Changing Soil Acidity For Rhododendrons

by Joe B. Parks

Rhododendron bureavii (No Chlorosis)
Picture by S & J Perkins
Rhododendron bureavii Lem's form in Salem, NH

If tests show that your soil acidity is not within the pH 5.0 to pH 6.0 range needed for the best rhododendron growth, then you should adjust it – that is, if you want your rhododendrons to do their best. What you need to do depends on the current soil pH;

  • it is below pH 5.0, then you will need to raise it to between pH 5.0 and pH 6.0,
  • if it between pH 6.0 and pH 7.0, then you will need to lower it to below pH 6.0,
  • if it is above pH 7.0, then you may have to construct a raised bed (an easy step described in another article).

Increasing the Soil pH

If the soil for your rhododendrons is below pH 5.0, then ground limestone is the material to use to raise the pH (reduce the acidity). Sufficient ground limestone (CaCO3) should be added to increase the soil pH to about 5.5. If you live in an area where the soil is deficient in magnesium (particularly New England), be sure to use dolomitic limestone so as to supply the needed magnesium at the same time. Sandy soil will require 2 to 3 pounds of limestone per 100 square feet (9+ sq. m), loam soils about 6 pounds (2.72 kg) and clay soil perhaps 7 to 9 pounds. These amounts are not very precise because the materials that make up your particular soil will determine the actual pH that results from your treatment. The increase in pH will also be quite slow. My recommendation is that you use no more than half these amounts and test the soil again a month to three months later before adding more.

Raising the pH is commonly considered to be the purpose of using limestone. Actually ground limestone (and lime) are important as a source of the calcium required for plant nutrition. If soil tests show your soil to be very acidic (below pH 5.0), it is almost certain to also be deficient in calcium. It is also common for soils in the pH 5.0 to pH 6.0 range to have a calcium deficiency – more on this in the next paragraph.

If tests show that the soil acidity is above pH 5.0, it still may be deficient in calcium. To correct this you should use gypsum (CaSO4) rather than limestone to provide the needed calcium. Gypsum provides the needed calcium without materially affecting the soil acidity. At the same time it will also supply another needed nutrient, sulfur. I add gypsum to my rhododendron plantings every other year.

Someone always asks if lime can be used instead of ground limestone. The answer is yes but it’s not a very good choice. The problem is that because lime (even after its slaked) works so rapidly it may "burn" plant roots. Thus it has to be used carefully in the right amount and mixed thoroughly into the soil, otherwise it may cause problems. Ground limestone works more slowly and thus there is less chance of making a mistake.

Reducing the Soil pH

If your soil tests between pH 6.0 and pH 7.0, you can use either sulfur or ferrous sulfate monohydrate (FeSO4) to increase soil acidity (reduce the pH). If you are working with unplanted soil, ferrous sulfate heptahydrate is quicker and may be safely used. The heptahydrate changes soil acidity in a few days. However, it is so fast acting that it is dangerous to use on growing plants because it can quickly "burn" the roots and kill a plant. For plants already in the ground, sulfur is the easiest and safest to use although ferrous sulfate monohydrate is also ok.

Never use aluminum sulfate to increase soil acidity as it is toxic to many plants including rhododendrons. This is a most important point! Under no circumstances should you use aluminum sulfate (Al2SO4) around your rhododendrons. As was pointed out in a previous article, soluble aluminum is toxic to rhododendrons. The chemical, aluminum sulfate, is soluble in water thus the aluminum component is immediately available – and toxic - to the plants. As the pH level of the soil decreases, the aluminum component becomes increasingly soluble – and thus increasingly toxic. Therefore, although aluminum sulfate will certainly reduce the soil acidity, it can, and will, quickly injure and may even kill your rhododendrons. Even though the plants may tolerate small quantities of this material, why should any quantity of a toxic material be added to the soil when other perfectly safe materials such as sulfur are available?

I once saw an entire bed of rhododendrons (no, not mine) killed within a week by a "proper" application of aluminum sulfate. Even small amounts of aluminum sulfate can stunt rhododendron (and azalea) roots and reduce their ability to absorb nutrients and water. True, organic materials in the soil will buffer aluminum and reduce its toxic effects, but the toxic effects occur immediately and the buffering effect is slow. Too, how do you know there is sufficient organic material available. Why take a chance with your valuable plants, do not use aluminum sulfate regardless of what anyone tells you!

