What Are The Best Soils For Rhododendrons And Azaleas

Factors affecting the growth of rhododendrons and azaleas are several, all of which are so bound together as to constitute a balanced system in which the plant’s growth is a measure of the whole.

In marginal situations, where one factor or another is unsuitable for these plants, the balance may be upset, the conditions complicated and the causes of ill-success may seem quite obscure.

rhododendrons and azaleas soilsPin

In other words, there is no one simple formula by which you can say good results may be secured everywhere.

Yet in situations where natural conditions are reasonably satisfactory—and such places are numerous — these plants may be grown with great ease and simplicity.

In many other places, good conditions may be created. Without going deeply into the complexities of it all, let us examine some of the cardinal points.


Nutrition is certainly one of the main factors, and this, in turn, is of significance only in relation to such accessory matters as soil acidity, soil aeration, temperature, moisture, and site.

Since the discovery, some years ago, that practically all ericaceous plants demand acid soil, this factor has been strongly presented as all-important.

Certainly, soil acidity is the first thing to look after; yet it is not the only important thing.

Amount of Acidity Varies

Rhododendrons in nature occur on acid soils, but the amount of acidity varies with species, some being much more tolerant of lime than others.

It has been generally observed, however, that those occurring in limestone regions are usually growing on a layer of peat or leaf mold above the lime and thus have their roots in acid soil on the immediate site.

Therefore, acid soil is almost universally prescribed as the first necessary condition in growing these plants.

Impact of Acidity On Rhododendrons

The effect of acidity is mainly to make iron available to rhododendrons.

The ill-effects of alkaline or even a neutral soil solution seem to be in causing iron to assume a metallic, insoluble form in which, although present in the soil, it becomes unavailable to rhododendrons.

The result of this is an iron deficiency within the plant, manifesting itself as a sort of “anemia”.

If chlorophyll is not maintained, pallor and yellowness (technically called chlorosis) appear and the plant gradually dwindles and dies unless the condition is corrected.

In passing, it should be noted that other things than alkaline soil can cause pallor in the foliage. 

These might include lack of nitrogen, waterlogged soil, the presence of disease or unbalanced nutrition, or even a too-acid condition – an unexpected possibility!

Have Soil Tested

Regarding soil acidity, several practical points should be observed.

The first is that it pays to test your soil occasionally or have it tested.

Simple and inexpensive colorimetric soil testing kits are easily available, or testing may be done by agricultural agents and experiment stations.

For accurate readings, the testing solutions should not be more than a year old, as they lose their potency with age.

pH Scale Meaning

On the “pH scale” by which soil acidity is measured, the neutral point is 7.0, which is too high for rhododendrons. The correct range is between pH 4.5 and pH 6.0, the best all-around point being around pH 5.0.

It is possible to get your soil too acid, and some azalea growers contend that the application of calcium salts in limited quantity benefits Rhododendron molle and certain other azaleas.

This should not be regarded as a rule, however, and should be considered experimental and of application only on calcium-deficient soils.

Planting Medium

Peat (usually granulated dried sphagnum deposits) is an almost universally accepted medium for growing rhododendrons and azaleas.

In some regions, such as in Rochester, New York, where calcium is abundant, pure peat moss to the depth of the plant roots apparently makes a safe and highly successful planting medium.

Elsewhere, peat moss mixed with the natural soil benefits it physically as well as chemically and is a far better means of acidifying soil than the use of aluminum sulfate or other chemical salts.

Acid leaf mold, especially from oaks, is useful for the same purpose, also sawdust and certain other mulching materials.

Unforeseen Factors – Lime, Water, and Drainage

As previously implied, excessive lime is the most common cause of insufficient acid in soils for rhododendrons.

Although soil may be artificially “made” to be correct in its acidity, it may be quickly neutralized by out-of-sight factors; such as “hard” water or improper fertilizer, or drainage from neighboring sources.

Water from deep wells or certain municipal systems may contain salts of calcium, sodium, or magnesium in quantity sufficient to ruin a peat bed in a few waterings, and this is one probable cause of failure with ericaceous plants in some cities where no one seems able to grow them well.

Drainage from neighboring areas, such as the wash from rain falling on the upper portion of a gravel bank and entering an acid peat-bed near its base, can quickly neutralize the peat and cause the death of plants.

Plantings near the base of a masonry wall may get sufficient lime from rainwater, running off the wall, to create havoc. One sure way to detect such trouble is to test the soil.

A cure is to set up raised beds of peat, so located that drainage cannot enter, and plant your rhododendrons in these.

Use Salt as Acidifier

Organic means of increasing soil acidity where needed by using acid peat moss as suggested are considered preferable to treatment with inorganic chemicals because with organic acids iron will remain available over a higher pH range.

