Clay Soil pH Levels

Clay soil is also known as "heavy" soil.

The relative acidity of a substance is measured by its pH; this is important to gardeners because certain plants require certain soil pH levels to survive and flourish. These requirements vary, but generally speaking, plants are most comfortable in neutral soil (pH 7) even if they are capable of doing fine in more acidic or more alkaline environments. Being aware of soil pH levels, and knowing how to change them, is essential.

Clay Soil

Almost all soil contains some clay, in combination with sand, silt, minerals and organic matter, but to be classified as "clay soil," 50 percent or more of a soil should be made up of clay particles. (Such soils are sometimes also called "heavy soils.") Clay soils can often be a problem for gardeners because the absorbent clay retains much more moisture and drains much more slowly than the sandy material of lighter soils. They also tend to be unusually alkaline--that is, they have a high pH--which can be a problem with certain plants.

Plant Requirements

Very high alkalinity--pH levels over about 8--is unhealthy for nearly every plant. But plants to whom alkalinity is particularly unfriendly include most household, greenhouse and fruit-bearing plants, as well as many vegetables.

Changing Soil Composition

If necessary, it's possible to very slowly change the pH of clay soil. The gentlest way to do this is to change the composition of the soil itself, making it lighter and less of an actual clay soil. Over time, add organic matter such as sawdust, composted leaves, wood chips, leaf mold and peat moss. As these materials decay, they will acidify, lowering the soil pH. This is best done patiently, over several seasons; a good rule of thumb is not to attempt a pH change of over 1 point in a single season.

Adding Sulfur

A more direct and drastic way to acidify soil is to add sulfur. Adding 3.6 oz. of ground rock sulfur per square yard will lower the pH of clay soil by 1 point. Make sure you mix the sulfur thoroughly into the soil before planting. There will be less potential for failure if you adjust pH levels gradually, through the addition of organic matter. And even sulfur modifications shouldn't exceed a few points in a single season; but sulfur will get the job done fast.

Giving Up

If you are dealing with a highly clay-based soil with a very high alkalinity--so high that extensive modification would be necessary to get almost anything to grow there--it may well be better to abandon the soil altogether and build a raised bed and fill it with more hospitable soil gathered from somewhere else or even purchased from a garden store.

References

• The Garden Helper: How to Test and Adjust your Soil pH
• Bachman's: Working With Clay Soil

Resources

• Home Harvest Garden Supply: Plant pH Preference Range