Soil pH - when is high too high?

It's funny how soil pH is often one of the last things considered when trying to diagnose problems in the field or on a crop. Typically we work our way through all the possible pests, both insects and disease, we might consider pesticide applications - not enough, too much or even pesticide residues, we will take tissue tests to look at the nutrients whether there is not enough fertility or too much, we may even consider lightening strikes before we look at soil pH. Soil pH though is one of the early signs of soil health problems. It is a test that is easy to do. There are even quick tests or portable pH meters that are quite accurate compared to the lab test.

Let's take a step back here. pH is a measure of acidity. The scale goes from 0 to 14 with 7 being neutral. Acids are below 7 while above 7 is considered alkaline. Typically most Ontario soils are in the range of 6 to 8 for pH. Many of our soils in southwestern Ontario are calcareous based and tend to have pH readings around 7 and above, particularly the clays and loamy soils. However, even in clay soils we can have pockets of lower pH. Many sandy soils have a low buffering capacity and tend to have lower pH.

Crops are responsive to soil pH. It influences the availability of nutrients and there are standards charts that you can get that give an indication of the relative availability of nutrients. Here is where some of the pH confusion comes in. How high is high and how low is low. pH is a logarithmic scale, this just means that each step up or down the scale represents a change by a factor of 10. A truly acidic soil pH that is suitable for blueberries is a pH of 4 or 5. High pH becomes a bit more of an interpretation. Generally in Ontario soils don't get much higher than 8.2 or 8.5. Usually this does not cause crop nutrient problems as long as there is nothing else interfering with the crop roots and nutrient uptake like disease or other pests. Other areas of the world do have soil pHs that are higher and their soils may also have high sodium levels that further complicate things.

Ontario soils can have quite a range of soil pH but our high pH soils that are in the 8's are usually still very productive with few nutrient problems. The low pH pockets are of more concern because they can interfere with crop consistency and yield. Don't guess - soil test!