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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: