Soil pH and Plant Growth

pH in soil is the measurement of the concentration of H+ ions in the soil. The concentration of H+ ions is a small number so when measuring the pH we are measuring the negative log10 of the concentration of H+ ions in the soil expressed in Moles/Liter. The pH scale runs from 0 to 14. Thus, a low number indicates a higher concentration of H+ ions and thus a more acidic soil. Conversely, a higher number indicates a lower concentration of H+ ions and a more basic soil. Also, because the pH scale is logarithmic a pH of 4 is 10 times more acidic than a pH of 5.

Soil pH and Nutrient Availability

Soil pH affects the availability nutrients for the plant. Generally speaking, the metal plant nutrients, Fe, Mn, B, Cu, Zn, whereas, other nutrients become unavailable at higher pH levels. P is a macronutrient that becomes unavailable at high and low pH levels. The ideal pH level is 6.5 for most vegetable plants. Some plants like Blueberries and Azaleas are “Acid Loving” plants and grow best at lower pH between 4 and 5. The table below shows where on the pH scale plant nutrients are most available.

Soil pH and Soil Microbe Activity

Soil microbes are essential for plant growth. Microbes break down plant nutrients and make them available to plants. This is especially true for one of the most important plant nutrients, Nitrogen. There are many forms of Nitrogen. The air we breathe is mostly N2. But this form of nitrogen is not available for plants. It takes microbes to make different forms of nitrogen available for plants. These microbes thrive at specific pH levels between 6.0 and 7.5.

Changing Soil pH

Having a soil sample tested is the best way to determine the pH level of the soil. Most states have a university that specializes in agricultural and will test soils samples for a modest fee. If the soil is too acidic lime can be added to raise the pH level. If the soil is too basic, elemental sulfur can be added to lower the pH level. The soil test will usually give recommendations on you much lime or sulfur is needed to achieve the desired pH level.