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Modifying Soil pH

Description/Purpose:

Soil pH is a measure of soil acidity. Soil pH affects a plant's ability to take up available nutrients. Modifying the pH of soil allows nutrients to be taken up by the plant. The ideal pH for most plants is between 6.0 and 6.8. Most soils in the Twin Cities area are alkaline (basic). Many evergreens, azaleas, rhododendrons and blueberries, however, benefit from acidic soil. Modifying the soil to a lower pH (more acidic) can create desirable color changes in Red Maple (Acer rubrum) and Bigleaf Hydrangea (Hydrangea macrophylla).

It is necessary to modify the pH of your soil if it does not fall within the ideal range, or if you would like to grow plants that require a pH that is outside the ideal range (evergreens, azaleas, rhododendrons, etc.). To find out if your soil needs modification, have it tested. For information on soil and soil testing see "Focus on: the Soil Testing Lab!," online factsheet from University of Minnesota Extension at www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Aug1599.html, or contact your county Extension office. Additional information on soil testing and pH is available at http://faq.gardenweb.com/faq/lists/soil/2000054914003462.html.

Acidic Soils:

Acidic soils are soils that have a pH of less than 6.5. The most common cause of acidic soil is heavy rainfall. The rainfall dissolves mineral nutrients and leaches them from the soil. Acidic soils are found in Northern Minnesota and in coastal areas that have high evergreen tree populations (dead evergreen needles are acidic). Acidic soils are also found in areas where drainage is poor and there is standing water, such as a peat bog.

There are several ways to raise the pH of acidic soil. It will take one to two years for the soil pH to reflect the change.

Agricultural lime is composed of calcium carbonate, magnesium carbonate, and other minerals, which neutralize soil acidity and furnish calcium and magnesium for plant growth. Calcitic lime is composed mainly of calcium carbonate. Dolomitic lime is composed mainly of magnesium carbonate. Dolomitic lime is preferable to calcitic lime because it is slow acting and contains magnesium, an essential element for plant growth. Calcitic and dolomitic lime are available in a pulverized form. Application rates for agricultural lime are given in Table 1.

               Table 1.

Pounds of Calcitic or Dolomitic Lime Needed to
Raise Soil pH to 6.5 (lbs. Per 100 square feet)
Soil Type
Soil pH Sand Loam Clay
6.0 2 lbs. 3.5 lbs. 5 lbs.
5.5 4.5 lbs. 7.5 lbs. 10 lbs.
5.0 6.5 lbs. 10 lbs. 15 lbs.
4.5 8 lbs. 15 lbs. 20 lbs.
4.0 10 lbs. 17.5 lbs. 23 lbs.

Adapted from Purcell Industries, Inc. "All About Fertilizers."

Hydrated lime, a fast acting form of lime, can raise pH quite rapidly. It is the "strongest" lime generally available. However, the use of hydrated lime should be avoided, except in extremely heavy clay soils, as it can easily burn plant roots.

Wood ashes from a fire pit or fireplace can be used to raise soil pH. The maximum amount of ashes that can be added per year is 2 pounds per 100 square feet of soil. If more than 2 pounds per year is added, the soil may become overloaded with potassium. Excessive amounts of potassium in the soil interfere with the plant's ability to take up essential nutrients such as magnesium and, to a lesser extent, calcium.

Alkaline Soils:

Alkaline (basic) soils have a pH of more than 7.0. Basic soils are generally found in areas with lower rainfall levels, in urban areas (due to large amount of cement), and in areas where there is a high concentration of clay in the soil (Midwest, Southwest).

There are several materials that can be used to lower the pH of alkaline soil.

Liquid soil acidifiers, such as Stern's Miracid, temporarily lower soil acidity. Miracid lowers the soil pH resulting in increased availability of micronutrients in high pH soils. Miracid can be applied to soil or on leaves for a fast foliar feeding. Because of the temporary nature of this product, it must be applied every two weeks. Liquid soil acidifiers, such as Miracid, are time consuming to apply and are therefore best used in small areas.

Application Rate: One tablespoon of Miracid to one gallon of water (one gallon of solution covers approximately 10 square feet).
Iron sulfate is a fast acting soil-acidifying amendment. Changes in pH level usually occur with 3 to 4 weeks. Table 2 shows the application rate of iron sulfate. If more than 7 pounds per 100 square feet is to be applied, split the applications in 1 to 2 month intervals and water frequently to avoid excessive levels of soluble salts.

               Table 2.

Pounds of Iron Sulfate Needed to
Lower Soil pH by One Unit (lbs. Per 100 square feet)
Soil Type
Sand, Loamy Sand, Sandy Loam Loam, Silt Loam
4.8 lbs. 19.2 lbs.

Adapted from University of MN Extension Service publication "Soil Acidification."

Aluminum sulfate can be used to lower pH. However, it is not recommended as a soil-acidifying agent because it can produce aluminum toxicity in plant roots.

Elemental sulfur reacts slowly with the soil. It should be applied and worked into the soil to a depth of 6 inches. Because elemental sulfur is a slow reactor, it should be applied the year before planting for best results. Table 3 shows the application rate of elemental sulfur used to lower soil pH by one unit. Table 4 provides the application rate of elemental sulfur used to lower soil pH to 4.5.

               Table 3.

