Have you ever tried to grow blueberries or azaleas only to have them turn yellow, then brown and eventually die? If you have, chances are you planted them in an alkaline soil.
"Acid loving" plants, like blueberries and azaleas, succeed only in acidic soils like those typically found in parts of Minnesota. In contrast, many plants that are native to Iowa are adapted to alkaline soils. However, on highly alkaline soils even some Iowa native plants grow poorly. These include pin oak, river birch, and white pine.
The standard measurement of alkalinity and acidity is known as pH. The pH scale ranges from 0 to 14. A pH of 7 is neutral, which is neither acid nor alkaline. Below 7 is acid and above 7 is alkaline. 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. A soil test will determine pH.
The soil pH is important because it affects the availability of nutrients in the soil. Many plant nutrients are not readily available to plants in highly alkaline or acidic soils. These essential nutrients are most available to most plants at a pH between 6 to 7.5.
Consequently, most horticultural plants grow best in soils with a pH between 6 (slightly acid) and 7.5 (slightly alkaline). Most Iowa soils are in this range. If your soil is not, 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.
But before attempting to raise or lower your soil's pH, you should first conduct a soil test to determine your current soil pH. Contact your local county Extension office for advice on collecting and sending a soil sample to a laboratory for analysis.
Some soils in Iowa (especially those in western Iowa) are slightly alkaline to very alkaline, with pH's that range from 7.2 to 9.5. 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.
If your soil is alkaline, you can lower your soil's pH or make it more acidic by using several products. These include sphagnum peat, elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, 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.
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.
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.
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.
Some types of fertilizers can help to acidify the soil and most of them are safe to apply. Acidifying fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea, and ammonium nitrate. Read the label on the fertilizer bag to determine if it is an acidifying fertilizer.
Research suggests that wood chips as a surface mulch may actually allow greater nutrient absorption by some trees. Spread a layer about three inches thick at least out to the dripline. Each spring add more mulch to keep the depth at about three inches.
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.
The pH of highly acidic soils can be raised by incorporating limestone into the soil. Hydrated lime works quicker, but over liming is more likely. The table below shows pounds of ground limestone needed per 100 square feet to raise the pH to 6.5 in the top 6 inches of soil.
|Soil pH||Sandy loam||Loam||Clay loam|
Wood ash will also raise the soil pH and make the soil more alkaline. 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.
This article originally appeared in the April 6, 1994 issue, pp. 1994 issue, pp. 42-43.