Soil pH is a measure of the acidity or alkalinity of the soil, and it is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil. The pH scale ranges from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral. Soil pH is measured on a negative logarithmic scale (base 10), and the lower the pH, the higher the acidity. Most plants are favored by a pHCa (measured in a 0.01M CaCl2 solution and denoted as pHCa) between 5.5 and 8. Changes in soil chemistry and microbiology when pH is below or above this range adversely impact plant processes resulting in reduced growth and yield. Because of the logarithmic scale, a small decrease in soil pH results in a large increase in acidity. For example, there is 2.5 times more acid at pHCa 4.4 than at 4.8.

Soil pH will influence both the availability of soil nutrients to plants and how the nutrients react with each other. For example, at a low pH, many elements become less available to plants, while others such as iron, aluminum, and manganese become toxic to plants. Therefore, it is important to maintain the correct pH for healthy plant growth. A pHCa range between 5 and 6 is considered ideal for most plants.

To measure soil pH, a laboratory analysis using a standard protocol is required. The pH is measured in soil-water (1:1) and soil-salt (1:2) solutions. A basic soil test kit is available through county offices of Penn State Extension for a nominal fee. The kit contains an information sheet for each soil sample, which is necessary for the lab to make lime and fertilizer recommendations.

In summary, soil pH is a measure of the acidity or alkalinity of the soil, and it is important for healthy plant growth. Most plants are favored by a pHCa between 5.5 and 8, and changes in soil chemistry and microbiology when pH is below or above this range adversely impact plant processes resulting in reduced growth and yield. To measure soil pH, a laboratory analysis using a standard protocol is required.