Soil pH

If you want your plants to grow strong, study up on your soil’s chemistry.

by Jessica Walliser
Knowing the pH level of your soil is important to growing healthy plants
Courtesy Stock.XCHNG
Soil pH indicates the acidity of your soil.

A healthy, productive farm is created from the ground up. The fertility, structure and biological activity of your soil are the keys to raising successful crops. Great soil grows great plants with increased vigor and pest tolerance, not to mention maximized yields. There’s a lot to know about your soil for sure, but the most critical measurement is its pH.

What is pH?
Represented on a scale of 0 to 14, pH is the measurement of the acidity of something—in this case, your soil. In a nutshell, the pH is the comparative measure of hydrogen and hydroxide ions present.

At neutral pH 7, there are equal numbers of hydrogen and hydroxide ions. A soil pH measurement below 7 is considered acidic and contains more hydrogen ions. Soil pH above 7 is alkaline and contains more negatively charged hydroxide ions.

The soil pH is an important number to know because it determines the availability of almost all essential plant nutrients. If the soil pH is not on track, plants will not have access to nutrients necessary for growth and, therefore, won’t perform at their best. Nutrients can get trapped in the soil and will not be released for plant use.

Amending Soil pH
When soil becomes too acidic, certain nutrients become less available (phosphorus in particular), good soil bacteria become less active and some elements (like aluminum and manganese) can become toxic. Soils that are too alkaline have different, but equally as important, nutrient availability issues.

Regular soil testing is the only surefire way to know your soil’s current pH level. Most soil-test results will also tell you how to raise or lower the soil’s pH to reach the desired level.

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When adjusting your soil’s pH, add only the recommended amount of any product as indicated by a soil test to make an effective pH change without going too much in the opposite direction. On the pH scale, a single-digit change (say from 5 to 6) translates as a tenfold change in acidity or alkalinity. That means a pH of 5 is ten times more acidic than 6.0, showing just how important the proper pH really is. 

Acidic Soil
Lime (aka limestone) is most often used to raise the soil’s pH and make it less acidic, but not all liming materials are created equal. First, look to your soil test results to determine if you need calcitic lime or dolomitic lime.

Calcitic lime is mined from natural limestone deposits and crushed to a fine powder. Also called aglime or agricultural lime, it supplies calcium to your soil as it adjusts the pH. Dolomitic lime is derived in a similar manner but from limestone sources that contain both calcium and magnesium.

If a soil test indicates high levels of magnesium, use calcitic lime. If the test shows a magnesium deficiency, then use dolomitic lime. Clay soils tend to hold onto magnesium so more often than not, calcitic lime is the more appropriate choice for clay soils and dolomitic lime for sandy soils. 
Alkaline Soil
If it’s necessary to lower your soil’s pH level and make it less alkaline, turn to elemental sulfur or aluminum sulfate. Elemental sulfur applied to the garden is oxidized slowly by soil microbes and takes a few months to alter the pH. Working it into the soil will yield better results as it is more rapidly processed beneath the soil surface.

Spring applications of sulfur are generally the most effective. Often found in pelletized form, sulfur may take awhile to work, but it’s less likely to harm plants than aluminum sulfate products. Aluminum sulfate reacts quickly with the soil and makes a rapid pH change, but there’s an increased potential to burn plant roots.

Soil pH in Your Garden

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In general, the broadest amount of nutrients is most available for plant use at pH 6.5—a perfect soil-pH target for the vegetable garden. There are, of course, exceptions to this rule, both inside and outside the vegetable garden. For example, blueberries and evergreens use a copious amount of iron, primarily available at more acidic pH values (generally 5.5 and below). 

To the left is a chart of some common vegetables and their pH tolerances. Remember, pH 6.5 is slightly acidic so those plants listed as enjoying acid soil will be quite at home at 6.5. Those listed as tolerant of alkaline soils don’t necessarily prefer to grow in them, they just perform better than some others do when the pH level is a bit too high. 

About the Author: Horticulturalist Jessica Walliser is the author of Good Bug, Bad Bug: Who’s Who, What They Do and How to Manage Them Organically (St. Lynn’s Press, 2008) and co-host of Pittsburgh’s top-rated gardening radio program, The Organic Gardeners, on KDKA Radio. Read about her gardening adventures in Dirt on Gardening.

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