How To Lower Soil pH

phValues Illustration 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.

  Understanding Soil pH

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 mineralization.

  Why Is Soil pH Important?

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.

Read more: pH and Nitrogen Availability

  Wisconsin's Marl-Based Soils

Map showing extent of Wisconsin's glaciers.
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.

  What You Can Do?

1.  Get Your Soil Tested

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.

2.  Add Compost

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.

3.  Add Elemental Sulphur or Aluminum Sulfate

Spreading salts to garden 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 products.

  Aluminum sulfate reacts quickly with the soil and makes a rapid soil pH change, but there is an increased potential to burn plant roots

  Soil pH Maintenance

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.

Further Information:

 Wisconsin Native Fruit Trees
 Wisconsin Native Berry Shrubs
 Lovely Native Violets
 Use Eggshells For Your Plants
 How to Use Banana Peels in Your Garden

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