Understanding pH

What is pH and how does it affect your plants?

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Soil pH level is dependent on many factorsCorrecting 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!
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