pH Testing for Orchids

Test Strip in Beaker

This clinic is an example of the pour through method of pH testing for orchid media. We have some Phalaenopsis which have been in their mix through the growing season, roughly for about 8 months. Their bloom spikes are nearing maturity and the plant will soon be in bloom. Two of these Phals are in Phal Monterey Imperial Orchid Mix and one is in Phalaenopsis Gold Classic Orchid Mix.

We know that the pH of orchid mixes changes over time. We would like to have our Phals in media with a pH that is between 5.5 and 6.0 which allows the plants to best use fertilizer for strong root development, plant strength, and healthy spikes with good flowering. Since we won't repot these Phals until they go out of bloom we want to test the media now.

First we gather together the supplies that we need. We have a small glass of RO water (bottled water), pH test strips, a very clean clear plastic saucer, and an orchid which was watered a little while ago so its media is nice and damp. We want to be especially careful not to throw off our test by using a collection container that has any kind of residue in it.

We measure the pH of the water by dipping a piece of pH test strip into it. We then tape the strip to the orchid's ceramic pot just to hold it while we perform the other steps. We don't want to set it down on our counter top because we don't want the damp strip to pick up any residue from soap, etc that may be on that surface.

Next we pour a small amount (2-3 ounces) of the water through the pot and allow it to drip out the bottom holes into the saucer. This water, called the leachate, is what we will measure the pH of. Its pH will have been affected by contact with the media in the pot.

We measure the pH of the leachate with another new strip of pH test paper. We then hang that piece on the ceramic orchid pot next to the other one.After orchid mix has been in a pot with an orchid for a while there is a lot happening that can have an effect on the pH of the orchid mix. The orchid mix breaks down over time and that causes changes in pH. That is one reason repotting is so important for these beautiful plants. Additionally, as fertilizer and water are regularly added to the mix, they have their own pH and will interact with the media which alters the pH.

For comparison we take another strip of pH test paper and use it to test our tap water. We prefer to use pure water, either RO or distilled, for leachate testing as it has less buffering and will provide us with a better estimation of pH when poured through the pot.

The RO water by itself tests at about 4.5.

The tap water tests between 5.5 and 6.0

The leachate tests at about 5.0

Since we want our Phals to be in media that is between 5.5 and 6.0 we are going to top dress this plant with some pellets of dolomitic lime. If the pH had been too high we would consider using Citric Acid in our watering regimen.

For more information on pH and testing see our pH Test Strips.

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