The pH level of your soil indicates its relative acidity or alkalinity. A pH test measures the ratio of hydrogen (positive) ions to hydroxyl (negative) ions in the soil water. When hydrogen and hydroxyl ions are present in equal amounts, the pH is said to be neutral (pH 7). When the hydrogen ions prevail, the soil is acidic (pH 1 to pH 6.5). And when the hydroxyl ions tip the balance, the pH is alkaline (pH 6.8 to pH 14).
Most essential plant nutrients are soluble at pH levels of 6.5 to 6.8, which is why most plants grow best in this range. If the pH of your soil is much higher or lower, soil nutrients start to become chemically bound to the soil particles, which makes them unavailable to your plants. Plant health suffers because the roots are unable to absorb the nutrients they require.

To improve the fertility of your soil, you need to get the pH of your soil within the 6.5 to 6.8 range. You can’t, and shouldn’t try to change the pH of your soil overnight. Instead, gradually alter it over one or two growing seasons and then maintain it every year thereafter. Liberal applications of organic matter is a good idea too because it helps to moderate pH imbalances.

Acidic Soil
If the pH of your soil is less than 6.5, it may be too acidic for most garden plants (although some, such as blueberries and azaleas require acidic soil). Soils in the eastern half of the U.S. are usually on the acidic side.

The most common way to raise the pH of your soil (make it less acidic) is to add powdered limestone. Dolomitic limestone will also add manganese to the soil. Apply it in the fall because it takes several months to alter the pH.

Wood ash will also raise the pH, and it works more quickly than limestone and contains potassium and trace elements. But if you add too much wood ash, you can drastically alter the pH and cause nutrient imbalances. For best results, apply wood ash in the winter, and apply no more than 2 pounds per 100 square feet, every two to three years.

To raise the pH of your soil by about one point:

• In sandy soil: add 3 to 4 pounds of ground limestone per 100 square feet.
• In loam (good garden soil): add 7 to 8 pounds per 100 square feet.
• In heavy clay: add 8 to 10 pounds per 100 square feet.

Alkaline Soil
If your soil is higher than 6.8, you will need to acidify your soil. Soils in the western U.S., especially in arid regions, are typically alkaline. Soil is usually acidified by adding ground sulfur. You can also incorporate naturally acidic organic materials such as conifer needles, sawdust, peat moss, and oak leaves.

To lower soil pH by about one point:

• In sandy soil: add 1 pound ground sulphur per 100 square feet.
• In loam (good garden soil): add 1.5 to 2 pounds per 100 square feet.
• In heavy clay: add 2 pounds per 100 square feet.

An excerpt from the article, “Building Healthy Soil“, by Kathy LaLiberte, courtesy of Gardener’s Supply Company.