Rhododendrons grow best in acid soil with a pH factor somewhere between 4.5 and 5.8. If the rhododendron has been grafted on to a tough, tolerant species, it may be able to cope with a pH factor of 6.5. The pH factor of most cultivated gardens is usually somewhere between 5 and 8, so, sometimes, the soil might be too alkaline for rhododendrons.
The pH factor (from Latin potentia hyrogenii - potency of water) defines the whole range of chemical reactions, from acid to alkaline, in the soil on a scale of 1-14. Factor 7 is a neutral reaction of the soil. Factors below this are more or less acid; factors above 7 indicate alkaline soil. The pH factor is largely determined by the chalk or lime content in the soil. The more chalk there is in the soil, the higher the pH factor will register, which means the lower the acid content will be.
A slightly acidic soil is the determining factor for the well-being of rhododendrons. If you are planting them for the first time, you should definitely test your soil first. Various testers are available in the gardening trade, so you will be able to determine your own garden's pH factor quite easily.
As described above, take soil samples from six different spots, mix them, put them in a plastic bag and send them to the soil institute.
If the pH factor is too high, plants which are accustomed to acid soils can no longer absorb iron or magnesium, both of which are essential for the formation of chlorophyll.
The result will be yellowing leaves, which may show green veins to begin with, followed by a complete cessation of growth in the entire plant. The higher the pH factor stands above 7, the more visible these symptoms of deficiency will become. This condition is called chlorosis. If the pH factor is too low (below 4), on the other hand, the roots will be damaged. The plant will not develop new roots and will eventually be unable to absorb nutrients.
The best way to lower the pH factor is by working peat into the soil. If you would like to lower the pH factor by one point, for example from 6.5 to 5.5, you would have to reckon on about six bales of moist peat worked in to a depth of 30cm (12in) over an area of 10sq m (12sq yd).
Use lime to raise the pH factor. To raise the factor by one point on the scale (for example, from 3.5 to 4.5), sprinkle the following amounts of lime on 10sq m (12sq yd):