Source: JARS V46:No.2:p77:y1992
Tips for Beginners: How To Adjust Acidity Levels in Your Soil
The rhododendron neophyte should know that rhododendrons will tolerate a fairly wide range of conditions with respect to acidity. Most people will have no insoluble problem. Different areas of the country, indeed different areas within the space of a few miles, however, may have different levels of acidity. The comments below, taken from Fred Galle's book Azaleas, Chapter 9: Planting and Care of Azaleas (Timber Press, 1987, revised and Enlarged Edition) should be helpful.
The soil for rhododendrons and azaleas should be acid, somewhere between very strong and medium, that is, a pH of 4.5 to 5.5 or 6.0. The pH scale is graduated from 0 to 14. A pH of 7.0 is neutral; reactions above 7.0 are alkaline and below 7.0 are acid. The degree of alkalinity or acidity increases or decreases by 10 times the previous level for each whole number change. Thus, a pH of 5.0 is 10 times more acid than a pH of 6.0 and 100 times more acid than a pH of 7.0.
Soils of the forest or originally forested areas are usually suitable for rhododendrons and azaleas. Soils originating from limestone are alkaline or close to it. Alkaline soils are usually found in up thrust sedimentary rock formed in ancient oceans. To grow rhododendrons and azaleas in such alkaline or neutral soils, it is best to improve the soil by adding large quantities or organic matter.
Your county agent or agricultural officer can provide information on how to have your soil tested for pH and nutrients in a soil analysis lab. The soil test report will indicate if either lime or acid forming materials are needed to adjust the pH of your soil. Soil testing kits are available, but the results can often be misleading, due to deterioration of the chemical reagents.
The best range is between 4.5 to 6.0. Azaleas and rhododendrons growing in a low pH of 3.5 to 4.5 will be healthy, but will often grow at a slower rate than normal. Foliage of these plants growing in a soil pH of 6.5 and above may appear yellow, an indication of chlorosis. If the soil pH is above 6.0, add ground sulfur (flowers of sulfur) or ferrous sulfate, not aluminum sulfate, to increase the acidity. Approximately 1.5 pounds of ground sulfur per 100 square feet mixed into the soil will lower the pH from 6.0 to 5.5 or 1/2 point. Sandy soil will require less sulfur, 0.5 pounds per 100 square feet, while heavy clay soils may require up to 2.5 pounds of sulfur, plus the addition or organic material. It is also advisable to add 1/2 pound per 100 square feet of ferrous sulfate with the ground sulfur. Do not add additional sulfur to the soil until you have had the soil retested.
Chlorosis is caused by a reduction of chlorophyll in the leaves. An improper relationship between soil pH and availability of iron causes chlorosis. Iron is most available to plants in an acid soil of pH 4.5 to pH 6.0. The symptom of rhododendrons and azaleas deficient in iron is chlorosis-leaves are yellow with prominent dark green veins-a warning sign to the grower for making the necessary adjustments. Chlorotic leaves may sometimes have a high iron content, but the iron has been converted to an unusable form due to an excess of calcium carbonate.
Chlorosis may be caused by other factors, however, such as poor root growth, over fertilization, soil nematodes, or poorly drained soils. If the plant is suffering from iron chlorosis, quick but temporary results can be obtained by spraying the foliage with iron sulfate (ferrous sulfate or copperas) at the rate of one ounce per two gallons of water. Chelated iron or iron sequestrenes are also effective as a foliage spray or soil application.
A plant deficient in magnesium looks very similar to one affected by iron chlorosis in the early stages. The yellowish leaves later develop reddish purple blotches, followed by a browning of the tip and margin. Epsom salts (magnesium sulfate) should be applied as a foliage spray at the rate of one ounce per gallon of water.
(With the permission of Mr. Galle, the wording of the above section, "Soil Acidity' from Azaleas, has been modified somewhat to include all rhododendrons.)
Test Your Beds
After having determined the acidity of alkalinity of your soil and, very importantly, the pH of the water that you may need to use during periods of dry weather, test your beds with actual rhododendrons or azaleas. They will let you know. In the long haul, oak leaves and pine needles are a wonderful mulch and promote acidity. If chlorosis does appear, add a tablespoon of ferrous sulfate to a two-gallon sprinkling can of water and sprinkle the plant, being careful not to get close to the main stem. Then wash the solution off the leaves with a hose. After a few weeks, repeat if it appears necessary. Different species and different hybrids may have somewhat different requirements.
Galle, Fred C. 1987. Azaleas, Timber