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Horticultural Myths Exposed:
Epsom Salts and Baking Soda
By Dr. Leonard Perry,
Extension Professor University of Vermont
Myths may be a polite way of referring to statements or beliefs that are believed
true, but are not accurate or totally true.  The Internet has made possible the
easy proliferation of such misinformation.   

Two of these myths relating to household products used in landscapes, often in
attempts to be sustainable, concern
Epsom salts and baking soda.
The common household ingredient Epsom salts (magnesium sulfate) has been
widely recommended for many garden uses including to control pests, to deter
slugs and voles, to make plants grow bushier, to increase chlorophyll (the green
in leaves) production, to help seeds germinate, to produce more flowers, and to
improve uptake of some nutrients.  Recommendations also are seen on how
much to regularly feed plants from trees and shrubs to houseplants, and in
particular tomatoes.  
The reality, if one examines the research on this chemical back to early in the
last century, is that it does correct and has been effectively used for magnesium
deficiency in soils or plants.
 Other than this, it is generally a myth that
applying Epsom salts will fulfill the often-seen claims to otherwise healthy
Some soils are naturally low in magnesium, such as those light and sandy, or
even clay soils if intensively cropped.  Most research, and applications of
Epsom salts, has been on intensive cropping systems of fruits and vegetables
that can deplete magnesium.  Magnesium may be deficient on pastures, and
may cause disorders in grazing cattle if deficient.
Nutrient chemistry in plants can be complex, an example relating to
magnesium.  An excess of potassium (the third number in a fertilizer formula)
can make magnesium less available to roots.  So under this situation, extra
Epsom salts won't help.  Reducing the potassium, or increasing nitrogen, may
be all that's needed to make the existing magnesium available if a plant is
deficient.  This illustrates why it is so important to do soil tests.
One claim is that Epsom salts won't accumulate in, and so won't hurt, the soil.  
What isn't mentioned is that it will be leached, and so potentially pollute water,
instead.  Epsom salts are highly soluble, and if applied in excess can leach into
water (49 percent in one study), the pollution negating its benefits. Other less
soluble forms of magnesium supplements often are better.  This is especially
true on soils under high rainfall or irrigation.
Another common household ingredient, baking soda (sodium bicarbonate), is
claimed and believed by many to kill fungi on plants and to reduce diseases.

The reality is that baking soda (by the way it's not legally labeled for use as a
fungicide) helps prevent rather than kill diseases.  The baking soda acts by
making surfaces more alkaline-- not conducive for growth of fungi.  So it is
technically fungistatic (prevents fungi growing) rather than fungicidal (kills

If some is good, more is not better, as an excess even of baking soda can cause
problems on fruits and leaves.  In addition to providing better prevention of
diseases, higher concentrations can cause discoloration of some fruits (as little
as a 2 percent solution) and interveinal chlorosis on leaves (as little as a one
percent solution).
Baking soda, while showing great results in lab studies, has been less consistent
in field trials.  Outdoors, where it has been tried on powdery mildew (including
my own studies on garden phlox in Vermont), it appears more effective
consistently when combined with horticultural oil. Other compounds, such as
the related potassium bicarbonate (which is in some legally registered
fungicides), often are more effective.
In a couple of scientific studies on roses, this combination of baking soda and
horticultural oil was effective on powdery mildew on roses but not blackspot --
a use often seen.  Siting roses (as well as other plants) properly with good air
movement and well-drained soil, keeping from stress through proper watering,
and using a coarse organic mulch such as wood chips, may provide adequate to
good control of leaf diseases such as black spot.
Another use of baking soda is to control postharvest diseases on fruits.  Even
here, it is more effective when combined with yeast organisms that prevent
diseases from growing.
More on these horticultural myths as well as many others, and better
alternatives, can be found on the website of Washington State University
Extension Horticulturist and Associate Professor Linda Chalker-Scott

Tips she provides for evaluating popular or "gray" (questionable validity or
basis in science) recommendations include asking the questions:

-- Does the author have reputable ties, such as to in academia or a scientific
-- Does the recommendation come from information or sources that can be
-- Does the recommendation refrain from, and is not tied to, selling a product?
-- Does the article appeal to reason rather than emotion?
Photo by Mateusz Stachowski