Soil pH And Rhododendrons
Important Facts To Consider
Weldon E. Delp, Harrisville, Pennsylvania
Reprinted from "The Rhodo Rooter", Great Lakes Chapter newsletter
First published in the Great Lakes Study Group Newsletter #7.
Soil dynamics are tied closely to soil pH. Microorganisms, especially beneficial bacteria, are not active when the pH drops below 5.0. Remember 7.0 is neutral.
Several nutrients, including phosphorus and iron are "locked up" in acid soils, essentially unavailable to rhododendrons. Phosphorus is one of the most complex and least understood of the three major nutrients. Used by a plant for cell division, seed and root development, its presence or absence is tied closely to plant maturity. When phosphorus is "locked up", iron, in turn, becomes unavailable, reducing a rhododendron's ability to photo-synthesize.
Rhododendrons will grow beautifully when soil pH is 5.5 to 6.5, but poorly at a pH of 4.5 (some recommend 4.5 for control root rot). This I feel is wrong because of "lock up". Most root rot problems are cultural.
Acid rain, fertilizers and some types of mulches acidify soil. Other factors, such as run-off from concrete structures create alkaline conditions. Most organic and inorganic fertilizers are acid. An exception is wood ashes, which raise soil pH as effectively as limestone. Peat moss and compost vary greatly in acidity. Leaf mold consisting of maple leaves will be slightly acidic while oak leaves and pine needles make very acid compost. Spent mushroom manure is a pH of 7.0 or above.
Soil and pH testing are the only way to accurately determine soil acidity. Check pH periodically.
If pH adjustment is indicated, limestone is used to "sweeten" the soil, or raise its pH, and sulphur is used to lower it. In western Pennsylvania and elsewhere, we are fortunate to have natural acidic soils, but beware, it could be too acidic.
We are confronted with a choice of products to raise soil pH. Which one is the right one? Following are a few tips on different liming products:
All lime contains calcium, an essential plant nutrient. The chemical formulation of this calcium is responsible for the difference in various types of lime.
Quick lime, also called burned lime, is calcium oxide. Being 80 to 95 percent calcium, it has twice the neutralizing power of crushed limestone, calcium carbonate. It's also rapidly available to plants, but is relatively expensive and doesn't last very long. Add these to the fact burned lime is difficult to handle. I do not recommend using it.
Hydrated lime is just as tricky to use. It's calcium hydroxide (calcium oxide plus water). This product is strong enough to burn growing plants.
Dolomite lime is safe and slow-acting. Its pink color comes from its magnesium content, found naturally in the mineral dolomite. Although it's more expensive than crushed limestone, it's a good source of the essential nutrient magnesium, often lacking in western Pennsylvania and other soils.
Gypsum is sometimes offered as a liming product. It is made of calcium sulfate, and will not raise soil pH. It just adds calcium to the soil - a good choice for those rare occasions which call for calcium, but no pH change. Gypsum is used primarily as a soil amender to break up heavy clay soils and counteract salt contamination.
Rhododendron growers probably are wise to choose either plain old crushed or agricultural limestone, or dolomitic lime, if soil tests indicate a need for magnesium Added in advance of the gardening season, it will ensure beneficial organisms, and your rhododendrons a good growing season.