The Homeowners Column

The Homeowners Column

How to Lower Soil pH

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

HOMEOWNERS COLUMN

by Sandra L. Mason

Unit Educator Horticulture/Environment

University of Illinois Extension

Champaign County

(217)333-7672

Release Date: July 5, 2008

Title Development Information

How to Lower Soil pH

Last week I discussed soil pH, and its importance in plant nutrient availability. Some plants such as blueberries, rhododendrons, and blue hydrangeas require a more acidic soil than our natural soils so we have to add products to the soil to for the plants to grow properly.

Before adding anything to the soil, get a soil test to determine the present soil conditions and therefore how much material to add to get to the desired pH. Several materials available at garden centers will lower soil pH; elemental sulfur; aluminum sulfate; iron sulfate; and sulfur coated urea. Each material has its advantages and disadvantages.

Generally materials should be worked into the soil after application to be most effective. Some can be used as a fertilizer sprayed on the plants while others can cause leaf burn if not washed off leaves immediately after application. Be sure to read and follow all label directions.

If dramatic shifts such as a pH 6.5 to 4.5 for blueberries are needed, than elemental sulfur is a better choice. It has more acidifying capability than the other products and lasts for years. However it reacts slowly and may take several months. Soil bacteria convert the sulfur to sulfuric acid. Since it is a biological process it is slower than a chemical reaction. The process occurs when the bacteria are most active in moist warm soil and are not active in the winter. Spring application and incorporation work best. Once plants are planted it's difficult to add much sulfur. Prepare beds in advance; ideally the season before planting.

Listed below is the amount of elemental sulfur required to lower pH. Elemental sulfur is sometimes labeled as flowers of sulfur and is available as a powder or pellet. The soil texture or percent sand, silt, or clay determines how much sulfur is needed to lower the pH.

Pounds of elemental sulfur needed to lower soil pH of a silt loam soil to a depth of 6 inches (for clayey soils increase amount by one-half)

Present pH

Desired soil pH

6.5

6.0

5.5

5.0

4.5

Pounds of sulfur per 100 square feet

7.5

2.0 lbs

3.5 lbs

4.5 lbs

6.0 lbs

7.0 lbs

7.0

1.0 lbs

2.0 lbs

3.5 lbs

5.0 lbs

6.0 lbs

6.5

--

1.0 lbs

2.5 lbs

4.0 lbs

4.5 lbs

6.0

--

--

1.0 lbs

2.5 lbs

3.5 lbs

Ammonium sulfate and sulfur coated urea fertilizers will have a small effect to lower pH. For example ammonium sulfate fertilizer 21-0-0 at 10 lbs per 1000 square feet can change the soil pH from 7.5 to 7.4. These are good choices for maintenance of acid loving plants. Do not use more than is listed for fertilizer needs.

Iron sulfate can be used to lower pH but requires six times more product than elemental sulfur. It reacts faster at 3-4 weeks than elemental sulfur but as with all the products can cause plant damage if over used.

Aluminum sulfate will change the soil pH instantly because the aluminum produces the acidity as soon as it dissolves in the soil. However aluminum sulfate should be reserved for use only with hydrangeas to promote blue flowers. Aluminum is necessary for the flower color change in hydrangeas but can cause aluminum toxicity in other plants such as blueberries. Many acres of land in the world are unusable for crops due to soil acidity and aluminum toxicity. Simply lowering the soil pH with any product will help hydrangeas flowers to be blue but the competitive deep blue will require aluminum sulfate additions.

So what is the moral of the pH story? Do your homework. Understand basic plant needs. Get a soil test. Add correct amount of proper product. Get yearly soil tests to determine success. Sit back and enjoy.

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