Sweet or Sour: The story behind soil pH
Soil pH is a measure of the acidity or alkalinity of your soil. A pH of 7 is neutral, a pH below 7 is acidic or sour, and a pH above 7 is alkaline or sweet.
Why is soil pH important?
Maintaining proper pH levels in your soil is as important to the overall health of your plants as fertilizing, watering, and pest control. Why? Soil pH affects nutrient availability for one thing. Certain nutrients like iron, copper, and aluminum become less available to plants in alkaline soils and other nutrients such as calcium and phosphorus become less available in moderately acidic soils. For example, the availability of aluminum in acidic soils is what gives hydrangea flowers a blue color. The optimum pH range for most garden plants is between 6.0 and 6.5, slightly acidic. Certain plants, however, prefer more extreme pH conditions. Shrubs such as rhododendrons, azaleas, and blueberries prefer a more acidic soil so be sure to check the preferences of your plants.
What causes my soil to be either acidic or alkaline?
One of the main factors affecting soil pH is the amount of rainfall that an area receives. Soils in climates with high rainfall and high humidity tend to be on the acidic side because the salts which tend to make the soil alkaline are leached out. In climates with low rainfall or where the soils have poor drainage, the pH tends to be higher due to the accumulation of these salts in the soil. In addition, soils formed on limestone bedrock, such as we experience here in the Shenandoah Valley of Virginia, tend to be more alkaline.
How can I determine my soil pH?
It is a good idea to have your soil tested. Kits to test soil pH are readily available at your local full-service garden center or order one
from the Cornell University Nutrient Analysis Laboratory. These come with detailed instructions on how to take the soil sample(s) to test. Alternatively, your local agricultural extension service or some full-service garden centers will test your soil for a nominal fee. The analysis from these tests will allow you to make informed decisions on how to improve the soil environment for your plants. If you choose to have your soil tested professionally, you will be provided with recommendations for amendments to improve the pH and also nutrient content if necessary.
Adjusting Soil pH
Amendments for acidic soil
- Ground limestone (calcium carbonate) is the most effective way to sweeten your soil (reduce acidity).
- Pelletized limestone is effective and very easy to apply.
- Dolomitic limestone has the added benefit of adding magnesium to the soil.
- Wood ashes are not as effective as adding lime but can raise pH if used carefully.
Amendments for alkaline soil
- Sulfur in some form is the most effective way to acidify your soil. Straight chemical or elemental sulfur is commonly used but it is slow to affect a change in soil pH.
- Aluminum sulfate causes a more rapid decrease in soil pH but the aluminum can become toxic to plants if it is overused.
- Iron sulfate adds iron to the soil as well as lowering pH.
- Organic amendments for lowering soil pH include peat moss, pine straw, or sphagnum moss. These amendments are beneficial to soil structure but in order to affect pH, they must be added in large quantities and then the pH change is very slow.
The Good News on Soil pH!!
The good news is that with the exception of azaleas, rhododendrons, and other ericaceous plants, most perennials, annuals, trees, shrubs, and evergreens grow quite well on soils with a wide pH range. For example, the flowering dogwood (Cornus florida) as well as the eastern redbud (Cercis canadensis) grow quite well on the acidic soils of Long Island, NY and equally well on the alkaline soils found in the Shenandoah Valley of Virginia.
The best all around range of soil pH to try to achieve in your flower gardens is 6 - 6.5.