JARS v56n3 - The Importance of pH in Seedling Culture

The Importance of pH in Seedling Culture
Dr. Mark G. Konrad
Sewickley, Pennsylvania

In the underworld of soil there is a science as mysterious as the universe itself. It is estimated that there are billions of organisms in very small samples of soil, nearly all functioning in support of the health and growth of plants. Beneath our footprints the following are found: fungi, bacteria, actinomycetes, protozoa, algae, lichens, nematodes, earthworms, mites and insects (including their larval forms) and burrowing animals. For the microorganisms fungi, bacteria and actinomycetes so important for plant life, the pH of the soil is highly important. Therefore, the significance of an ideal pH in rhododendron seedling culture cannot be stressed enough. Two key related issues emerge.

  1. First is the proper pH range to insure availability of nutrient elements.
  2. Second, and related to the same pH range, are the conditions conducive to the growth and sustainability of microorganisms beneficial to the plants.
Fig 1
Figure 1. pH Value

Figure 2 illustrates how pH affects both nutrient availability and the ability for soil microorganisms to survive. From Figure 2 it becomes obvious that fungi are available to the plants over a wide pH range, while the bacteria and other organisms fall off precipitously below a pH of 5. It is important to have a pH that encourages bacterial as well as fungal growth for the overall health of the soil.

Fig 2
Figure 2. Ph effect on nutrient availability and microorganism activity.

How Microorganisms Help
The microorganisms fungi, bacteria and actinomycetes are necessary in the decay of organic material and are vital in the mineralization of plant and animal residue. For instance, some organisms can fix nitrogen from the air or can release it from decaying organic matter, which is then available for root absorption. The key to a healthy soil is humus, a byproduct of microbial activity. Artificial mixes have become very popular, probably in response to eliminating disease problems. However, with the decrease in organic food (humus), the natural benefit of soil organisms has been sacrificed.

The Fungi
The mycorrhizal fungi penetrate the roots and then grow as extensions. Apparently there are still thousands of mycorrhizal fungi to be identified. The symbiotic relationship is beneficial both to the host and fungus. Apparently sugars are supplied to the fungi with an exchange of nutrient ions to the plant. It has been estimated that 80 percent of the world's plants would not thrive without mycorrhizal fungi. In the future, the management of micorrhizae may be as important as the use of fertilizer.

Gary W. Watson, a plant physiologist at the Morton Arboretum, explains how fungi benefit plants (6) :

"Plant root systems cannot penetrate every area of the soil. Even the smallest root tips are much larger than soil micropores. Soil micropores are the tiniest of soil spaces and surround individual soil particles. These microscopic openings make up the bulk of soil pores and could provide access to tremendous amounts of nutrient ions if plant roots were small enough.

"A symbiotic (or mutually supportive) relationship that overcomes this problem has evolved between plant roots and soil fungi. Certain fungi and plant root systems grow together, each becoming an extension of the other. The fungi produce structures that are called mycorrhizae. Mycorrhizae are not roots, but for ease of comprehension can be thought of as such. They go unnoticed because they usually cannot be detected with the naked eye." Mike Amaranthus, chief scientist for Mycorrhizal Applications Inc., states (1) :

"These mycorrhizal fungi increase the surface absorbing area of roots 10 to 1,000 times, thereby greatly improving the ability of the plants to use the soil resource. Mycorrhizal fungi release powerful chemicals into the soil that dissolve hard-to-capture nutrients, such as phosphorous, iron and other "tightly bound" soil nutrients."

The Bacteria and Actinomycetes
The microorganisms bacteria and actinomycetes also play an important part in plants' good health. John Paul Bowles, horticulturist at the Dawes Arboretum, Newark, Ohio, explains how they work (2) :

"Bacteria are the most numerous and arguably most important soil organisms. There are many kinds of bacteria. Some of the most beneficial types convert atmospheric nitrogen (all plants require nitrogen to live) into forms plants need. This is called nitrogen fixation - something plants cannot do for themselves."

"Other bacteria, and another group of microorganisms called actinomycetes, are responsible for the decomposition of dead things, a useful task, if not for the sake of good housekeeping, then certainly for the advantages of recycled nutrients."

Weldon Delp and pH
Some of the work of the late Weldon Delp regarding pH is presented (see below). He experimented many years in an attempt to develop an ideal mix for rhododendrons. He and many other people have had great success.

Weldon E. Delp info
Soil dynamics are tied closely to soil pH. Microorganisms, especially beneficial bacteria, are not active when the pH drops below 5.0. Remember 7.0 is neutral.

