Soil pH

Different types of plants require varying degrees of soil acidity. In fact, some plants are very sensitive to soil pH.

Rhododendrons and heathers will not tolerate lime in the soil. On the other hand Clematis prefers an alkaline soil.

What does soil pH mean?

The acidity or alkalinity of the soil is measured by pH (potential Hydrogen ions).

Basically it is a measure of the amount of lime (calcium) contained in your soil, and the type of soil that you have. Generally, soils in moist climates tend to be acid and those in dry climates are alkaline. A soil with a pH lower than 7.0 is an acid soil and one with a pH higher than 7.0 is alkaline.

The soil must be adjusted to suit the plant which will occupy that area if it is not already within that plants requirement range.

Testing your soil pH

Most good garden centers will even gladly pH test a soil sample for you, or you can buy an inexpensive pH test kit at most nurseries, or hardware stores.

These test kits generally consist of a test tube, some testing solution and a color chart. You put a sample of your soil in the tube, add a few drops of test solution, shake it up and leave it for an hour or so to settle.

The solution in the tube changes color according to the pH of your soil. Compare the color of the sample with the color chart that came with the kit. Matching colors will tell you the pH of your sample. The better kits will also advisory booklets about how to interpret your result.

Adjusting your soil pH

Once you have determined the pH you can amend the soil, if needed to accommodate the plants in your garden using materials commonly available at your local garden center.

Raising the soil pH to make it more alkaline

Generally speaking, it is easier to make soils more alkaline than it is to make them more acid. Because different soil types react in different ways to the application of lime you will have to add more lime to clay soils and peaty soils than you will in sandy soils to achieve the same result.

To increase your pH by 1.0 point and make your soil more alkaline:

Add 4 ounces of hydrated lime per square yard in sandy soils

Add 8 ounces of hydrated lime per square yard in loamy soils

Add 12 ounces of hydrated lime per square yard in clay soils

Add 25 ounces of hydrated lime per square yard in peaty soils 

Correction of an overly acid soil should be considered a long term project, rather than trying to accomplish it in one year. It is better to test your soil each year and make your adjustments gradually. The addition of hardwood ash, bone meal, crushed marble, or crushed oyster shells will also help to raise the soil pH.

Lowering the soil pH to make it more acidic

If your soil needs to be more acidic, sulfur may be used to lower the pH if it is available. To reduce the soil pH by 1.0 point, mix in 1.2 oz of ground rock sulfur per square yard if the soil is sandy, or 3.6 oz per square yard for all other soils. The sulfur should be thoroughly mixed into the soil before planting. Sawdust, composted leaves, wood chips, cottonseed meal, leaf mold and especially peat moss, will lower the soil pH.