What Is Soil pH And How To Test And Adjust It

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This article provides a definition for soil pH and how it can be adjusted or corrected to benefit plant growth
by Brett · All Zones · Growing Basics · 0 Comments · March 29, 2013 · 12,336 views

About Soil pH

Soil pH is a measurement of the alkalinity or acidity of soil and is measured on a scale of 1-14, with 7 as the neutral mark. Any measurement below 7 is considered acid and anything above 7 is considered alkaline. Acid soils are often referred to as "sour" and alkaline soils as "sweet." A pH of 5.5 is 10 times more acidic than a pH of 6.5. Conversely, a pH of 8.5 is 10 times more alkaline than a pH of 7.5.

Technically, pH is a gauge of the hydrogen-ion concentration in the soil. However, for the gardener's needs, it is enough to know whether your soil is alkaline, nuetral or acid because certain nutrients plants need to thrive can only be absorbed by the plants roots when the soil pH falls into a certain, acceptable range, which depends on the specific needs of a particular type of plant.

As you'll notice when researching the specific needs of plants, most, including most vegetables and lawn grasses, will thrive in a pH ranging from 6 to 7.5. However, there are some plants, such as azaleas, blueberries, gardenia and Irish potatoes, which prefer a highly acid soil in the 4 to 5 range on the scale. If the soil is too alkaline, or "sweet," the foliage of these acid-loving plants may turn yellow, they will not flower or produce fruit as well if any at all, and can eventually die. Other plants, such as lilacs, prefer a more alkaline soil.

Some soils are more acidic while others are slightly alkaline to very alkaline. If more alkaline, this is due mainly to the limestone parent material from which the soils were formed. In addition, home builders may remove topsoil during construction and replace it with more alkaline subsoil. Alkaline building materials, such as limestone gravel and concrete, and high pH irrigation water may also contribute to a soil's alkalinity.

Testing and Adjusting Soil pH

If the pH of your soil is at a range not acceptable or beneficial to a type of plant or plants you are growing, then you will need to make a choice. Either choose plants adapted to your soil's pH or alter your soil's pH to fit the plants needs.

You can test your own soil by purchasing a test kit from your local nursery and garden center or you can buy a soil test kit online here. Your local Extension office may also provide soil testing services.

Making Soil More Acid (lowering pH)

If your soil is alkaline, you can lower your soil's pH (or make it more acidic) by using several different products. These include, elemental soil sulfur, aluminum sulfate, iron sulfate, chelated iron, acidifying nitrogen, sphagnum peat, and organic mulches. An excellent way to lower the pH of small beds or garden areas is the addition of sphagnum peat. (The pH of Canadian sphagnum peat generally ranges from 3.0 to 4.5.) Sphagnum peat is also a good source of organic matter. On small garden plots, add a one to two inch layer of sphagnum peat and work it into the top 8 to 12 inches of soil before planting. The addition of sphagnum peat to large areas would be cost prohibitive. If you have a native heavy clay soil that holds too much moisture during the cool season, avoid using peat moss to amend the clay as it holds 9 times its weight in water, exacerbating the drainage problem. With the exception of bog or aquatic plants, most plants prefer a well-drained but moist soil that does not stay constantly wet or soggy.

Granular sulfur is the safest, least expensive but slowest acting product to use when attempting to lower your soil's pH. The table below shows the pounds of elemental sulfur needed per 10 square feet to lower the pH of a loam or silt-loam soil to the desired pH indicated in the table. Reduce the rate by one-third for sandy soils and increase by one-half for clays.

Pounds of Sulfur Needed to Lower the Soil pH (make it more acid)

Chart source: http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html

To avoid plant injury, don't exceed 2 pounds of sulfur per 100 square feet per application. Wait at least 3 months to make another application.

Other products you can use to make soil more acid:

Aluminum sulfate and iron sulfate react more quickly with the soil than elemental sulfur. However, aluminum sulfate and iron sulfate must be applied at a 5 to 6 times greater rate. Do not apply more than 5 pounds per 100 square feet of aluminum or iron sulfate at any one time. Excessive amounts of these two sulfates can also injure plants.

Research suggests that wood chips as a surface mulch may actually allow greater nutrient absorption by some trees. Spread a layer about two inches thick at least out to the dripline. Each spring add more mulch to keep the depth at about two inches.

Note: If the pH of your soil is greater than 7.5, then the soil may contain a large amount of free calcium carbonate. This compound strongly resists changes in soil pH. Lowering the pH becomes difficult or impractical on soils that have a pH above 7.5.

Making Soil More Alkaline (raising ph)

The pH of highly acidic soils can be raised, or made more alkaline, by incorporating limestone into the soil. The table below shows pounds of standard ground limestone needed per 100 square feet to raise the pH to 6.5 from 5, 5.5, and 6 in the top 6 inches of soil.

NOTE: There are new, specially formulated lime products, such as Green N Grow Lime, that are 10 times stronger that standard pelletized lime. You can use these products at 10 times less the rate of standard pelletized lime.

Wood ash will also raise the soil pH, making the soil more alkaline.

Other Tips

  • Do not apply wood ash, limestone, hydrated lime, or other liming materials to alkaline soils.
  • Modifying a soil's pH is usually a slow process and may require repeat treatments. It is often most effective to use a combination of treatments. However, don't expect a quick fix or a miracle cure.

Hope this info was helpful to you!


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