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The Importance Of Soil Acidity For Rhododendrons

by Joe B. Parks


Rhododendron bureavii (Chlorosis)
Picture by S & J Perkins
Rhododendron bureavii Lem in Salem, NH


Although rhododendrons survive and grow in some incredibly poor soils in the wild, too often these wild plants are thin scrawny things we would not be satisfied with in our garden. For beautiful healthy plants, provide good soil. It may not be necessary, but it is well worth the effort to amend any soil (it is particularly important to increase the organic content) in which you wish to grow rhododendrons. You’ll be repaid many, may time over.

Soil Acidity and Rhododendrons

Rhododendrons require an acid medium in which to grow. In saying that they must be grown in an acid medium, emphasis has to be placed on the word "must". In neutral or alkaline soils, rhododendrons do poorly and will eventually die simply because most nutrients are available to them only in an acid soil. Anything between pH 5.2 and pH 6.2 is ideal though a range between pH 5.0 to pH 6.0 is certainly satisfactory.

Why is pH so Important?

What is it, and who cares? You do, if you want to grow rhododendrons (or many other choice plants). The term pH is a measure of how acid or alkaline your soil is. Soil acidity (the pH level) determines whether a plant can grow in a given situation. Specifically soil acidity determines;

  • what nutrients are available to plants,
  • what toxic materials naturally in the soil will, or will not, affect plant growth,
  • the microbial activity in the soil (what bacteria will be active, if any) and
  • the ability of the root cells to absorb water and nutrients.

In simplest terms, some plants can utilize only those nutrients that are soluble (available) in an alkaline soil. Others, such as rhododendrons, are able to utilize only those nutrients available in an acid soil. Thus when we speak of "soil acidity" with regard to our rhododendrons, we are really talking about the availability of the nutrients in the soil. Of course if the required nutrients are missing from the soil, then the fact that the soil is "acid" will have little bearing on the health of the plant – but that is another problem.

The pH scale runs from 0 to 14. Neutral is 7.0. Soil that measures above 7.0 is alkaline and that below 7.0 is acid. Note that each division of the pH scale is a ten-fold difference from the next one; thus, pH 5.0 is ten times more acid than pH 6.0 and a hundred times more acid than pH 7.0. However, the only really important things you need to know about soil acidity are;

  • what pH range is optimum for your rhododendrons (pH 5.2 to pH 6.2),
  • how to determine the pH of your soil, and
  • how to change the soil acidity so it will be optimal for rhododendron growth. (For the technically inclined the footnote provides more detail about soil pH.)

Soils tend to be acidic in those areas where rainfall is adequate and also in areas where granite rock is dominant, e.g., the eastern part of the United States tend to be acid and test below pH 7.0. Some parts of my New Hampshire garden actually tested around pH 3.5 before they were modified. In areas where the rainfall is low, such as those west of the Mississippi River, soils are commonly alkaline (above pH 7.0). There are also areas in New England, such as along the Connecticut River and in central Maine which are underlain with limestone, however, where the soil is alkaline. These are but generalities and have little meaning for a specific location such as your garden. The only sure way to find out whether your soil is acid or alkaline, and how much so, is to test it.

A rough test of soil acidity can be made using a solution made from purple cabbage. Chop or grind some purple cabbage leaves. Squeeze out some juice or dissolve out the purple dye. To dissolve it, pour distilled (or neutral - pH 7.0) water over them in an amount about equal to the volume of the chopped leaves. Let this soak for an hour or so until the water becomes purple. Test the solution by mixing a few drops with an equal amount of vinegar (standard 5% cooking vinegar). The mixture should turn quite red indicating a pH of about 3.5.

To test for acid soil, put some fairly dry soil into a glass and add about twice as much of the solution (2 parts of solution to 1 part of soil). Stir thoroughly and let sit until the soil settles. If the solution has turned pink or red your soil is acid, if it remains purple the soil is neutral (about pH 7.0) and if the solution is blue, the soil is alkaline. Note that in the case of extremely alkaline soil the solution may turn greenish. This is however, only a rough test. While it will give you a feel for what type of soil you have, it is really too imprecise to tell you much – except that you had better get a precise test made. Note: if you wish to keep the cabbage solution, it will have to be sterilized (by boiling) and sealed in a jar. Otherwise it will sour and turn red.

So don’t guess! Test! Most county extension service offices and state universities provide a soil testing service. Upon request, they commonly furnish boxes that you fill with soil (collected as they prescribe) and return to them. They test the soil and provide you with a complete analysis of your soil. There is usually a charge for this service.

