This piece came about because of a technical session I attended at scientific meetings in Saskatoon in July 2019. Rick Engel of Montana State University presented a paper showing soils on the Highwood Bench near Fort Benton, Mont., that had become so acidic crops like lentils and sunflowers would barely grow. The soils were at a pH level where aluminum toxicity was a problem. Excellent photos showed situations with great variability within fields with relatively level typography.

Soil pH (acid or base) measurement is to a soil scientist as blood pressure is to a medical doctor — it is fundamental.

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The pH is a measurement of the hydrogen ion concentration (acidity) in a solution. In pure chemistry, the scale goes from 0 to 14. A pH of 7 is neutral. Values less than 7 are acidic and values more then 7 are basic (alkaline). For soil science purposes, the limits of pH are from about 4 to 9. The lower the number, the more acidic the soil. The pH is a logarithmic scale, so a pH of 5 has 10 times as many hydrogen ions as a pH of 6.

A high pH of 7.5 or greater results in problems with phosphorus availability and many of the micronutrients. At a pH less than 6, alfalfa performs poorly and other crops can be affected. At a pH less than 5 aluminum toxicity kicks in with big problems for most crops.

The age-old solution for acidic soils is the application of lime to neutralize the acidity.

Soil acidification: a natural process

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Acidification is a natural process. Some soils are “born” that way — that is to say the soil parent material was acidic. In parts of the world, soils that have never known rejuvenation by glaciation or volcanoes, soils can be acidic from the get-go. The nature of the parent rock will determine if the rock weathers to form acidic or basic soils.

In our soils of Western Canada, Mother Nature’s giant lime spreader was the several glaciations that passed over the countryside. The continental glaciers picked up limestone from the East and spread it West over the countryside. The Third Meridian is just east of Saskatoon and is about the dividing line. Natural acidic soils are rare east of the Third Meridian but more common west of it.

The mountain glaciers that came from the west had no such limestone, so soils in parts of Alberta and western Saskatchewan are more acidic. The belt of solonetzic soils that runs north–south from about Vegreville to Brooks, Alta., had “once over lightly” glaciation that left shallow glacial deposits over the sodic, acidic pre-glacial shale.

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I remember a road trip across that solonetz soil belt with the famous glacial geologist Earl Christiansen. As we passed through creeks, the pre-glacial shales were visible in the road cuts. His remark was, “What this country needs is another glaciation.” The solonetzic soils are often acidic.

Soils that are deposited with ample calcium and magnesium (base forming) can become acidic by the leaching of water through the soils carrying the calcium and magnesium down. In many landscapes, even with low relief, the depressions can be leached and slightly acidic. Liming to fix such soil types would be a great example of the need for variable rate to deal with the problem parts of the field only.

The Peace River Country

The Peace River Country of Alberta and northeast British Columbia is a special case. One acre in three is acidic in that area and liming research goes back to the 1970s. Paul Hoyt (1930–2011) spent much of his career researching the problem from the Agriculture and Agri-Food Canada station at Beaverlodge, Alta.

I had the pleasure of working with Paul on the slopes of Kilimanjaro in Tanzania for two years in the 1970s and learned much from him. He was the main author of Farming Acid Soils in Alberta and Northeast British Columbia. If you Google that title you should soon have a PDF of that excellent bulletin. Photo 1 (above) shows the effect of lime on alfalfa in Peace River Country.

Manitoba is also a special case. For the most part, acid soils in Manitoba were a rare beast — until recently. In potato- and corn-growing areas of sandy soils, the high nitrogen (N) rates have acidified the soil and liming programs are in place on some farms.

The effect of ammonium fertilizers in soil acidification

Ammonium fertilizers applied at high rates of N over long periods of time will acidify soils. That is what is happening in some areas now. The banding of N near the seed at relatively shallow depth is very efficient but can lead to acidity. With the capability to place the fertilizer precisely and to plant this year’s crop between the rows of last year’s crop perhaps the acidity is enhanced. Most studies show the drop in pH to be greatest at shallow depths.

Ammonium sulphate (21-0-0-24) is the most acidifying per pound of N applied. Theoretically, anhydrous ammonia (82-0-0) and urea (46-0-0) are about the same per pound of N but urea is sometimes affected by volatile loss, which would reduce the pH effect. Elemental sulphur will also acidify soils, so caution is needed with high rates.

Not all soils are equal in the effect natural processes or fertilizers will have on soil pH. Sandy soils will become acidic much easier than high-clay soils.

Clubroot

Much of the renewed interest in soil pH of recent years has been related to the very scary clubroot disease problem in canola. The heartland of that problem has been the acid soil region in the area of Edmonton, Alta. The Meadow Lake area of northwest Saskatchewan is also affected and some areas in Manitoba.

Work in the United Kingdom from as early as the 1940s showed clubroot related to acid soils was caused by long-term ammonium sulphate fertilization.

Existing maps of clubroot from field surveys in the three Prairie provinces do not always match the soil pH maps.

What next

At the farm level, soil test, soil test, soil test. Make sure the soil pH is known and checked over time. At the research level, what is needed now is a co-ordinated, problem-focused, three-province program. It should document the geography and agronomy of acid soils on the Prairies and establish practices to solve the problem with the age-old practice of liming.

Short term, local project funding operating in isolation will not do the job. In the past, I have referred to such projects as “peanut scrambles.” The funding agencies hold out a pot of money and ask for individual proposals to use up that money. Individual projects operating in isolation can add some light, but not solve the problem.

In this situation, work must start with a thorough review of all historical data and a preparation of a three-province, generalized, soil pH map based on soil survey data. With the demise of public soil test labs, a system should be devised to utilize summary data from private labs as the International Plant Nutrition Institute did prior to its untimely demise. Soil test summary data is most useful if it can be assembled on the basis of soil type rather than geographic or political boundaries.

I am sure good work will show that many farms are at no peril, but will identify soils and cropping/fertilization practices that are not sustainable as presently operating. Liming is older than Prairie agriculture but will need to be adapted to our situation. The main thrust must be to establish sources of liming materials that can be accessed and transported on an economic basis.

In a future column, I will present maps of soil pH that exist in Alberta and Saskatchewan. Owners of Henry’s Handbook of Soil and Water can check out pages 72–75 for more information on acid soils.