EPSOM SALTS FOR RHODODENDRONS
In soils that are deficient in readily available magnesium, rhododendron leaves may tend to be yellowish with a little more green color along the veins. This sometimes gives rise to a distinct herring-bone pattern which follows the direction of the secondary veins in the leaf. This condition may be counteracted by using some form of magnesium, the most readily available and cheapest form usually being magnesium sulfate or Epsom salts.
In many cases, however, yellowish color of the leaves is simply due to lack of readily available nitrogen. The probability of this being so is enhanced by the rather widespread use of mulch for rhododendrons. Many people are using sawdust, shavings, ground bark and other organic materials to conserve moisture and keep down weeds, and eliminate the need for cultivation which would do considerable damage to the shallow roots of rhododendrons. Such organic mulches are fine but they do tend to tie up the available nitrogen with the result that the leaves may become quite yellowish green. Usually this is a rather uniform color and not with the darker green veins as is true with magnesium deficiency.
Some gardeners tend to confuse the two and consider nitrogen and magnesium as somewhat interchangeable remedies for yellow leaves. Both are essential elements, of course, but in many soils magnesium is not deficient and its use will give no visible benefit. The conditions under which the plants are growing, and the color pattern, will usually give a clue as to which element is deficient, although there are other deficiencies, less common perhaps, which occasionally cause off-color of rhododendron leaves.