Field Growing Rhododendrons - Soil Test Now
Oregon State University Extension Service
Many rooted rhododendron cuttings will be moved to the field during the next couple of months or so. Now is the time to test and ready the soil for these plantings.
Generally, a soil pH near 5.5 is best for nutrient availability for rhododendrons. Phosphorous, calcium and magnesium levels should be relatively high. Soil iron levels, important to rhododendrons, are usually sufficient if these other requirements are met, although iron in a fertilization program does no harm.
Well drained soils are a must. This may seem elemental, but it is sometimes given inadequate consideration. Various root diseases, such as Phytophthora, and nutrition are less likely to be a problem in well drained soils.
Most of our soils will grow better rhododendrons if an organic material, such as sawdust is added to the soil. Remember that decomposition of soil organic materials does increase soil nitrogen needs, beyond that needed just for rhododendron growth. As a starter, apply 60 pounds of actual nitrogen (equivalent to 190 pounds ammonium nitrate or 300 pounds ammonium sulfate) for each acre-inch of sawdust. It's best to apply this to surface of sawdust prior to incorporation. Since it takes about four years for sawdust to completely break down in soil, make reduced applications of about 30 pounds of actual nitrogen in second year for each acre-inch of sawdust originally mixed with soil, and 15 pounds in third and fourth years.
NOTE: These nitrogen applications are over and above the needs of rhododendrons themselves, and adjustments may be needed to keep plants growing well.