Understanding pH

What is pH and how does it affect your plants?

May 16, 2011

You’ll often read the recommendation to check soil pH, but what does that really mean? pH is simply a measure of how acid or alkaline a substance is, and soil acidity or alkalinity (soil pH) is important because it influences how easily plants can take up nutrients from the soil. Many gardening books and catalogs list the preferred pH for specific plants. The good news for gardeners is that, with a few exceptions, most plants will tolerate a fairly wide range of soil pH. 

Nutrient uptake and pH: Plant roots absorb mineral nutrients such as nitrogen and iron when they are dissolved in water. If the soil solution (the mixture of water and nutrients in the soil) is too acid or alkaline, some nutrients won’t dissolve easily, so they won’t be available for uptake by roots. 
Most nutrients that plants need can dissolve easily when the pH of the soil solution ranges from 6.0 to 7.5. Below pH 6.0, some nutrients, such as nitrogen, phosphorus, and potassium, are less available. When pH exceeds 7.5, iron, manganese, and phosphorus are less available.
Regional differences: Many environmental factors, including amount of rainfall, vegetation type, and temperature, can affect soil pH. In general, areas with heavy rainfall and forest cover such as the Eastern states and the Pacific Northwest have moderately acid soils. Soils in regions with light rainfall and prairie cover such as the Midwest tend to be near neutral. Droughty areas of the western United States tend to have alkaline soils. However, the pH of cultivated and developed soils often differs from that of native soil, because during construction of homes and other buildings, topsoil is frequently removed and may be replaced by a different type of soil. So your garden soil pH could be different from that of a friend’s garden across town. 
Changing pH: Most garden plants grow well in slightly acid to neutral soil (pH 6.0–7.0). Some common exceptions include blueberries, potatoes, azaleas, and rhododendrons, which prefer moderately acid soil. You can make small changes to soil pH by applying soil amendments. However, you’ll have best success if you select plants that are adapted to your soil pH and other soil characteristics. Adding organic matter such as compost to the soil buffers the pH, which means that it tends to bring both acid and alkaline soils closer to neutral. 
If you have your soil analyzed by a lab, the lab report will include soil pH. You can also test soil pH yourself with a home soil test kit or a portable pH meter. Home kits and portable meters vary in accuracy but can be helpful in assessing the general pH range of your soil.
The quantity of liming or acidifying material needed to change soil pH depends on many factors, including current pH, soil texture, and the type of material. A soil lab report will contain recommendations on types and quantities of amendments to use.
You can spread liming or acidifying materials with a garden spreader or by hand for small areas. If hand spreading, be sure to wear heavy gloves to protect your skin.
Correcting acid soil: If your soil is too acid, you must add alkaline material, a process commonly called liming. The most common liming material is ground limestone. There are two types: calcitic limestone (calcium carbonate) and dolomitic limestone (calcium-magnesium carbonate). In most instances, you’ll use calcitic lime. Apply dolomitic lime only if your soil also has a magnesium deficiency. 
Ground limestone breaks down slowly in the soil. Apply it to the garden and lawn in the autumn to allow time for it to act on soil pH before the next growing season. A rule of thumb for slightly acidic soils is to apply 5 pounds of lime per 100 square feet to raise pH by one point. In general, sandy soils will need less limestone to change pH; clay soils will need more.
The amount of lime you must add to correct pH depends not only on your soil type but also on its initial pH. For example, applying 5 pounds 464of limestone per 100 square feet will raise the pH of a sandy loam soil from 6.0 to 6.5. It would take 10 pounds per 100 square feet to make the same change in silty loam soil. However, if 5.6 was the initial pH of the soil, 8 pounds per 100 square feet would be required for the sandy loam soil, and 16 pounds per 100 square feet for the silty loam soil. There is no simple rule of thumb that applies to all soils. The safest approach to take if you plan to apply limestone is to have your soil tested and follow the lab recommendations. 
Applying wood ashes also will raise soil pH. Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace minerals. Because it has a very fine particle size, wood ash is a fast-acting liming material. Use it with caution, because overapplying it can create serious soil imbalances. Limit applications to 25 pounds per 1,000 square feet, and apply ashes only once every 2 to 3 years in any particular area. At this rate, your soil will get the benefits of the trace minerals without adverse effects on pH.
Correcting alkaline soil
If your soil is too alkaline, add a source of acidity. The most common material to add is powdered elemental sulfur. As a rule of thumb, add 1 pound of sulfur per 100 square feet to lower pH 1 point. But as with lime, the correct amount will depend on your soil type and its initial pH. Testing your soil and following lab recommendations is the best approach if you want to lower the pH of an entire bed or area of your yard. 
Mixing peat moss with the soil will also lower pH, but peat moss is not a sustainable resource and has been overharvested in many areas; incorporating ample organic matter (such as shredded leaves) is a more environmentally friendly option.
Why Is It Called pH?
What does pH stand for, and why is it spelled in that odd way? Thank chemistry for this incomprehensible abbreviation. It stands for “potenz Hydrogen” (“potenz” means “the potential to be”). In chemis-try, the elements of the periodic table—remember that from school?—such as Oxygen and Hydrogen, are capitalized; that’s why it’s pH rather than ph, Ph, or PH. But what does hydrogen—much less the potential of hydrogen—have to do with soil acidity or alkalinity? Well, the activity of hydrogen ions in solution—and soil is actually a solution at the microscopic level—determines the acidity or alkalinity of the solution. Acidic solutions have a high concentration of hydrogen ions; alkaline solutions have a low concentration. This may all seem arcane, but here’s a fun fact: The inventor of the pH scale developed it to determine the acid content of his beer!