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Roger Weller, geology instructor   

Soil pH
by Tianya Shemanski
Physical Geology
Fall 2007


                   The Ups and Downs of Soil pH


            Soil pH is an extremely important factor in any kind of agriculture, farming,
gardening, or plant growth today.  It affects the health and production rate of
plants as well as dictating what can grow in a particular area of similar earth. 
Thankfully, technology has made detecting and altering pH levels much easier
and simpler than it’s been in the past, but the basic process is still a tedious and
arduous one.

            PH literally stands for the French phrase “puissance d’hydrogene”, which
can be directly translated to English as “the strength of hydrogen”.  We use this
term because pH itself is defined as the negative logarithm of the hydrogen ion
concentration, pH=-log[H+], which determines the acidity or alkalinity of a particular soil.



Figure - The pH scale

How acid or alkaline a soil may be is measured on a scale of 1 to 14, 7 being neutral,
such as distilled water or milk, lower than 7 becoming increasingly more acidic, and higher
than seven becoming increasingly more alkaline.  Because the equation uses a logarithmic
scale, a one-unit decrease in pH is a ten-fold increase in hydrogen ions. For instance, a pH
of 5.5 is ten times as acidic as a pH of 6.5, and a hundred times more acidic then a pH of 7.5.
A pH of 7.0 is considered neutral because there are equal concentrations of H+ and OH- molecules.

Text Box: ·         Extremely acid: < than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2–4 
·         Very strongly acid: 4.5–5.0; beer=4.5–5.0; tomatoes=4.5 
·         Strongly acid: 5.1–5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3 
·         Moderately acid: 5.6–6.0; potatoes=5.6 
·         Slightly acid: 6.1–6.5; salmon=6.2; cow's milk=6.5 
·         Neutral: 6.6–7.3; saliva=6.6–7.3; blood=7.3; shrimp=7.0 
·         Slightly alkaline: 7.4–7.8; eggs=7.6–7.8 
·         Moderately alkaline: 7.9–8.4; sea water=8.2; sodium bicarbonate=8.4 
·         Strongly alkaline: 8.5–9.0; borax=9.0 
·         Very strongly alkaline: > than 9.1; milk of magnesia=10.5, ammonia=11.1; lime=12 














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          A soil’s pH value is naturally predetermined by geology and weather patterns.
Much of the earth’s soil is formed from eroded rock particles and minerals, which have
a pH value of their own. Because of this, the pH of a soil on a particular area may not
correspond to that of the surrounding bedrock simply because the matter that formed
that particular soil was transported by weathering from a different location. For instance,
all of the earth that was eroded away in the creation of the Grand Canyon has been
transported away by the Colorado River to be deposited along its banks or in the Gulf
of California.



An aerial view of where soil has been displaced in the Grand Canyon.


           Rainfall also affects soil pH. Water passing through the soil leaches out basic
nutrients such as magnesium and calcium, which are then replaced by more acidic
elements such as iron and aluminum. Because of this, soils formed under conditions
of high rainfall tend to be more acidic than those formed in arid climates. Other
causes of acidic soil conditions are carbon dioxide from decomposing organic matter
and root respiration dissolving in soil water to form a weak organic acid as well as the
formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from
decaying organic matter and oxidation of ammonium and sulfur fertilizers.

            Soil pH is so incredibly important because it directly affects the availability of
plant nutrients in the soil. Fourteen of the seventeen essential plant nutrients are obtained
directly from the soil. A nutrient must be and remain soluble long enough to travel
successfully through the soil solution and into the plant’s roots. Most of minerals and
nutrients tend to be more available in acid soils, with a pH between 6.0 and 6.5, rather
than those which are neutral or alkaline. Still, the lower acid limit for most plants remains
between 4.5 and 5.5 on the pH value scale. This is because soil which is that acidic can have
high concentrations of soluble aluminum, iron, and manganese, which may be toxic to the
growth of some plants.

Text Box: Nutrient Availability based on soil pH.


                             Figure Two

Nutrient Availability based on soil pH.


           Soil pH also affects the growth and function of many important
microorganisms which assist in the creation of fertile soil. Microorganisms that change
organic nitrogen, or amino acids, from the ammonium form of nitrogen to the nitrate
form that a plant can absorb depend on the pH. Certain bacteria which convert
atmospheric nitrogen into a form that plants can use live in the root nodules of
particular plants, called “legumes”. These bacteria function best when the pH of the
plant they live in is within an acceptable range.

Determining the pH of a particular soil is fairly easy to do these days. PH kits,
containing the necessary color charts and chemicals, are available from most garden
stores. Once you have determined the pH of the soil you are working with, you need
to look at the types of plants you are trying to grow, and what pH soil they will thrive
in. For instance, blueberries and rhododendrons grow best in an acidic soil with a pH
between 4.5 and 5.2, while evergreen trees and shrubs prefer a pH between 5.0 and 6.0.


Plants that prefer acidic soil:

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          Changing the pH of a particular soil is possible, though it can be quite tedious
and tricky. It is also much more difficult to change the pH of a clay soil rather than
a sandy soil, though it is still possible. To lower the pH of a soil, one can add iron
chelate or powdered sulphur. This is broken down by bacteria in the soil, forming
sulfuric acid, which, in turn, increases the acidity of the soil and lowers the soil’s pH.
The lime found in limestone is extremely alkaline, and can be added in ground form
to raise the pH of a soil. Lime not only changes a soil pH, it also provides two major
nutrients, calcium and magnesium, to your plants.

Though generally unheard of, soil pH values are constantly affecting the world
around us. They determine which plants grow naturally in our surrounding area as well
as those which will grow well in our own private gardens. Next time you look out your
car window, remember that all the plants you see growing, whether you are on a highway
in the middle of no where or driving down the middle of Main Street, are a direct result
of the pH value of the soil in which they grow. Thus, soil pH is a complicated web, working
constantly behind the scenes and dictating silently the intricacies of existence for the
flora on our planet.



Other Sites and Sources on Soil pH: