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.
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.
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 go to MyTurfandGarden.com and click on "Soil Testing" in the top menu bar. 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.
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.