Soil pH and Fertilizers

The pH of a soil refers to how acid or alkaline the soil is. The letters “pH” mean “potential hydrogen.” The availability of nutrients is directly affected by soil pH. If the soil’s pH is too high or too low, some nutrients become insoluble, limiting the availability of these nutrients to the plant root system.

The acidity-alkalinity scale ranges from 0 to 14. Soils are referred to as being acid, neutral, or alkaline, depending on their pH levels. A pH of 7 is neutral, while a pH lower than 7 is acid, and a pH higher than 7 is alkaline (basic).

A logarithmic scale is used to measure a soil’s pH. That is, a change of one unit in the pH scale represents a 10-fold change in acidity or alkalinity. A soil with a pH of 5.0 is 10 times more acidic than a soil with a pH of 6.0 and 100 times more acidic than a soil with a pH of 7.0. This is one good reason to be very careful in trying to increase or lower soil pH. Some factors, including soil type and organic matter, affect the amount of materials needed to change soil pH. Fertilizers and rain affect soil pH. Organic matter and soil microorganisms are a few other factors that affect soil pH.

Agricultural limestone normally is used to increase the soil’s pH. Sulfur is normally used to lower the soil pH. But fertilizer and water normally change the soil pH more rapidly. Plants have specific pH requirements for normal growth. Most ornamental plants grow well in a pH range of 6.0 to 7.0. But azaleas, gardenias, camellias, and related plants require a soil pH level between 4.5 to 5.5. It is important to know the pH levels and nutrient requirements of your ornamental plants to ensure normal growth and success. The following table lists some woody and herbaceous landscape plants and their desired soil pH ranges.