(Spud Demon) wrote:

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?

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.
--
Pardon my spam deterrent; send email to
Visit my Rhododendron and Azalea web pages at:
http://home.earthlink.net/~rhodyman/rhody.html
Also visit the Rhododendron and Azalea Bookstore at:
http://home.earthlink.net/~rhodyman/rhodybooks.html
Cheers, Steve Henning in Reading, PA USA Zone 6