In requesting a test, be sure to point out that you expect to be planting (or have planted) rhododendrons and that you know that they grow best in a soil between pH 5.0 and pH 6.0. The report will state the pH level of your soil and the amount of each nutrient in it. It will also show the amount of nutrients (fertilizer) that will have to be added to the soil for best plant growth plus the amount of limestone that will have to be added to make it alkaline.

It is very important to understand that if your soil is found to be acid and if you failed to tell them that the test is being done because you intend to plant rhododendrons, they may advise you to add lime or limestone to your soil. They may anyway. Do not lime your soil unless the test shows soil acidity to be below pH 5.0. If the soil acidity is already between pH 5.0 and pH 6.0 there will of course be no need for the pH to be changed.

However, it is important to note that even if the pH value is within the proper range for rhododendrons there still may be a nutrient problem about which you should do something. The problem is that acid soils are typically deficient in calcium. Thus it is possible that even though your soil is properly acidic, your rhododendrons may be suffering from a calcium shortage. Fortunately you can add calcium to the soil without changing the pH. (see "Correcting Soil Acidity for Rhododendrons").

Since there is a range of acceptable pH levels and since rhododendrons have low nutrient requirements, you might wish to do the soil testing yourself. There are soil test kits available in almost any garden shop. These usually come in two basic types – one for just determining soil acidity (this one can also be found in pool supply shops) and a more expensive one for determining both soil acidity and nutrient levels. Since I already know that my soil is acidic, I prefer to use a third type of material (pH indicator paper) for determining the actual level of soil acidity. Indicator paper changes color, depending on the exact pH of the soil. It is thus simple and quick to use – and cheaper, though probably not as accurate as the chemical kits. Lest it be thought that I only use pH indicator paper, it should be noted that when a more accurate measure is required, I use a hand-held, direct reading, microprocessor based pH meter that reads pH to the nearest tenth.

If your soil acidity needs to be adjusted for rhododendrons, you may want to read the next section, Correcting Soil Acidity for Rhododendrons. However, if you live in an area where the soil is actually alkaline, then it may be easier to use a raised bed and you should read the section on Building and Using Raised Beds.

Why Be So Specific About the pH Range for Rhododendrons?

(The Hard Truth about Iron and Aluminum)

The reason for being so specific about the soil pH for rhododendrons is simple:

  • If the pH is too high (over pH 6.0), several nutrients (principally iron) become unavailable (even though tests may show them to be plentiful) and in effect, your rhododendron starves.
  • On the other hand if the pH is too low several materials naturally in the soil may become poisonous causing your rhododendrons to languish or even die. The principal culprit is aluminum but some necessary nutrients may actually become available in toxic quantities.

SOIL FACTORS THAT LIMIT RHODODENDRON GROWTH

pH4. -- pH 5.1 pH5.2 -- pH 6.2 pH6.3 -- pH 7.
Aluminum, which is toxic to rhododendrons, begins to become soluble and thus poisonous at this point. The rate of increase in toxicity is rapid below pH 5.2.

AREA OF BEST RHODODENDRON PERFORMANCE

Iron becomes almost insoluble at this point and thus unavailable to rhododendrons.

© 8/1/98 Joe Parks

The Iron Problem. Rhododendrons require iron for use in several of the enzymes involved in photosynthesis. Iron is also involved in reducing oxygen (O2) to water during plant respiration. In a soil with a pH of about 6.2 and below, iron is in a soluble form that is readily available to rhododendrons (and other acid loving plants). As the soil becomes less acid and enters the pH 6.2 to pH 6.5 range, iron in the soil rapidly changes to a form that is insoluble and virtually unavailable to rhododendrons. Since iron is absolutely essential to rhododendron growth, the leaves of a rhododendron that does not receive enough of it will soon turn yellow (become chlorotic) and the plant will eventually die.

The Aluminum Problem. A pH level that is too low (below pH 5.0) is equally undesirable. The problem is that the toxicity of aluminum increases as the pH level drops. Aluminum is a major constituent of nearly all soil – actually the third most common element on the surface of the earth. Aluminum is not toxic to rhododendrons as long as it is insoluble in water ("locked up"). But as the soil acidity drops to about pH 5.2 and below, aluminum rapidly begins to change to a form that is soluble in water (becomes "unlocked"). The roots then absorb the soluble aluminum and are poisoned by it.

Specifically aluminum, when it is absorbed by the roots, slows or prevents cell division in the root tips. Thus, being poisoned, growth of the roots is slowed and even stopped. As roots become stunted and are less able to absorb water and nutrients, plant growth is affected. As the soil acidity increases (pH becomes lower), aluminum becomes more soluble at an increasing rate and thus even more toxic.


Note that this article has been adapted for the benefit of the members of the Massachusett Chapter from work being prepared by Joe Parks. This does not constitute an official endorsement by the American Rhododendron Society Massachusett Chapter of the material being published by Joe Parks.


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