Certain chemical salts, however, have been utilized as acidifiers. Ordinary sulfur, applied at rates not exceeding one pound per 100 square feet per application, is one method of correcting alkalinity.

In sandy soils, it requires a pound of sulfur to lower the soil one pH degree, but in medium-loam soils, two to three times as much is needed.

Actually, in heavy-clay soils, correction by inorganic chemicals seldom is feasible.


A four-week interval between applications is advised where more than one pound per 100 square feet is needed. Tannic acid, applied in a weak solution, might also be used.

In all cases, the application of these soil amendments should be followed by soil testing to measure the pH.

Aluminum sulfate has been widely recommended as a soil amendment. It will increase acidity, but its continued application may produce aluminum toxicity, so proceed cautiously.

Other Elements

Essential elements that are likely to be deficient in soils supporting these broad-leaved evergreens are:

  • Iron
  • Nitrogen
  • Potassium
  • Phosphorus
  • Magnesium and sometimes Calcium
  • Boron or Manganese.

Nitrogen is perhaps best supplied in the form of ammonium, either in well-rotted animal manure or as ammonium sulfate.

Be careful, though, not to overdo the latter. Applications should be made in early spring only.

One tablespoon of ammonium sulfate to 10 quarts of water, applied through a watering pot is often the only fertilizer needed on a well-balanced, well-mulched soil.

Phosphorus may be applied in the form of superphosphate, potassium as sulfate or chloride, and magnesium in the form of Epsom salt.

It is unlikely that calcium, boron, or manganese will be deficient in your soil. These mineral deficiencies manifest themselves by symptoms of chlorosis similar to those of iron deficiency.

It may come as a surprise to some growers to realize that insufficient calcium can produce symptoms similar to those of excessive lime.

Soil Should Be Spongy

It should be understood that the physical condition of the soil can be equally as important as its chemical condition.

Good aeration of the soil is absolutely essential. Remember that almost all the species occur naturally on soils that are spongy, fibrous, and filled with air holes

Some, in the tropics, are actually epiphytes, living in tree-tops like orchids. Like a damp sponge, the soil should be moist at the same time that it is airy.

Organic is Not a Must

Now, organic matter is not necessary for this effect.

Phenomenal growth has been produced quite successfully in the laboratory, where rhododendrons were cultured on pure quartz sand and inorganic mineral salts, from seed to bloom.

Yet the most feasible garden method for getting this result is through the use of acid-reacting organic matter, such as peat, leaf mold, sawdust, and other media in the soil itself, to lighten it up and give it a spongy character.

Look out for Heavy Clay

The soil itself should not be heavy clay but should have enough sand and loam mixed in to afford good drainage.

This is especially important outdoors in regions of very heavy soil, where either a gravel base or special drainage should be installed beneath the rhododendron bed to insure the rapid percolation of water out of the bed and through the subsoil.

Fortunately, most garden soils are well-drained, so this need not be a problem everywhere. But on heavy soil, it should be checked.

Add Nitrogen

Whenever sawdust or similar material is used in the soil, nitrogen should be applied because the bacteria and fungi causing the decomposition of sawdust use up quantities of nitrogen and will deplete the soil of this element, if more is not supplied.

Add 150 pounds of ammonium sulfate per ton of dry sawdust (or one pound per bushel) to offset this loss of nitrogen.

Phosphorus is also depleted in the same way, so it is wise to add a small dose of superphosphate.

In a good many cases, the addition of ammonium sulfate and superphosphate, as noted above, will constitute about the only fertilizer treatment required.

Rhododendrons and Azaleas Easy To Ruin

Rhododendrons and azaleas can easily be ruined by too much attention; one of the common mistakes is to give them the same kinds of fertilizer and cultivation accorded ordinary plants.

These plants are different and should neither be cultivated nor treated with common garden fertilizers.

Choose Fertilizers Carefully

Judiciously used, so as to avoid destroying the acidity of the soil, well-rotted cow manure may be very helpful, but the soil should be tested because manure is alkaline.

Under no circumstance uses ordinary commercial fertilizers unless these are especially recommended for acid-loving plants.

Avoid the use of nitrate of soda altogether. There are several acceptable formulae, but only one or two will be mentioned here.

On the light soil of his Cape Cod garden the late CO Dexter successfully used one part of potassium nitrate to two parts of superphosphate and applied one trowelful (about ½” pint) of this dry mixture to each 3-foot plant.

Organic materials, such as tankage, are also good sources of nitrogen. Ordinarily, a high-nitrogen fertilizer is advised.

