The genus Rhododendron, from the family Ericaceae, include species and hybrids of azaleas as well as rhododendrons. They are considered ornamental shrubs and are small to medium in size. Rhododendrons are hardy in United States Department of Agriculture (USDA) zones 4 to 8. Soil preparation and maintenance is important to the care of this genus.
Rhododendrons prefer partial shade and filtered sunlight--enjoying early sunlight, but needing protection from the afternoon sun. They require moist, well-drained acidic soil. Rhododendrons flower in a variety of colors including white, pink red and lavender. Flowers are 1 to 3 inches in diameter and cluster in a small area, evenly spread across the top leaves. They have green leaves that are oblong to oval.
Soil conditions are important for proper plant growth. There are key elements in the soil that plants need to obtain nutrients for growth, flowers and fruit production. These key elements, to name a few, include the mineral nutrients nitrogen, phosphorus, potassium and magnesium. Soil pH is another key soil element; pH is a measure of the acidity or alkalinity of the soil. Rhododendrons require more acidic soil, with a pH of 5.0 to 6.5. This can be achieved by amending the soil with aluminum sulfate prior to planting or making sure the plant receives adequate acidic fertilization. Soil acidity or alkalinity affects a plant's ability to take in the important elements. Soil may be missing these elements due to conditions such as leaching, caused by heavy repeated rains or overwatering; or it may simply have an over-abundance of one element over another. Additionally, high levels of potassium can interfere with the roots' ability to take in magnesium.
A soil test analyzes the soil of a particular site to determine what essential elements are present. Soil tests are performed on lawns and gardens. The most basic tests determine soil texture and pH, as well as nutrient profile, including phosphorous and potassium content. Other more specific tests can be performed upon request. University Extension Offices offer soil testing for a minimal fee. Once the test is performed, the results are sent out and amendments can be made accordingly. If it is determined that the soil is magnesium-deficient or not acidic enough for a rhododendron, then the soil can be properly prepared before planting. Once the rhododendron is planted, the soil must be monitored and maintained with the proper elements, such as magnesium, and the proper acidic pH level, via aluminum sulfate, by for the plant to thrive.
Magnesium for Rhododendrons
If rhododendrons are grown in proper soil conditions, they are not likely to need any fertilization. However, if soil is poor--light and sandy--it will require an additive to boost acidity. Or if an overabundance of mulches like sawdust or woodchips have been added, decomposition of these materials will cause a nitrogen deficiency which, in turn, leads to magnesium deficiency. As a result of the need for magnesium, the rhododendron will undergo chlorosis, a condition that results in red to purple spots on the leaves and yellowish browning of the leaf edges on older rhododendrons. In some cases, a complete acidic fertilizer is enough for the plant to recover, but in other cases further measures may be required.
Epsom Salts for Magnesium
It has been debated whether using Epsom salts to boost magnesium levels in garden soil is beneficial. Epsom salts are magnesium sulfate (MgSO4). According to Washington State University's horticulturist Linda Chalker-Scott, Epsom salts do relieve some magnesium deficiencies in the garden. However, they cannot completely solve problems such as too much rain causing magnesium to be leached away--as it will also be washed away. But in cases where the soil itself is magnesium-deficient, such as in light and sandy soils, Epsom salts can help increase the missing amounts of magnesium.