How To Lower Soil pH
Many soils in Wisconsin especially in south-eastern Wisconsin are alkaline
(high pH), and may contain free calcium carbonates. These carbonates are a
source for alkalinity.
Soil pH generally refers to the degree of soil acidity or alkalinity. Chemically,
it is defined as the log10 hydrogen ions (H+) in the soil solution. The pH scale ranges from
0 to 14. A pH of 7 is considered neutral. If pH values are greater than 7, the soil is
considered basic or alkaline. If the pH of the soil is below 7, the soil is acidic.
pH expresses the acidity or alkalinity of a solution on a logarithmic scale on
which 7 is neutral, lower values are more acid and higher values more alkaline.
It is important to recognize that because the pH scale is in logarithmic units, a change of just
a few pH units can induce significant changes in the chemical environment and sensitive
biological processes. For example, a soil with pH 5 is 10 or 100 times more acidic than a
soil with pH 6 or 7, respectively.
Sources of H+ (hydrogen ions) in soil solution include carbonic acid produced when carbon dioxide
from decomposing organic matter, root respiration, and the soil atmosphere is dissolved
in the soil water. Another important source of H+ ions us from fertilizers and organic matter
pH is so important to plant growth because it determines the availability of almost all essential
plant nutrients. At a soil pH of 6.5, the highest numbers of nutrients are available for plant use.
Soil pH affects the soil's physical, chemical, and biological properties and processes, as
well as plant growth. The nutrition, growth, and yields of most crops decrease where pH
is low and increase as pH rises to an optimum level.
Map showing extent
of Wisconsin's glaciers.
Soils in Milwaukee, Waukesha, Kenosha and other eastern Wisconsin counties are a special
case because they are marl-based. Marl or marlstone is a carbonate-rich mud or mudstone
which contains variable amounts of clays and silt. Marl is a finely divided calcareous material
deposited when the area was an old lake bed.
Glaciers and Marl. Marl-based soils are extremely common around lakes in areas
where they were covered during the glacial periods.
Clay soil is composed of the finest rock particles — less than .002 mm. Clay is the result of
extreme rock weathering over time and then being deposited by bodies of water. These particles
tend to compact together. This compaction can cause quite a few problems.
The only way to tell if your garden soil pH needs to be adjusted is to get a soil test. In Wisconsin,
you can contact the US Soil and Forage Lab to request a soil test. Their web
site contains instructions on how to collect and submit soil samples.
Home pH soil litmus paper test kits are available at your local garden supply or hardware store, however they
are extremely inaccurate and completely useless for measuring the pH level of soil.
When you add compost to the soil the microbes and bacteria in the soil break the compost down
into its basic building blocks, called humus. Humus coats the soil particles lessening the cohesion
and electrical charges that hold them together. Humus also feeds the bacteria and microorganisms;
and it stimulates an increased population and activity of those good, helpful bacteria and
microorganisms. For healthy soil, we need to keep the soil at least 5% humus.
The Best Solution
Maintaining a changed pH is difficult and requires annual attention. The soil you already have
supports thousands of different plants, so consider selecting plants that will already grow in your
soil. Doing so will be less work for you and better for the plants, and you won’t need to test the
pH of your soil.
If you need to lower soil pH, you may need to lower the soil pH into the acidic range. If this is
necessary, turn to elemental sulfur or aluminum sulfate.
In general, it is best to reduce soil pH before planting sensitive landscape ornamentals, rather than
attempting to reduce soil pH after plants have become established. Use about 4 to 6 lb. of aluminum
sulfate per plant for most medium- and fine-textured Wisconsin soils in order to decrease soil pH
by about one unit.
If elemental sulfur is applied, decrease the total recommended application by
one-sixth. One pound of aluminum sulfate or elemental sulfur is equal to about 2 cups.
If plants are already established, use a top-dress application limited to about 1 lb. (2 cups) aluminum
sulfate or 1/6 lb. (1/3 cup) elemental sulfur per typical landscape plant. Lightly incorporate the
aluminum sulfate or elemental sulfur into the soil, or water-in well. Repeat applications monthly until
the total recommended amount of aluminum sulfate or elemental sulfur has been added.
Because lowering soil pH is a very slow process, have the soil pH checked about three months after each
application to determine if additional applications will be needed. Several applications may be needed
on some soils before the soil pH shows any significant change.
Elemental sulphur is applied to the garden and is eventually oxidized by soil microbes.
It takes a few months to adjust pH. Working it into the soil will yield better results than adding
it to the surface because it is more rapidly processed when it’s mixed into the soil. Spring
applications are generally the most effective. Elemental sulfur is often found in pelletized form,
and while it may take some time to work, it is far less likely to burn plants than aluminum sulfate
Aluminum sulfate reacts quickly with the soil and makes a rapid soil pH change, but
there is an increased potential to burn plant roots
It’s important to remember to only add the recommended amount of any pH adjusting product
as per the results of a soil test. Adding too much may shift the pH too far and cause a different
set of problems.
Because both lime and sulfur will eventually be processed out of the soil, the pH will revert to a
less-than-ideal level every few years. To keep the vegetable garden soil pH at the optimum 6.5, a
new soil test should be performed in the vegetable garden every four to five years.
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