Provide the Essential Nutrients with this Solution

For complete coverage of all essential nutrients where rhododendron seedlings were growing on pure peat moss, agar or other non-nutritive soils, we have used, with success, the following formula which is Dr. Lewis Knudson’s “solution C”, devised for orchid work:

  • Calcium nitrate, Ca (NO3)2.4H20 = 1.00 grams
  • Monobasie potassium phosphate, KH2P04 = 0.25 grams
  • Magnesium sulphate, MgSO4.7H20 = 0.25 grams
  • Ammonium sulphate (NH4)2SO4 = 0.50 grams
  • Ferrous sulphate, FeSO4.7H2O = 0.025 grains
  • Manganese sulphate, MnSO4.4H2O = 0.0075 grams
  • Water = 1 liter

If desired, 1/10 N hydrochloric acid may be added to adjust the solution to pH 5.0 for rhododendron work, but this may not be necessary.

This is a liquid that may be diluted and poured on.

Several proprietary fertilizers, specially adapted for acid-soil plants, are also quite satisfactory.


Some growers say that no fertilizer should be applied later than the month of May.

We have found that the old idea that rhododendrons need no fertilizer except what they obtain from a decomposing mulch is not always adequate.

Wild conditions are not a measure of a good environment, but rather the successful survival of plants under conditions of harsh competition.

Nitrogen is almost always desirable, and some foliage troubles, often diagnosed as winter injury or disease, may be due to malnutrition.

Bacteria in Soil

In addition to its chemical and physical aspects, the soil also has a biological side.

Good horticultural soil, or the woodland soil where rhododendrons grow wild, is teeming with life.

Bacteria and fungi are present, breaking down organic matter into organic acids and turning insoluble elements into soluble salts. Some of these fungi, called mycorrhiza, live on and within rhododendron roots.

Soil Fungi

It has been said that rhododendron; along with certain other ericaceous plants, have no regular root hairs which, in most plants, are the tiny microscopic cells attached to roots, through which soil water and solutions enter the plant.

Investigations have not yet entirely cleared the matter, but it is suspected that soil fungi may play an essential role as substitutes for root hairs, functioning beneficially as absorbing organs.

Their inclusion within the very cells of the root could make them symbiotic instead of parasitic.

It is assumed that fibrous, peaty soil, full of organic matter, is essential for their welfare in nature. Much remains to be proved as to their total effect, but it is a fact that most wild rhododendrons are equipped with a mycorrhiza.

Mulching Important To Rhododendron Care

The matter of mulching is important. The roots of rhododendrons are relatively shallow, and they are very delicate. They must have both air and moisture.

They need protection against extremes of brat and cold.

In nature, leaves, moss and organic remains furnish a protective mantle which at once supplies this requirement and, also, adds nutrients to the soil as it slowly decomposes.

In the Northeast, the ideal mulch is a covering of oak leaves, applied to the surface of the soil in the autumn and allowed to remain throughout the year.

For large species, the initial application may be eight inches deep. It will settle under the winter snow and slowly decay.

Oak leaves are fibrous, contain much acid, and decompose slowly. Pine needles, likewise, make a satisfactory mulch.

The leaves of thin-leaved sorts, such as elms and maples, decompose quickly and, while acid at first, they soon disintegrate and become alkaline in reaction, losing their fibrous character in the process.

It is advised, therefore, to use oak leaves or pine needles for mulching and to avoid the use of the other kinds of leaves unless they are applied for winter protection only and removed in spring.

Mulch with Oak Leaves

Since root protection is needed in summer too, spring removal is not advised, however.

It is best to use a mulch of oak leaves and allow it to remain permanently.

This is especially important in regions where water is unfavorably “hard”, for the summer mulch, coupled with a moisture-retentive peaty soil below, will obviate the need of watering the plants at all during the summer, except in times of severest drought.

Other Types of Mulch

If oak leaves are not available, other materials can serve for mulching.

The use of sawdust appears to be quite successful when nitrogen is added to replace losses, as suggested earlier.

Peatmoss is also useful but is better if incorporated into the soil. As a mulch, it becomes dry on top, causing rainwater to run off rather than soak in.

To counteract this, a light covering of leaves above the peaty mulch will keep it from drying and prevent such run-off.

Shredded cornstalks and many other substances such as spent tanbark, peanut hulls, and tobacco stems have been used.

Grass clippings are not recommended, nor are buckwheat hulls.

High Shade Is the Aim

The planting of rhododendrons under “high shade”, as often recommended, is desirable in that a mulch is regularly provided by the natural falling of tree leaves above, and excessive drying is prevented by reason of the shade.

This automatically provides satisfactory and cheap maintenance.

Rhododendron roots are delicate and located very near the surface.

Cultivation of the soil over them is often harmful. With the use of a proper mulch, cultivation becomes unnecessary.

Weeds may be removed by hand-pulling.

42948 by Clement Bowers