What is pH?
How does it Affect Orchids?
These questions keep coming up. Many orchid growers, new to these endeavours, have asked me about pH and how it applies to their orchids. So I had to look it up:
Webster’s dictionary explains:
“pH is the negative logarithm of the effective hydrogen-ion concentration (or hydrogen-ion activity) in gram equivalent in liters, used in expressing both acidity and alkalinity on a scale from 1 – 14.”Duh…
Or the World Book Encyclopaedia:
“pH, a symbol used (with a number) to indicate acidity or alkalinity in testing soils for suitability to specific crops, in analyzing body secretions, and in various industrial applications. It represents the relative concentration of hydrogen ions (in gram atoms per liter) in a given solution, usually determined by the use of a substance (indicator) known to change color at a certain concentration. The pH common scale ranges from 0 to 14, pH 7.0 (the hydrogen-ion concentration, (10 to the power of minus 7) or .0000001, in pure water) being taken as neutral, below 7.0 increasingly acid, and above 7.0 to 14 increasingly alkaline. Most soils range between pH3 and pH10”.Hmm - is this better?
What we can glean from the scientific jargon above is that pH is a measuring system based on percentages of Hydrogen ions that runs from 0 to 14. A high pH (from 8 – 14) means the substance is alkaline (sweet) and a lower pH (from 0 – 6) means that the substance is acidic (acid). The neutral point is at the pH value 7.
Most orchids (in my experience) like to have their roots in a slightly acidic medium (pH value about 5); others need to have slightly alkaline conditions (pH value about 8). We must keep in mind that certain plants have evolved in lime (alkaline) soils and they are better off in neutral surroundings. So far, I have not found a list of which orchids need to be in acidic media and which ones need alkaline media. It is up to the grower (who wants to know) to study her/his plants’ requirements. Please note: if plants grow in limestone soils and get cold rainwater or cold water seepage from underground, the lime gets dissolved and becomes available to the plants. When the limestone receives only warm water, then it does not dissolve much and the soil may not show a high pH value.
Here is a graphic that may help more than many words:
This is from a very useful book about composting called “Let It Rot” by Stu Campbell
How can we find out what our orchids are dealing with, inside their pots? Since rainwater is slightly acidic (pH around 5) and our tap water in Victoria, BC is pretty much the same, we could say that as long as we use tap water, things will be good for most orchids. That would be correct here on Vancouver Island. Other geographical areas may have to deal with different water. Check with the local water department for information about your water.We also apply different fertilizers (having different pH levels) and as long as we flush the pots on a regular basis, things are still ok. When somebody tells you that some (not all) Paphiopedilums and some (not all) Cypripediums need an alkaline environment, you might add some crushed oyster shell, crushed egg shells or a little pinch of Dolomite limestone to a particular plant or mix something of that into the growing medium or you can use calcium nitrate with your fertilizers. You have to read the instructions that come with the different mineral fertilizers. Please make certain that this lime is really needed by your plant! For instance, I have learned that Paphiopedilum delenatii, in contrast to most Paphiopedilums, prefers rainwater and NO lime.
But then – how do I know what pH the potting mix has?
What you can do is: take your orchid plant, flush the medium with lukewarm tap water, saving this water in a dish. Immerse either one of those pH-testing strips and see what colour it turns; instructions come with the kit. Or you can purchase a pH meter, that will need to be calibrated regularly, adjust it to the same temperature as the drained-off water in your dish, immerse the testing tip in the water and get an accurate reading of the pH of the mix in that pot. Flush the pot a second time and repeat the test. That would give you a good idea of the average pH in that pot. If you find that it is extremely high or extremely low, you can then try to bring things back to about pH 6-7. This may give you peace of mind. However, remember that orchids (as a family) are much older than humans and will put up with a lot of nonsense from us before they retire to “Orchid Heaven”. So, don’t worry too much about this pH thing. If the plant really looks sick, it will probably be a better idea to re-pot it instead of trying to adjust the pH. I suggest that as long as plants look healthy, leave good enough alone.
Ingrid Schmidt-Ostrander - Canadian Orchid Congress