How to Change the pH of Soil

Soil pH is measured on a scale of 1 to 14. Soil below 6.5 is said to be acidic, while soil above 7 is alkaline. A pH of 6 to 7.5 will make essential nutrients available to most plants. To learn whether amendments are needed to raise or lower your soil pH, you can test it with a kit available at most garden supply centers.

Acidic soil is typically found in areas with heavy rainfall. You may need to raise the pH of acidic soils. Alkaline soils are typically found in dry, arid areas or areas with heavy, clay soil. You may need to lower the pH of alkaline soil.

Dolomitic lime, magnesium carbonate, is the amendment of choice for raising soil pH because it adds an essential nutrient, magnesium, to the soil. It is sold at most garden supply centers. You need to till the amendment into the top 6 inches of soil.

How much lime you add depends on the quality of your soil; sand typically needs less and clay the most. Loam, soil with roughly equal amounts of sand, silt and clay, lies somewhere in the middle.

You should begin the process of increasing soil pH least a year before planting anything. Increasing pH is a slow process, typically taking one to two years.

As an example, you can use dolomitic lime to increase sandy soil from a pH of 6 to 6.5:

 Dolomitic lime

Tiller

Till 100 square feet of soil 12 inches deep.

Scatter 2 pounds of dolomitic lime over the soil.

Till the lime into the soil 6 inches deep.

Use the same method to move loam or clay soil from a pH of 6 to 6.5. Add 3 1/2 pounds of dolomitic lime to loam and 5 pounds to clay.

If your soil ranges from a pH of 4 to 5.5, add these amounts of dolomitic to raise the pH of 100 square feet of soil to 6.5.

To go from a 4 to 6.5 pH, add 10 pounds to sand, 17 1/2 pounds to loam and 23 pounds to clay.

To go from a 4.5 to 6.5 pH, add 8 pounds to sand, 15 pounds to loam and 20 pounds to clay.

To go from 5 to 6.5 pH, add 6 1/2 pounds to sand, 10 pounds to loam and 15 pounds to clay.

To go from 5.5 to 6.5 pH, add 4 1/2 pounds to sand, 7 1/2 pounds to loam and 10 pounds to clay.

You can also add up to 2 pounds per year of wood ashes to 100 square feet of soil. If you add more than that, you risk overloading the soil with potassium that can suppress the ability of plants to obtain magnesium and calcium from the soil.

Elemental sulfur, available at most garden supply centers, is the amendment of choice for lowering soil pH.

Here's how to lower a soil pH of 7.5 to 6.5 in 100 square feet of sandy or loamy soil:

Elemental sulfur

Rototiller

Till the soil 12 inches deep.

Spread 2 pounds of sulfur on the soil.

Till the sulfur into the soil 6 inches deep.

Here are the amounts of sulfur necessary to lower soil with a pH of 7.5 to 6 and lower in 100 square feet of sandy or loamy soil.

Add 3 1/2 pounds for 6 pH.

Add 4 1/2 pounds for 5.5 pH. 

Add 6 pounds for 5 pH. 

Add 7 pounds for 4.5 pH.

These are the sulfur amounts to lower 7 pH soil:

Add 1 pound for 6.5 pH. 

Add 2 pounds for 6 pH.

Add 3 1/2 pounds for 5.5 pH.

Add 5 pounds for 5 pH. 

Add  6 pounds for 4.5 pH.

Sulfur amounts to lower 6.5 pH soil:

Add 1 pound for 6 pH. 

Add 2 1/2 pounds for 5.5 pH. 

Add 4 pounds for 5 pH. 

Add 4 1/2 pounds for 4.5 pH.

Sulfur amounts to lower 6 pH soil:

Add 1 pound for 5.5 pH.

Add 2 1/2 pounds for 5 pH. 

Add 3 1/2 pounds for 4.5 pH.

Increase these amounts by 50 percent for clay soil.

Commercial liquid acidifiers are available that will temporarily lower soil pH on small areas. A typical application rate is 1 tablespoon per gallon of water to treat 10 square feet. Repeat the application every two weeks.

To lower soil pH organically, add 14 pounds of compost, 5 pounds of manure or 2 1/2 pounds of peat moss per square yard.