pH and Plants

Soil pH indicates the acidity or alkalinity of soil and is measured in pH units on a scale of 0 to 14 with 7 as the neutral point. From pH 7 to 0, the soil is increasingly more acidic, and increasingly alkaline from pH 7 to 14. It is important to understand that the pH scale is not linear. The intervals between numbers are logarithmic, which means every number of the scale represents a solution with ten times less H⁺ ions than the number below. Soil with a 5 pH is ten times more acidic than soil with a pH of 6. Following is the pH for many familiar items.

pH Ranges

Classification	pH Range	Common Examples
Extremely acid	Less than 4.5	Lemon juice, vinegar, stomach acid
Very strongly acid	4.5-5.0	Beer, tomatoes
Strongly acid	5.1-5.5	Boric acid, cabbage, black coffee
Moderately acid	5.6-6.0	Potatoes
Slightly acid	6.1-6.5	Cow’s milk, salmon
Neutral	6.6-7.3	Saliva, blood
Slightly alkaline	7.4-7.8	Eggs
Moderately alkaline	7.9-8.4	Sea water, sodium bicarbonate
Strongly alkaline	8.5-9.0	Baking soda, borax
Very strongly alkaline	Greater than 9.1	Ammonia, milk of magnesia, lime

Plants have preferred pH ranges for optimal growth and health. From a technical standpoint, pH is an indicator of the relative abundance of hydrogen ions (H+) and hydroxyl ions (OH-) in the soil. The levels of these ions play an important role in soil fertility because pH affects the ability of the soil to hold nutrients as well as the availability for plant uptake. Soil pH can also impact microbial populations as well as some aspects of soil structure. The following table illustrates the availability of certain nutrients under a range of pH conditions. As you can see in the orange bar range, most nutrients that plants need are readily available when the pH of the soil solutions ranges from 6.0 to 7.5, or slightly acid to neutral.

The majority of plants grow well in the 6.0 to 7.0 pH range; however, there are some that require more acidic or basic conditions. This is in part due to the nutrients made available under these conditions.

For example, rhododendrons, azaleas and other ‘acid-loving’ plants prefer more acidic conditions because higher pH levels can reduce their ability to take up sufficient quantities of some important nutrients (see blue block on chart).

Another important point is that pH can vary dramatically from one location to another.   You have a better opportunity for plant success if your soil pH is matched to the pH range needed by whatever you are planting. The most accurate pH testing is offered by the Soil, Plant and Pest Center at the University of Tennessee’s Institute of Agriculture. No special equipment is needed, and you can collect and mail your soil sample using containers you have at home or those provided at your county Extension office. Just carefully follow the instructions posted by the Center at https://ag.tennessee.edu/spp/Pages/soiltesting.aspx

References:

“Soil pH: What It Means.” http://www.esf.edu/pubprog/brochure/soilph/soilph.htm
Cullina, William. Native Trees, Shrubs and Vines. 2002. The New England Wild Flower Society.
“Acid or Alkaline? What pH means in gardenspeak.” http://www.plantea.com/pH.htm