507-289-3741
GARDEN TOPICS
Acidifying Soil
Growing acid-loving plants in our
area can be a challenge because the
soils here tend to be more alkaline
than most plants like. In fact, the vast
majority of ornamental plants prefer a
soil pH of 6.0 to 6.5, slightly acidic.
To complicate the matter, the water
we use in our gardens tends to be
alkaline. Often municipal water
systems buffer the water to a pH as
high as 8.0 to help our pipes last
longer. That's great for the pipes, but
can be hard on our plants. Ideally, we
would all grow acid-loving plants in
naturally acid soil and alkaline loving
plants in alkaline soils. But that's so
limiting, we have learned some ways
to change the soil to suit the plant.
Are all plants sensitive to soil pH?
While most plants prefer slightly acid
soil, most plants are also very tolerant
of a range of 6.0 to 7.5 pH. There are,
however, some plants that require acid
(or alkaline) soil.
If the pH is way off, what happens
to the plants? When the pH is too
high for a plant, it limits the plants
ability to make use of many nutrients.
In soils that are too alkaline, plants
cannot make use of the boron, copper,
iron, manganese and zinc even if
there is enough in the soil.
How can I find out the soil pH? The
best way is to send a sample to a
laboratory for testing. There are home
kits, but they aren't quite as accurate
and can be influenced by the pH of
the water you use in testing. Locally,
soil can be tested at the soil labs at the
University of Minnesota. Testing
forms are available at Whiting’s.
What exactly does the number
indicate? The pH scale ranges from 1
to 14. A pH of 7 is considered neutral.
Anything below 7.0 is acid and
anything above is alkaline. While it
507-289-3741"
may not seem like there is much
difference between a pH of 5.0 and
6.0, there is. The values are
logarithmic like the Richter scale for
earthquakes. That means soil with a
pH of 5.0 is ten times as acid as soil
testing 6.0.
What are some of the plants that
grow in our area and require acid
soil? Require is the key word. In
addition to all the plants that prefer it
slightly acid, there are a few that
require acid. Azaleas, rhododendrons,
blueberries and pin oaks all require
acid soils. Probably the most common
plants in the landscape that prefer it
are river birch, gray birch, magnolias,
hydrangeas, serviceberries, spruce,
pines, mountain ash, clethra and holly.
(Whitespire birch would like it
slightly acid too, but they are more
tolerant of higher pH.) There are also
perennials that prefer acid soil. Some
of the more common perennials that
prefer acid soils are ferns, most
woodland plants, lupines, garden
lilies, butterfly weed and primroses. If
you aren't sure what pH your plants
prefer, ask one of our horticulturists.
Should the soil be acidified before
the plants are put in? Whenever
possible, this is the best way to start.
But even if the soil pH is adjusted
before planting, it will need to be
monitored and may need acidifying
on a yearly basis.
What should be done to acidify the
soil before planting? That will
depend on the soil type. If you are
lucky enough to be starting with a
well-drained soil, it can be acidified
by adding large amounts specific
organic materials that acidify as they
break down, such as peat moss and
composted oak leaves.
What if the soil isn't well-drained?
If the soil has lots of clay and is
poorly drained, adding lots of organic
material can make the problem worse
by holding more moisture. In that
situation, it is better to acidify the soil
using elemental sulfur or iron sulfate.
Apply the recommended amount of
sulfur or iron and incorporate it into
the top several inches of soil.
What is the best way to safely
acidify soil where I already have
plants? Applying elemental sulfur or
iron sulfate is the best way. Sulfur is
very slow acting (it may take as long
as 3-4 months to be effective).
Applied at recommended rates, it's
safe for your plants. Iron sulfate acts
more quickly (it takes about 2-3
weeks to affect the pH) but it takes a
lot more of the chemical compared to
using elemental sulfur. Be especially
careful with iron sulfate. It will leave
rusty stains on your clothes, sidewalks
and patios. It's even a good idea to
wash your gardening clothes
separately from your other laundry
after spreading iron sulfate.
Is aluminum sulfate an option? It is
often recommended as a method of
lowering the pH enough to turn Nikko
Blue hydrangea blooms from pink
(bloom color when the soil is too
alkaline) to blue (indicating the soil is
acid enough), but it should not be
used on a regular basis. The heavy
metals will accumulate and become
toxic to the roots of the plants.
Is there anything else that will
acidify as quickly without the
drawbacks of aluminum sulfate?
An optional way to make a quick
change to soil pH is to water the plant
several times with a solution of 2
tablespoons vinegar to
WWW.JIMWHITINGNURSERY.COM
507-289-3741
Is there anything else that will
acidify as quickly without the
drawbacks of aluminum sulfate?
An optional way to make a quick
change to soil pH is to water the
plant several times with a solution
of 2 tablespoons vinegar to
application rates vary widely
depending on soil type as well as the
initial pH. If you don't feel confident
after reading the instructions, check
with one of our horticulturists.
They'll be glad to help.
Additional Information Sheets
a gallon of water. This is a great way Plants for Acid and Alkaline Soils
Understanding Soils
to adjust pH in containers indoors
Soil pH
and out.
Are there any other options for
Recommended Products
acidifying? There are several soil
Soil Testing Kits and Information
supplements that will help acidify,
Baled Peat Moss
such as cottonseed meal and
Elemental Sulfur
ammonium phosphate, but the
Iron Sulfate (Ironite)
amounts needed make them
Aluminum Sulfate
impractical for large areas.
Ammonium Sulfate
Are there any products that will
both acidify and fertilize? There
are a few. The best known is the
water-soluble product called
Miracid. product will help keep the
pH problem from getting worse, but
they are not strong enough to lower
the pH. Another option would be to
fertilize with ammonium sulfate, but
it is strong and can easily burn
plants if not used carefully.
Cottonseed meal can be used as an
acidifying fertilizer.
Doesn't lime have something to do
with changing pH? Lime, calcium
carbonate, will do the exact opposite
of acidifying. Lime is used to
neutralize or sweeten a soil that is
too acid, raising its pH level. Lime
is rarely needed in our area.
What about gypsum? Gypsum is
calcium sulfate and would slightly
acidy soil if there wasn't already a
high level of calcium. In our area,
the soils are extremely high in
calcium and adding gypsum would
not be an effective way to lower the
pH.
Is there an easy way to know how
much of an acidifying product to
apply? Unfortunately the
instructions on packages of products
such as elemental sulfur seem vague
or confusing. That's because
507-289-3741"
WWW.JIMWHITINGNURSERY.COM