Influences of Soil pH  

Soil pH is a measure of the concentration of hydrogen ions within the soil. This is interpreted through a pH scale that ranges from 0-14. A pH value of 7.0 is neutral, anything below is acidic and anything above is alkaline or basic. The natural pH of soil is influenced by how the soil was formed, through parent material (rock or organic matter in which most soils originate from), time, topography, climate and organisms (plant or animal). Additionally, soil texture also plays a role, where those that have higher amounts of clay or organic matter can hold onto H+ ions due to their large surface area and negative charge. Soil pH can affect the soil’s physical, chemical, and biological characteristics of soil. In addition to soil characteristics, pH has a large influence when it comes to plant health and success. The soil in Ontario typically has a pH range from 4.0 - 8.0, depending upon geographic location, climate, and soil history. Most crops do the best in pH levels that are near neutral, however there are exceptions for some crops like blueberries which thrive in more acidic environments of 4.5 - 5.5, or cranberries, that like a pH between 5.5 - 6.5. For crops that we are interested in growing, these low pH levels can drastically limit success in regard to nutrient and mineral availability, and microbial diversity.  

Limitations with Soil pH in Agriculture 

Nutrients that plants require to grow are soluble at different pH levels, therefore keeping an eye on your soil pH is essential to maximize nutrient uptake throughout the season. Optimal conditions are in between 6.0 - 7.5, where typical crops can have access to the nutrients they need to survive. In more acidic soils, there is a reduction in the availability of calcium, phosphorous, magnesium, boron, and molybdenum. In contrast to that, aluminum, iron, zinc, and manganese can increase in their availability, potentially reaching levels of toxicity within the plant (see the image below for more details). When it comes to microbes, they aid in the breakdown of organic matter into corresponding nutrients that can be utilized for plant growth and development. When pH gets to more extreme levels, this microbial breakdown rate is reduced, resulting in less nutrients being released into the soil and less available for plant uptake. 

In addition to the things discussed above, pH can also have an influence on herbicide efficacy. This can affect the performance and/or breakdown within soil; however, pH does not influence every herbicide in the same way. Two chemical families in particular that are impacted by soil pH are the triazines (Group 5) and sulfonylureas (Group 2). In more acidic soils, these chemicals bind with the soil, making them less available and increases the risk of carryover for the next season. In higher pH soils, the opposite is true, where the chemical is more available to the plant, however we are likely to see more crop damage in this case.  

How to control soil pH 

In conditions where soil pH is high, sulfur can be used to lower the pH to more neutral conditions. Depending on the desired change in soil pH, and what the current levels are will change the amount of sulfur recommended. When it comes to raising a pH that is too acidic, there are a few more factors to consider. Not all liming materials are similar, the main ones used are calcitic or dolomitic lime. The use of calcitic lime is favored, when your soil test shows a high magnesium percentage. This is because calcitic lime contains calcium carbonates, whereas dolomitic lime contains both calcium and magnesium carbonates. Therefore, applying more Mg on an already high concentration through the use of dolomitic lime could potentially create an unsuitable plant environment. If soil test levels show lower Mg levels, that is when dolomitic lime will be more effective. In general, dolomitic lime is the better option if you are able to use it, as it has more available and typically the least expensive choice. Like the application of sulfur, the amount of lime that is recommended will depend on the current levels and timeframe for application.