If your rhododendron and azalea leaves look yellowish and are smaller than normal, they could need a more acidic environment.
The plant could be growing where it is too wet or too dry or could need additional nutrients, such as nitrogen, sulfur or iron. However, rhododendrons, azaleas, blueberries, heathers and hydrangeas typically need a strong acidic environment, and if the soil has the wrong pH, they are unable to absorb nutrients.
John Hart, soil scientist with Oregon State University Extension Service, recommends the following sequence of strategies to help keep plants healthy and in optimal condition.
"The most effective way to correct iron deficiency is to acidify the soil," Hart says. "Cold and wet times of the year, such as winter or early spring, are a good time to acidify soil. But first, be sure that acidification is necessary. Have the pH of your soil tested, or test it yourself before you acidify."
To acidify an established tree or shrub, dig four, small, foot-deep holes per shrub or tree, around and away from the trunk. Dig the holes a foot from the trunk of a shrub, 2 feet away from a small tree, 3 feet away from a medium tree and just inside the drip line of a larger tree. Using a soil auger or bulb planter can help to make these small holes deep enough. Add a half cup of elemental sulfur (S), phosphoric acid or sulfuric acid, divided equally among the four holes.
"Wait until next spring or summer to gauge the plant's response," Hart warns. "Again, check your soil pH before proceeding."
Iron can be sprayed directly on plants' leaves to complement a soil-acidification program if a severe iron deficiency exists. Leaf (foliar) iron sprays usually are short-lived and need to be applied every other week during rapid growth.
For more details, an OSU Extension Service publication, "Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden West of the Cascade Mountain Range in Oregon and Washington" (EC 1560-E), can be found at http://extension.oregonstate.edu/catalog/pdf/EC/EC1560-E.pdf