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THE MEANING OF pH

 

by

Wilella Stimmell

(Reprinted from "The Arizona Orchidist", Nov., 1995)

 

The idea for presenting a brief,but useful segment on pH was a natural outgrowth from a recent discussion I had with one of our valued OSA members who is no longer able to be an active grower. Vic Polk adjusted the pH of the water used on his orchids by adding an 8 ounce cup of tea made of cut black/orange pekoe loose tea, brewed to medium strength (suitable for human consumption). The "brew" was added to a 30 gallon container of reverse osmosis water. The pH of the "tea-water" was 6-6.5. One watering was pH adjusted; the following watering was unadjusted for pH, but with fertilizer (Miracid) added. His orchids were "happy" (the plants were healthy and flowered well), and his water acidifier was neither dangerous to handle, nor complicated. (Fir bark was Vic's choice for a potting medium.)

 

In a search of articles on the subject of pH in the AMERICAN ORCHID SOCIETY BULLETIN, I located 72 articles in which pH was discussed. The first one was published in 1932. Also, every book on orchid culture, and every book on orchid biology, addresses the subject of pH of the water used on orchids.

 

There is a consensus that in nature, epiphytic orchids grow on decomposed organic matter which has an acid reaction. Terrestrial orchids grow in soil that has a high humus content, and, therefore, is also acid. Since we orchid growers are attempting to duplicate, so far as is possible, the growing conditions of orchids in their native habitats, we must address, as one of those conditions, the subject of pH.

 

pH is not an abbreviation for phooey! All acidity is due to hydrogen ions. The pH is the measure of the concentration present of these ions, but in reverse: the lower the pH, the higher the acidity. On a scale of 0 -14, a pH of 7 is neutral. Decreasing numbers indicate increasing acidity. Each number is ten times more acid than the preceding number (a negative logarithm is involved, but we need not understand the formula to understand the principle). Pure water has a pH of 7. Our metro Phoenix city water has a pH of 7.5 - 8.5. A spot-check of the pH of the water available from vending machines in front of most supermarkets has always measured 7. (Vending machine water is also free of salts. I conduct my own quality control tests with a conductivity meter.)

"Most, but not all orchids, are acid-loving plants and like to grow in the range of pH 5.0-6.0, but they are surprisingly tolerant of fluctuations around this optimum..."* Rebecca Northen, in her book, HOME ORCHID GROWING, Fourth Edition, p. 32, states: "...water that is above pH 8 should probably be treated to bring it closer to neutral, not necessarily below pH 7.5." The differences of opinion regarding optimum pH are not as confusing as they seem. Another factor comes into play here, the potting medium. Most of us use fir bark or a combination thereof. FRESH fir bark is slightly acidic. (As it ages in the pots and is in contact with higher pH from the water, the fir bark becomes less acidic. Other factors which affect the rate of decomposition of fir bark will be discussed in a future discussion of fertilizers.)

 

Even if you choose not to adjust the pH of the water you use to irrigate your orchids, you would be wise to know the pH of your water and periodically recheck to detect any changes. There are pH indicator strips available locally at a chem lab supply store. The strips are very easy to use and give a reliable measure of pH. Some of your fellow OSA members have pH meters, and we would be happy to test your water for you. Should you wish to own your own "toy", you should know that a pocket pen-type pH meter with a non-replacable electrode has a short life span. When the electrode fails, you discard the entire meter. There is another pH meter available with a replacable electrode. Initially this meter is more expensive, but when the electrode fails, you only need to replace the electrode.

 

Assuming you have checked the pH of your water and discovered that it is too alkaline, you have several methods available to acidify your water. The simplest and least expensive is probably Vic's tea. You could also use citric acid, an organic acid in granular form, which is not dangerous to handle (and which some people find makes a very refreshing drink!). Acetic acid, the principal acid in vinegar, has also been used to acidify water. However, since there have been reports that acetic acid may be toxic to orchids, I do not recommend the use of vinegar for adjusting pH. Some growers use hydrochloric acid, which is also known as muriatic acid. Phosphoric acid is highly recommended, although for the hobbyist with just a few plants, it is very expensive, and very small quantities are needed to adjust pH. One quart of phosphoric acid costs about $40.00. Several growers could split the quantity and cost by sharing a bottle of phosphoric acid. This product is available locally. There used to be a product called Green Magic which was used to acidify water. I think my plants responded well when I used one drop of Green Magic to one gallon of reverse osmosis water. If anyone knows whether this product is available again, perhaps under a different name, I would appreciate hearing about it.

Whichever method you use to acidify your water, write down exactly what you did - in as much detail as possible. Closely observe your plants, and note beneficial effects, if any. Don't be afraid to experiment. They are YOUR plants, and experimentation is a learning process. Select a few plants to be used as "guinea pigs", not your entire collection! The most successful cultural practices are the result of a combination of knowledge, observation, experimentation, and good old fashioned COMMON SENSE.

 

*AOS BULLETIN, Vol. 58, No. 12, December 1989, p. 1217.

 

 

 

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