Pounds of Elemental Sulfur Needed to
Lower Soil pH by One Unit (lbs. Per 100 square feet)
Soil Type
Sand, Loamy Sand, Sandy Loam Loam, Silt Loam
0.8 lbs. 2.4 lbs.

Adapted from University of MN Extension Service publication "Soil Acidification."


               Table 4.

Pounds of Elemental Sulfur Needed to
Lower Soil pH to 4.5 (lbs. Per 100 square feet)
Soil Type
Soil pH Sand, Loamy Sand, Sandy Loam Loam, Silt Loam
7.0 1.9 lbs. 5.8 lbs.
6.5 1.5 lbs. 4.6 lbs.
6.0 1.2 lbs. 3.5 lbs.
5.5 0.8 lbs. 2.4 lbs.
5.0 0.4 lbs. 1.2 lbs.

Adapted from University of MN Extension Service publication "Soil Acidification."

Organic compounds such as sphagnum peat moss, compost, and manure can be used to lower soil pH. Table 5 provides the approximate application rate used to lower soil pH by one unit.

               Table 5.

Pounds of Organic Compound Needed to
Lower Soil pH by One Unit (lbs. Per 1 square yard)
Compound Type
Peat Moss Compost Manure
2.5 lbs. 14 lbs. 5 lbs.

Adapted from Smartgardening.com "Soil pH."

Tools and Equipment:

Materials necessary for amending the soil:

  • Shovel
  • Wheelbarrow or Tarp
  • Rototiller
  • Spreader
  • Amendment material
  • Paint to mark off area
Site Considerations:

Make sure that the area you will be amending has adequate drainage, and that the soil doesn't have an extreme pH problem. A soil with an extreme problem would be one that needs to have a drastic change in pH in order for a plant to grow (eg. changing a soil ph from 8.5 to 4.5). Keep in mind that landscaping materials can affect pH. Materials such as cement or limestone walks and patios will raise the pH of soil.

Process for Application:

For both acidic and basic soils, it is best to incorporate the amendments at least six inches into the soil. For alkaline soils this should be done a year before planting time. In acidic soils this can be done anytime prior to planting. The easiest method for incorporating amendments is:

  1. Mark off the area to be amended.

  2. Till the top one foot of soil with the rototiller.

  3. Apply amendment at the recommended rate (from the above charts).

  4. Till the amendment into the soil.
An alternate method for incorporating the amendments into the soil is double digging.
  1. Measure off the area to be amended. Dig a trench 1 foot wide by 1 foot deep. Place soil alongside trench on a tarp or into a wheelbarrow (Fig. 1). Break up the subsoil on the bottom of the trench (Fig. 2).

  2. Dig another trench of the same size next to the first one. Place soil from this trench into the first trench (Fig 3.). Break up the subsoil on the bottom of this trench.

  3. Repeat these steps until the whole area has been double dug. Fill the last trench with the soil from the tarp or wheelbarrow (Fig 4).

  4. Spread the recommended amount of amendment (from charts above) over this area. Dig it into the top foot of soil.

Figure 1 Figure 2

Figure 3 Figure 4
Adapted from Stefren Buczacki Conran's Basic Book of Home Gardening.

A third method of application is to topdress.

  1. Apply the recommended amount of amendment over the area to be amended.

  2. Distribute amendment evenly over soil using the spreader.
The disadvantage of this method is that only the upper 1-2 inches of the soil will be affected by the amendment.



References:

Buczacki, Stefren. n.p. Conran's Basic Book of Home Gardening, A Complete Guide for the First-Time Gardener. Viking Penguin, Inc. New York.

Eliason, Roger D. Aug. 15, 1999. "Focus on: the Soil Testing Lab!" Yard & Garden Line News, Vol. 1 Number 9. University of Minnesota Extension Service. http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Aug151999.html.

Greshuny, Grace. 1994. Start With the Soil. Rodale Press, Pennsylvania.

Illinois Cooperative Extension Service. 1995. "pH Lower." Horticulture Solution Series. http://www.ag.uiuc.edu/~robsond/solutions/horticulture/docs/phlow.html

Magdoff, Fred. 1992. Building Soils for Better Crops. University of Nebraska Press.

Purcell Industries, Inc. 2000. "Soil & pH." All About Fertilizer.
www.fertilizer.com

Smartgardening.com. 1998-2000. "Soil pH."
http://www.smartgardening.com

Sterns' Miracle-Gro Products, Inc. Miracid Soil Acidifier Plant Food. Port Washington, New York.

University of Iowa - Integrated Pest Management. Apr. 1994. "How to Change Your Soil's pH." http://www.ipm.iastate.edu/ipm/hortnews/1994/4-6-1994/ph.html

University of Minnesota Extension Service. n.p. "Raising Soil pH." http://www.extension.umn.edu/distribution/horticulture/components/1731-04.html.

University of Minnesota Extension Service. n.p. "Soil Acidification." http://www.extension.umn.edu/distribution/horticulture/components/1731-05.html.

University of Minnesota Extension Service. n.p. "Soil pH Modification." http://www.extension.umn.edu/distribution/horticulture/components/1731-03.html.

Wolf, Benjamin. 1999. The Fertile Triangle. Food Products Press, New York.


This implementation report was developed by Laura Ducklow and Daniel Peterson, students, University of Minnesota Department of Horticultural Science.

 
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