Soil and plant microbial communities have a mutualistic relationship where they provide what the other needs to thrive. There are many factors that can influence this relationship, such as soil structure, composition, and humidity, but soil pH has one of the most profound impacts on the growth and survival of plants and microbes.
When we measure pH, we’re actually measuring the local concentration of protons (hydrogen ions, known as H+). When there are too many H+ ions, the soil is acidic, resulting in a low pH. When the proton concentration is low (or a scarcity of H+ ions), the soil is alkaline (or basic) and has a high pH. Soil chemistry largely determines which microbial communities can get established in the soil.
Soil microbes are just as much affected by high pH as they are by low pH. When soil pH is in a neutral range (6.0-7.5, which is close to the pH of water), plants are more easily able to uptake the nutrients they need. For that reason, this is generally their optimal range. However, there are some plants such as blueberries or Rhododendron, that favor a more acidic soils. Not all microbes necessarily favor a neutral pH – fungi actually prefer soils with a lower pH. As acidic soils reduce the availability of certain nutrients, fungi are able to increase access to those limited nutrients for the plant to uptake. Still, challenges can arise for microbial communities when the soil pH deviates from its optimal range. While soil pH will naturally lower in time due to geochemical and metabolic processes, the soil pH can decrease rapidly with the input of certain fertilizers.
For example, when ammonium-based fertilizers are applied to the soil, bacteria use oxygen to convert the ammonium nitrogen (NH4+) to nitrate (NO3–), a more plant-usable form, and H+ ions, through a process called nitrification. Bacteria do this to obtain energy. If a plant needs some potassium, it will exchange one of its positive hydrogen (H+) ions for a potassium cation (K+) in the soil. This exchange helps maintain electrical balance through a natural process called cation exchange. However, this balance can be thrown off by over-application of fertilizers. As plants take up certain nutrients, it leaves a deficit of other nutrients, which can cause a decrease in pH levels.
If more fertilizer is continuously applied, more H+ ions are released, which can cause an imbalance in the soil and lead to soil acidification. The increased amount of H+ ions will directly affect the functionality and structure of the soil microbial communities. A low soil pH can disrupt plant cell membranes – altering enzyme production and limiting cell reproduction. Enzyme production and activity work best in more neutral pH ranges. As well, low soil pH can limit the exchange signals between root bacteria (rhizobia) and plants, which reduces root nodule formation, and alters the symbiosis that allows plants to fix nitrogen.
Overall, maintaining a balanced soil pH is crucial for ensuring nutrient availability and more importantly, microbial diversity in your soil. As deviations in pH can disrupt essential processes like nutrient uptake, enzyme activity, and the communication and functioning of symbiotic relationships between plants and soil microbes.