Sometimes it seems that there is so much to understand when
dealing with the garden and landscape that it can be overwhelming. You finally figure out
all that stuff about hardiness, mulching, pruning and fertilizing and then they throw in
pH. It really is important. But don't despair… it isn't all that complicated. You
just need to be aware that pH is a factor that can influence your gardening and learn the
indicators of a pH problem. Since you have much more important things to keep in your head
than how much lime per 1000 square feet or whether it is better to use elemental sulfur or
aluminum sulfate, you just need to know where to find the answers!
Just what is pH and why is it important? pH is a measurement of acidity
or alkalinity in soil. Your soil pH influences whether or not plants can make use of
available nutrients.
How can I measure the pH in my soil? Soil can be tested by the University
of Minnesota or many other independent soil testing labs. There are also several
easy-to-use home pH tests available that will give you a general measurement. Most home
tests involve putting a pinch of reactive agent and a pinch of soil into a little test
tube that you fill with water and shake. Match the color of the water with a color scale
that is provided to determine your pH. Since water also has a pH, it can influence the
outcome of your test. To increase the accuracy of home tests, just use distilled water
instead of tap water.
What do the different readings mean? PH is measured on a scale of 0 to
14. A pH of 7.0 is considered neutral. All readings with a number value higher than 7.0
are considered alkaline. 7.0-8.0 is slightly alkaline and 8.0 to 9.0 is moderately
alkaline. Readings with number value lower than 7.0 are considered acid. 6.0 to 7.0 is
slightly acid and 5.0 to 6.0 is moderately acid.
Will the pH vary much in different locations in my yard? It can vary
significantly. If you suspect that might be the case take several soil samples and use a
home test.
Once I have my soil tested, do I need to do it again? It's a good idea to
test your soil each season, especially if your plants are showing symptoms that might
indicate a problem. While pH doesn't change very quickly, several factors can change the
pH in your soil over a season. For instance, city water in much of our area has a pH of
7.5-8.5 and watering with it gradually makes your soil more alkaline. Similarly, some soil
amendments will acidify your soil as they break down.
What kind of symptoms will my plants have if the pH is not what they like? Since
pH can make nutrients in the soil unavailable, plants will look like they need
fertilizing, the leaves yellowing mid-season and not responding to fertilizer.
What pH do most plants like? The vast majority of plants grow well with a
pH from 6.0 to 7.0. When you find out a specific plant prefers a pH of 6.5, keep in mind
that most plants are tolerant of a fairly wide range and that value of 6.5 represents the
middle of the range. Only a few require a specific pH.
What is the difference between tolerating and requiring a certain pH in the soil? Tolerance
means the plant will not show a significant difference when the pH isn't what is ideal.
For instance, delphiniums do best in a slightly alkaline soil, but will grow in neutral or
slightly acid pH. A few plants require a certain pH and simply won't thrive without it.
Azaleas and blueberries require very acid soil.
Are there plants that actually like it acid? Almost all garden plants
prefer a slightly acid pH. If your pH is moderately acid, try growing Azaleas,
Rhododendrons, Clethra, Holly, Foxglove, Trillium, Bleeding Heart and Bluebells.
Is there anything that does well where the soil is alkaline? For alkaline
soil you don't want to adjust, try delphiniums, dianthus, boxwoods and spirea. Many salt
tolerant plants grow in moderate alkalinity.
How can I adjust pH? To change the soil pH, you need to take into
consideration the type of soil and what you want to use to change the pH. Sandy soils are
easier to acidify than clay soils. To make acid soil less acid, you need to add alkaline
materials such as lime. To make soil more acid, you need to add acid materials such as
peat moss, sulfur, iron sulfate or aluminum sulfate. (Use of aluminum sulfate is
associated with damaged plant roots and heavy metal contamination of the soil.)
How do I choose which material to use when changing pH? Consider
cost, how quickly the material will work and how you want to apply it. Most materials used
for changing pH are organic, but not all. Sulfur is usually the least expensive way to
acidify soil. Lime is the least expensive way to make soil less acid. University soil test
results will recommend the right materials and amounts for your situation.
The garden center sells different types of lime. What is the difference? Most
lime is ground limestone, either calcitic lime (calcium carbonate) or dolomitic lime
(calcium-magnesium carbonate). The type of lime available at local garden centers usually
depends on what is found naturally in the area. Dolomitic lime is best in soils low in
magnesium, but calcitic lime is usually less expensive. All lime is very slow to break
down and won't affect your pH for months. You may also find hydrated (or quick) lime. It
is more alkaline than ground limestone so it takes less to change the pH, but it is a fine
powder that is harder to work with and you'll need to be especially careful not to let the
caustic powder get into your eyes, nose and mouth or on your skin.
What are the differences between pelleted, granulated and powdered lime? To
make lime easier to spread and quicker to dissolve, lime can be ground very fine and then
made into pellets. Granular lime usually flows through a spreader fairly well, but is much
slower to dissolve. Powdered lime is quick to dissolve, but can be very difficult to
spread.
Is gypsum different from lime? Gypsum is calcium sulphate and can be
applied to alkaline soils to move the pH toward neutral. It is sometimes recommended for
clay soils.
Is there a best way to apply these products? Is there a best time? To
determine how much to use and how best to apply it, check the instructions on the package.
If you have your soil tested by a soil laboratory, it may provide you with specific
recommendations.
Can I bring my soil test results in and get some help? We'd be glad to
help you. If our horticulturist isn't available when you stop by, just leave the
information and we will get back to you as soon as possible.
Additional Bachman's Information
Acidifying
Soils
Understanding
Your Soil
Soil Tests
Working
With Clay Soils
Recommended Products
Pelleted and Hydrated Lime
Baled Peat Moss
Elemental Sulfur and Aluminum Sulfate
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 in 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 Bachman'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 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” (Asclepias incarnata) 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 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 it good, too. 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 sometimes recommended as a method of
lowering the pH enough to turn Nikko Blue hydrangea blooms from pink (bloom color when the
soil is 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
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 a gallon of water.
This is a great way to adjust pH in containers indoors and out.
Are there any other options for acidifying? There are several soil
supplements that will help acidify, such as cottonseed meal and ammonium phosphate, but
the amounts needed make them impractical for large areas.
Are there any products that will both acidify and fertilize? There are a
few. The best known is the water soluble product called Miracid. Bachman's also has an
acid version of their own Excel Gro™. Both products 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 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 Bachman's Information
Packaged
Soils and Soil Amendments
Understanding
Soils
Soil pH
Plants
for Acid and Alkaline Soils