country has a large area of acid soils and some as low as pH 5.5-6.0. Central Alberta (Calgary/Edmonton) also has many acid soils and that is the main area of concern for clubroot in canola.
An Alberta pH map was prepared in 1970 by computer processing of 63,000 farm samples of the Alberta Soil Testing Lab from 1962-69. That map showed much the same trend as the 2018 map. Keeners from CCA or PAg can access that data at Cameron and Toogood, 1970, Canadian Journal of Soil Science, Volume 70, pages 1-7.
The pH map of Saskatchewan soils (Figure 2 below) is based on samples and analysis dedicated to that mapping. In 1982, 8,000 surface samples were selected using a modified grid pattern. At each site, samples were taken from three slope positions in the field. From 1982-86, another 15,000 soils were sampled as part of routine soil survey activities.
The Saskatchewan map is much more complex than the Alberta map. In general, soils west of the Third Meridian are more acid than soils east of it. In the brown and green areas, acid soils will be rare and not likely a problem. In the brown area, there are many soils where a slight reduction in soil pH would be a boost to phosphorus availability.
In the red category, it is prudent to make sure you have lots of soil test data. Current data shows more red in the Meadow Lake area than this map shows. In the yellow area, especially in the southwest, there is evidence that acid soils are becoming a problem.
Variation within a field
When it comes time for liming, variable rate could be a big winner. In general, pH values are highest on knolls and lowest in depressions (Figure 3 below), where much water has passed through the soil to leach out the basic ions. That kind of variation is not limited to hilly land. Even fairly level landscapes can have large pH variations, with even slight depressions being much more acid.
Alberta and Saskatchewan maps
As you look at the maps from Alberta and Saskatchewan, you might wonder which one has the right approach. Alberta has a fairly straightforward legend with clearly defined pH categories, however, the Saskatchewan map is much more complex and hard to follow.
So, who is right? In my opinion, they are both right. Readers who have Henry’s Handbook of Soil and Water can check out page 29 for a complete explanation. In short, Alberta has a much thinner layer of glacial deposits, so very little pothole country. There are much more well-defined natural drainage systems, which lead excess water off to an outlet that will lead to a major river and eventually an ocean. Much of Saskatchewan has very poorly developed natural drainage, so the local water distribution leads to more complex soils, including variation in pH.
The Manitoba map (Figure 4 below) is a high-level view but the brown and green areas dominate and will have very few acid soil problems. The most acidic (yellow) areas are mainly in forested regions, mostly non-ag.
Manitoba has generally been considered to have very few acid soils, however, the University of Manitoba Research Farm at Carman, Man., has some plots with low pH values. Sandy soils with high nitrogen rates for certain crops do now have pH problems. At this time, the extent of the problem is not known.
Soil survey reports and maps
In all three provinces, Soil Survey was a very active program for about 70 years — from the 1920s to 1990s. Almost all of the detailed Soil Survey Reports produced over those many years have been retained and are available online at CANSIS (Canadian Soil Information System). Most reports also include a section providing detailed lab analysis that was conducted for selected soil profiles.
Table 2 (below) illustrates examples of the data that is readily available with a few mouse clicks.
Clearly, these soils were not made acid by nitrogen fertilizers. The Waseca soil has special meaning for me. My first job at the University of Saskatchewan was as a lab instructor in vocational ag labs. They were all farm kids (boys then) and they were instructed to bring a surface soil sample from a field on their home farm.
In one lab exercise was simple lab tests including pH. Vic Hult from Waseca had a soil that showed pH around 5.6. My response was that the pH meter must be broken. At that time, and many years after, we had a mental block about admitting that acid soils did exist in Saskatchewan.
Table 3 (below) shows similar data for Manitoba. Blackstone is a clay soil developed on shale clay till with moderate lime content and challenges to annual crops. Kenville soils are developed on medium texture glacial lake deposits and compare with the best in Manitoba for grain and forage crop production.
Clearly, these Manitoba soils were acid before cultivation, so it was a natural condition.
The bottom line
At the farm level: soil test, soil test, soil test. It is important that each farm knows where they are at on the pH scale. Any farm that has been growing great crops with high rates of nitrogen fertilizers should be sure to have soil test data for your individual situation.
At the research, extension and consulting level, it is important all professionals are aware of the decades of pH and other soil analysis data that is readily available in Soil Survey reports and easily accessible at CANSIS.
In addition to the data available in reports, there are also many analyses of individual soil profiles resting very uncomfortably in the guts of federal computers. In Saskatchewan, the soil maps are already available online at SKSIS (Saskatchewan Soil Information System). It should be easy to append those profile analysis data to that platform.