Potted plant pH too high -- how to adjust?

I have a plant that has been getting lighter green as it gets bigger. I thought the problem was nitrogen so I added some Miracle-Gro to the water. It didn't seem to have any effect.
Today I tested the pH of my soil, and it's 9. Recommended value is 5-7. How can I safely acidify a potted plant?
(Pointers to a FAQ page welcome ... if this is a FAQ.)
-- spud_demon -at- thundermaker.net The above may not (yet) represent the opinions of my employer.
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On 2005-11-30 12:10:34 -0500, snipped-for-privacy@OMEGA.MITRE.ORG (Spud Demon) said:

http://www.savvygardener.com/Features/soil_ph.html
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snipped-for-privacy@OMEGA.MITRE.ORG (Spud Demon) wrote:

It sounds like you have an azalea. Anyway, what ever it is, you can use powdered sulfur to lower the pH. Do not use aluminum sulfate. It is good for hydrangeas but eventually kills plants like azaleas. They can't tolerate high aluminum levels.
Also, there is a product MirAcid put out by MiracleGro. I am not a fan of it, but it would be better than MiracleGro. However over-fertilizing can be the problem*. First, try some sulfur. It is slow acting, so wait a while. You might try some Epsom salts (magnesium sulfate). It is good at greening up plants and will provide a little short term acidification. Magnesium is an essential element and lack of it will cause yellowish areas between the leaf veins on older leaves. If the leaves are a solid green the addition of Epsom salts (magnesium sulfate) would not be useful.
*Yellowing of a leaf between darl green veins is called chlorosis. Many conditions can be responsible. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency or magnesium deficiency. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer.
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The best remedy for a potted plant is to change the potting soil. What could you have put in the pot to go pH 9. I don't know of any potting soils that are more than neutral. I f you have an acid loving plant requiring a pH below 6. there are specific potting mixtures for that situation.
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Thanks to all who replied, especially Stephen -- lots of useful info!
-0800:

The plant has been in the same soil for over a year but the problem is more recent. All I added was water (through a new hose) and fertilizer. But it's an undrained pot, maybe that has something to do with the pH going crazy.
Anyway, last night I added half a cup of Epsom salt (Magnesium Sulfate) mixed with a gallon of water. Later today I will re-check the pH.
I don't want to mess up the root ball by transplanting it in different soil, but I might rinse it (overwater and then dump out the excess) if the chemical/bacterial route doesn't work. That should bring the pH close to 7.
-- spud_demon -at- thundermaker.net The above may not (yet) represent the opinions of my employer.
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Spud Demon wrote:

Plants should *never* be planted in a pot without drainage.
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In my opinion and experience, you'd be far better off getting that plant out of the pot, soil rinsed from the roots, and then repotted into fresh medium in a pot with proper drainage. Put the drained pot into the undrained one, if you want to show it off -- and consider looking up "double potting", where the space between the clay pot and the display pot is packed with a fibrous, water-retentive medium like long strand sphagnum.
Plants experience many more stresses in a pot than they do planted outside. The root volume is restricted, the soil tends to collapse and become oxygen deficient as organic matter is depleted, the plant tends to experience drought-and-drown watering (especially a problem in an undrained container) and salts tend to accumulate in teh soil, leading to tip burn of leaves and eventually to the plant being unable to get adequate moisture from the soil. Adding that half cup of mag sulfate (a salt) is going to accelerate that salinization problem.
I don't know the specimen size you're dealing with here... I've repotted lots of big plants myself, up to about 60 gallon containers, and borrowed muscles for bigger containers. It's definitely work. But the plant will do better in better growing conditions.
Kay snipped-for-privacy@fern.com
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snipped-for-privacy@OMEGA.MITRE.ORG (Spud Demon) wrote:

An undrained pot is going to kill the plant. First, salts in the water build up to toxic levels. Second, the roots drown in the water in the pot. Unless you have a bog plant, the plant is doomed. If it is a bog plant, it will take a little longer to kill. Either drill a hole in the bottom or move to a pot with a hole.
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Quit screwing around, repot with fresh soil and provide pore space and drainage. Spud Demon wrote:

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My experience with high pH says to check the water you are watering with. The soil tends to take on the pH of the water used. In my case my well water tends to run with a pH > 8 and pretty soon so is the soil in my house plants. About once a month I use a acid type fertilizer (miracle grow for acid plants). Seems to balance everything out over time. Put some holes in the pot too. Using a fertilizer will build up salts real fast. Watering with rain water is also helpful.

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