Several nutrients, including phosphorus and iron are "locked up" in acid soils, essentially unavailable to rhododendrons.

Phosphorus is one of the most complex and least understood of the three major nutrients. Used by a plant for cell division, seed and root development, its presence or absence is tied closely to plant maturity. When phosphorus is "locked up," iron, in turn, becomes unavailable, reducing a rhododendron's ability to photosynthesize.

Rhododendrons will grow beautifully when soil ph is 5.5 to 6.5, but poorly at a pH of 4.5 (some books recommend 4.5 for rhododendrons to control root rot). This I feel is wrong because of "lock up." Most root rot problems are cultural.

Acid rain, fertilizers and some types of mulches acidify soil. Other factors, such as run-off from concrete structures create alkaline conditions. Most organic and inorganic fertilizers are acid. An exception is wood ashes, which raise soil pH as effectively as limestone.

Peat moss and compost vary greatly in acidity. Leaf mold consisting of maple leaves will be slightly acidic while oak leaves and pine needles make very acid compost. Spent mushroom manure is a pH of 7.0 or above.

Soils and pH testing are the only way to accurately determine soil acidity. It's a good idea to check pH periodically. We tend to overlook this important fact. If pH adjustment is indicated, limestone is used to "sweeten" the soil, or raise its pH and sulfur is used to lower it.

In western Pennsylvania, we are fortunate to have natural acidic soils, but beware - it could be too acidic. We are confronted with a choice of products to raise soil pH. Which one is the right one? Following are a few tips on different liming products:

All lime contains calcium, an essential plant nutrient. The chemical formulation of this calcium is responsible for the difference in various types of lime.

Quick lime, also called burned lime, is calcium oxide. Being 80 to 95 percent calcium, it has twice the neutralizing power of crushed limestone, calcium carbonate. It's also rapidly available to plants, but is relatively expensive and doesn't last very long. Add these to the fact that burned lime is difficult to handle, and I recommend not using it.

Hydrated lime is just as tricky to use. It's calcium hydroxide (calcium oxide plus water), and this product is strong enough to burn growing plants.

Dolomitic lime is safe and slow acting, very much like crushed limestone. Its pink color comes from its magnesium content, found naturally in the mineral dolomite. Although it's more expensive than crushed limestone, it's a good source of the essential nutrient magnesium, often lacking in western Pennsylvania soils.

Gypsum is sometimes offered as a liming product. But beware. Although it is made of calcium sulfate, it will not raise soil pH. It just adds calcium to the soil - a good choice for those rare occasions which call for calcium but no pH adjustment.

Gypsum is used primarily as a soil amender to break up heavy clay soils and counteract salt contamination.

Rhododendron growers probably are wise to choose either plain old crushed or agricultural limestone or dolomitic lime, if soil tests indicate a need for magnesium. Added in advance of the gardening season, it will ensure beneficial organisms, and your rhododendrons a good growing season.

Delp Potting Mix for Rhododendrons

  • 1 5-gallon bucket Michigan peat
  • 1 5-gallon bucket perlite
  • 1 5-gallon bucket coarse peat moss
  • 1 5-gallon bucket pine bark*
  • 2 ounces Aqua Gro
  • 2 tablespoons agricultural sulphur
  • 2 tablespoons gypsum
  • 2 tablespoons 18% rock phosphate
  • 1 tablespoon Epson salts
  • 1 tablespoon Peters acid fertilizer (21-7-7)

* Preferred composted shredded pine bark under product name Aged Pro Base.

It is interesting to note at this time that Premier Horticulture has a PRO-MIX BX with MYCORISE, but it is not specific for rhododendron culture.


  1. Amaranthus , Mike. 2002. Declaration of interdependence. Digger, Oregon Association of Nurserymen , April 2002: 48-53.
  2. Bowles, John Paul. 1990. Living things in the soil. Soils, Plants & Garden, Brooklyn Botanic Garden Record , Record Handbook #110: 19.
  3. Curtis, Helena. 1983. Biology Fourth Edition . Worth Publishers Inc.
  4. Shewell-Cooper, W.E. 1975. Soil, Humus and Health , Canada: Douglas David and Charles, Ltd.
  5. US Department of Agriculture. 1957. Soil, The 1957 Yearbook of Agriculture
  6. Watson, Gary W. 1990. Living things in the soil. Soils, Plants & Garden, Brooklyn Botanic Garden Record , Record Handbook #110: 18.

Dr. Konrad, a member of the Great Lakes Chapter, is a frequent contributor to the Journal.