Epsom salts contain two elements magnesium and sulfur. It's considered
a neutral salt, having little effect on soil pH. If your soil is
acidic add dolomite lime, it wil add magnesium while raising the soil
Being a sulfate, Epsom salts (magnesium sulfate) are somewhat acidic.
Unless you specifically want to add magnesium, however, I would not use
Epsom salts to adjust the pH.
To reduce the pH, add elemental sulfur to the soil. Granular or
powdered soil sulfur is available at low cost in most nurseries.
Bacteria in the soil convert the sulfur into sulfuric acid.
To increase pH, add lime to the soil.
You want to add Epsom salts when perennials and shrubs fail to produce
new shoots from the base. Today, I divided about a 1/4 cup of Epsom
salts around seven rose bushes to promote new canes for this year's
growing season. I then added another tablespoon to one rose that has
only a single cane (the result of an accident that broke down the bush
last summer). Magnesium will help the roses create new canes from the
Epsom salts is Magnesium Sulphate. Its solution is roughly neutral as
it is the salt of a strong acid and a strong base. In the sort of
quantities that you are likely to add to soil it will have no effect
on pH at all. As a soil additive it is used in small quantities as a
quick fix to supply soluble Magnesium to plants with that deficiency,
there is no other reason I can think of to use it as a soil additive.
Garden lime (calcium carbonate) and dolomite (magnesium/calcium
carbonate) both raise pH but be aware that it will take weeks to years
to have a significant effect depending on the nature of your soil, its
current pH and how much you need to raise it. Raising pH is often
required as many soils are naturally too acid for the plants we like
in our gardens and because compost and rotted vegetation is slightly
acidic and if added to soil regularly it will slowly lower the pH over
a period of time.
Lowering pH is not done so often except (for example) if your district
is an alkali basin or has lots of limestone (calcium carbonate)
around. Sulphur can be used but this will be dfficult and expensive
in large areas and you may be better off just growing plants that like
the pH that you have or importing some good topsoil.
Cheap probe type pH testers (if that's what you have) are often
unreliable. If yours tells you that your soils pH is way off I would
cross check with a dye indicator test kit before spending much time
and effort trying to amend soil that possibly isn't so bad, Indicator
kits are quite reliable within the accuracy required by the home
gardener and quite cheap.
Thanks everyone, for the helpful replies. I'm sure what I have is a
cheap tester, so I won't rely on the results. I think I'll play
around with it on my houseplants.
I was hoping to use it to help me turn my hydrangeas blue, as the
blooms thus far have been beigey -pink.
The advice for using epsom salts on roses is great. Thanks
I didn't read all the replies, but if you put sulfur at the base of
your hydrangea it may or may not help with blueing the flowers. Some
varieties are always pink or white or beige. Do you know what kind of
hydrangea you have?
Use the following to determine final bloom color (determination of
color is set approx. late Aug-early Sept.):
pH >7.0 = red
6.5 - 7.0 = white
5.5 - 6.5 = pink
4.5 - 5.5 = blue
<4.5 = purple
pH control for plants: to raise pH, use hydrated powdered
lime at a rate of 1.1#/100sq.ft. to raise pH 1pt. for heavy soil.
To lower pH, use ironite or iron sulphate at a rate of 2.5#/100sq.ft.
to lower pH 1 point.
For 10sq.ft. (size of large hydrangea) area use 1/4 lb. of
which is 1/2 cup.
To acidify the soil, use aluminum sulfate, which provides aluminum as
well as lowers pH, mixed 1 pound to 7 gallons of water. Soak the
ground with it after the plant starts growing in spring and repeat
twice at two-week intervals as necessary to lower the pH 1/2 point. To
raise the pH 1/2 point, spread ground limestone in spring or fall at a
rate of 5 pounds per 100 square feet.
Picked this off the NG several years ago and found a cup of Epsom
salts turn my normal blue to a deep purple. I usually apply in
October for the following year.
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