Testing Soil For pH
Sandra McDonald, Ph.D.
Reprinted from the "Mid-Atlantic Rhododendron, News and Notes", Mid-Atlantic Chapter
We all know that azaleas and rhododendrons need acid soils to do well, and that we should have our soils tested by the Agricultural Extension Service, but we are not told how to take samples except for general crops.
Winter is usually a good time to take soil samples, though they can be taken at any season. It is best to take samples from areas that have not not been recently fertilized. The sample should not be taken from the soil surface, but rather from one or two inches below down to about 6 inches deep, where the plant roots will be. Take several samples from around the plant and mix them together. Do not take the sample from too near the trunk, as you may injure a large root. Samples from near the edge of the foliage canopy are ideal. Samples should be taken when the soil is dry enough to plow. Use a small trowel or spade and take a slice of soil about 1 inch thick. It is also advisable to take a separate sample from the subsoil, especially if you are having problems growing plants in that particular area of the garden. The samples from the subsoil and the samples from the top 6 inches should be analyzed separately. Use a clean container or clean plastic bag for your samples. Be sure to label all samples so that you will know the locations from which they were taken when you get the soil test results back weeks later.
A pH of 5.5 is usually recommended for azaleas and rhododendrons, but anywhere in the range of 5 to 5.7 is probably adequate. Dolomitic ground limestone or lime should be added for pH readings below 5, the amount depending on how low the pH is. Your extension agent can make recommendations. Dolomitic lime should be used if your soil is deficient in magnesium.
If your pH is too high (above 6) for your rhododendrons to do really well, sulfur (flowers of sulfur) can be used to lower the pH. Do not use aluminum compounds as some people suggest, as aluminum is toxic to rhododendrons.
Your water supply can affect your soil pH if you irrigate. Some local water supplies have very alkaline water which can raise the soil pH when used for irrigation. Have the pH of your water tested if you think there is a problem with it.
Fertilizers and decomposition of organic matter (mulches) can affect your soil pH, usually making it more acidic. Therefore pH tests should be run every few years to make sure that your soil is not changing too much for your plants, especially if you use fertilizers.