In theory, approximately 1.5 pounds (0.7 kg) of sulfur per 100 square feet (9 square m) mixed into loam soil will lower the pH about a point, i.e., from pH 6.5 to pH 5.5. It will require about 5 pounds (2.25 kg) of ferrous sulfate (monohydrate for existing beds, heptahydrate for new beds only) to do the same thing. Sandy soil requires less and clay soil may require more to achieve the same result.

These amounts, however, are not very precise. The problem is that the materials that make up your particular soil will determine the actual pH that results from your treatment. My recommendation is that you apply no more than half these amounts to fresh soil, water them in well, wait for several weeks and then test (ferrous sulfate heptahydrate requires only a few days to react so can be tested within a week). Under no circumstances would I apply more than half the suggested amounts at one time around growing plants. I usually put on less than half the recommended amount, then test several weeks later. If necessary, I make a second and even a third application.

It is also worth repeating again that there are two forms of ferrous sulfate; the "monohydrate" and the "heptahydrate". The monohydrate form is slow acting and as such, is safe to use around your plants. The heptahydrate is very fast acting and will likely "burn" and may kill any roots it contacts. Use the heptahydrate only for making new beds as it reacts with the soil in a few days and can acidify a new planting area in less that a week.

Buffering the Toxic Effects of Aluminum

Fortunately there are ways in which, to some extent, aluminum’s toxicity can be offset. One is automatic. If rhododendrons are grown in a highly organic soil, the organic materials in the process of breakdown (composting) "tie up" soluble aluminum salts. Since this means that less aluminum reaches the roots, the toxic effect is lessened. Put another way, even though the soil pH is quite low (say pH 4.5), plants in a soil with a high organic content will tend toward more normal growth since the organic material acts as a buffer.

A second method for reducing the toxic effect of aluminum involves the use of phosphorus. As soil acidity drops below about pH 5.5, phosphorus increasingly binds to aluminum as aluminum phosphate (AlPO4) thus rendering the aluminum insoluble (note that this also reduces the available phosphorus). Thus soil with an excess of phosphorus can be quite acidic and still support adequate root growth. Maine potato growers have used this method for years so that scab-free potatoes can be grown. This also might be the reason that some rhododendron growers have found that excess phosphorous in the soil appears to increase bud set. It could be that the extra phosphorous has "tied up" an excess of aluminum in the soil and thus indirectly promoted an increase in root growth – and a healthier plant.

What has been said above might well lead you to say, "Why not put plenty of organic material or phosphorus in the soil and not worry about aluminum at all?" My answer is simply this. In the first place, keeping soil acidity above the pH level where aluminum is a problem (pH 5.2) is relatively easy. In the second place, it would seem better to consider the organic matter in the soil as a sort of backup, a safety net if you like. If you allow the soil to remain highly acid and rhododendron growth is affected, there’s nothing left to do but raise the pH anyway. Lastly, in a highly acid soil, several plant nutrients, among them nitrogen, become "locked up" and unavailable to plants. So why not keep the soil acidity within the proper range (about pH 5.0 to pH 6.0) and not worry if your plants are growing as well as they might?

Other Problems with Soil That is too Acidic

As mentioned above, another problem with highly acid soil is that important nutrients become insoluble ("locked up") and thus unavailable. At about pH 4.5, nutrients such as potassium, nitrates and magnesium begin to become insoluble and thus unavailable to rhododendrons.

In highly acid soil, the opposite happens with some trace elements (micro-nutrients) According to Samuel Tisdale writing in Soil Fertility and Fertilizers (Macmillan Publishing Company), trace elements (micro-nutrients) become more soluble (and thus more available) in a highly acid soil. Although absolutely necessary for plant nutrition, excess amounts of trace elements such as copper may begin to become "unlocked" as soil acidity drops below about pH 5.6. At some point as the pH drops lower, these excessive amounts can become so large that they are 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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