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Soil pH

Soil pH to a soil scientist is like blood pressure to a doctor — it’s the first thing we want to know. The pH affects nutrient availability, and acid (sour) soils are generally unproductive.

In many parts of the world liming acid soils to raise its pH is a fundamental part of farming.

pH and clubroot

Soil pH is also a factor in residuals of certain herbicides but the biggest issue bringing soil pH to the front burner is clubroot in canola. Low pH soils are more susceptible to clubroot.

High pH can be an issue in volatile loss of surface-applied ammonium fertilizers but, for this issue we are most interested in low pH.

Clubroot is not a new disease in Brassica species. The historic British Research station of Lawes and Gilbert fame (Rothamsted, near London) gave data that led to this statement many decades ago: “By the late 1940s there was increasing concern that the soils in a number of plots getting ammonium sulphate in the Classical experiments were becoming so acid that yields were adversely affected. … In the Agdell experiment, acidity was so severe on the NPK plots that the disease club root so decreased yields of turnips that the experiment had to be extensively modified in 1951.”

The modification was applying lime to raise the pH. It was not a quick fix and a further quote is, “Once established this fungus (clubroot — also called “finger and toe”) is difficult to eradicate and turnips could not be grown satisfactorily.”

Keeners of the CCA or P.Ag type can find that and other Rothamsted gems at Go to “publications” under “research” and click on “Guide to the Classical…”

The solution to acid soils is to apply lime, but tonnes per acre of lime are required to do the job. High freight costs have limited the practice in Western Canada.

Alberta and Manitoba

Alberta has lots of acid soils. One acre in three in the Peace River country is acid. Liming was carried out in the Peace River country when trucking subsidies were in place. Acid soils also occur in the Edmonton area, and acid surface soils are also common in the Solonetzic soil belt at Vegreville.

An extension bulletin —old but good — is “Farming Acid soils in Alberta and northeastern British Columbia” by Paul Hoyt (1930-2011), Marvin Nyborg and Doug Penney. Paul Hoyt was a soil chemist at Beaverlodge (Agriculture and Agri-Food Canada) for many years. He and I played tennis often at the Lyamungu Research Station on the slopes of Kilimanjaro Mountain in Tanzania circa1973. He was a grand fellow, a good friend and a great scientist. We miss him.

Paul Hoyt taught me a lot about soil chemistry. Put “Hoyt farming acid soils” into Google, click on first item and you will soon have a PDF of that bulletin on your desktop.

If you are from Manitoba you can skip this article. Most Manitoba soils are on the high side of pH rather than the low side and acid soils are rare.

pH 101

The pH scale is a different beast. It is a log scale, and the smaller the number the greater the acidity (i.e. hydrogen (H) ions). Soil with a pH of five has 10 times as many H ions as soil with a pH of six, 100 times as many H ions as soil with a pH of seven and 1,000 times as many H ions as soil with a pH of 8.

We often make the general statement that Saskatchewan soils have a neutral to slightly alkaline (basic) pH.

But, that is not always true.

My first experience with soil pH was as a very green instructor in Voc Ag labs at the University of Saskatchewan. We had the students bring in soil samples from their home farms and determine salt levels and pH in the student lab. A sample from the Lloydminster area of Sask., (it was likely brought in by Vic Hult of Waseca) gave us an answer of pH about 5.5. I dismissed it as a faulty lab result and carried on. It was probably a true result!

The map “pH of Saskatchewan Soils” was made by Harold Rostad and others from the Saskatchewan Soil Survey. Harold was a federal employee (AAFC). He and I were two-thirds of the Soil Science graduating class at the U. of S. in 1964 (the other third was Jim Bole, retired Director of Winnipeg Research Station of AAFC).

I have always been very proud of the job done by my classmate Harold Rostad —it is a nice piece of work. Harold carried pH equipment and did many measurements in a field lab and on the tailgate of a pickup truck.

The Saskatchewan map shows that low pH is not much of an issue east of the third meridian. In West Central and Northwest Saskatchewan acid soils (red on the map) are common. In the most acid area (A1 map unit: Unity, Macklin, Luseland) 20 per cent of soils have pH less than 5.5, 40 per cent have pH between 5.5 and 6.0, and 40 per cent have a pH between 6.1 and 6.7.

It would seem prudent to maintain extra vigilance for Clubroot in canola in the red areas of the Saskatchewan map. †

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



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