We are interested in soil pH because it plays an important role in plant growth. Soil pH influences many facets of crop production and soil chemistry, including availabilities of nutrients and toxic substances, activities and nature of microbial populations, solubility of heavy metals, and activities of certain pesticides. The soil pH is easily determined and, like taking your temperature when you are sick, it gives us some quick, valuable information that will enable the "Plant Doctor" to prescribe corrective procedures.
pH is defined as the negative logarithm of the hydrogen ion (H+) concentration. When water ionizes to H+ and OH- (a neutral solution), both H+ and OH- ions are in equal concentrations of 0.0000001 moles per liter. That is a very small concentration.
HOH <—> H+ + OH-
[H+]=[OH-]=1 x 10-7 moles/liter. The H+ ion and OH- concentrations in water are very small.
The pH scale has been devised for conveniently expressing these small
concentrations by expressing
pH=Log 1/[H+]
See a simple definition of pH at pH Simplified
When the hydrogen concentration isgreater, such as 0.0001 moles per liter, the pH is 4; when it is smaller, such as 0.00000001, the pH is 8. One thing to remember is that when the pH changes from one unit to another, the change in the hydrogen ion concentration is a ten-fold change, not just one. So a pH of 5 is ten times more acid than a pH of 6 and 100 times more acid than a pH of 7.
The pH of a soil is dependent on the parent material, the climate, the native vegetation, the cropping history (for agricultural soils), and the fertilizer or liming practices.
The pH range for most mineral soils would be from 5.5 to 7.5. This is also the range for most soils found in Minnesota.
The above map indicates that the eastern half of the state has mostly acid soils, while the western half has alkaline soils. Soils become acidic when precipitation leaches away basic cations (which provide the OH- ions). Eventually all the Ca++, Mg++ and other cations are are replaced by H+ ions.
Exchangeable hydrogen is the principal source of H+ until the pH of the soil goes below 6. Below 6, exchangeable aluminum becomes the source of hydrogen ions, due to the dissociation of Al from clay minerals. For simplicity, we will use the term "exchangeable H" for the cause of acid soils. See Chemical Reactions. for the equation with aluminum.
Soils tend to become acidic as a result of: (1) rainwater leaching away basic ions (calcium, magnesium, potassium and sodium); (2) carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid; (3) uptake of positive ions by plant roots and the resulting release of H+ by the root to balance internal charge; (4) formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids.
Sources of H+ ions in the soil :
1) dissociation of carbonic acid (H2CO3), which forms readily in soils when CO2 is present;--- H2CO3- ---> CO2 + H + +HCO3-
2) organic acids formed during the decomposition of organic matter;
3) the burning of coal in electrical power plants releases sulfur to the atmosphere which is added to soils during precipitation as sulfuric acid, and fertilizers containing sulfur, which adds H+ ;
4) the conversion of NH4+ to NO3- releases H+ during the nitrogen cycle or when nitrogen fertilizers are added to soils.
5) uptake of positive ions by plant roots and the resulting release of H+ by the root to balance internal charge
pH is < 4.0=indicates that the soil contains
free acids probably as a result of sulfide oxidation
pH is < 5.5=indicates that the soil's exchange complex is dominated by Al
pH is < 7.8=soil pH is controlled by a range of factors
pH is > 7.8=indicates that the soil contains CaCO3
Where leaching is minimal, the concentration of basic cations (Ca++, Mg++,
K+, and Na+) on the exchange complex will be large. These basic cations will
come from the weathering of rocks and minerals, from dust blown on soils, from
irrigation water or runoff water. When basic cations dissociate in the soil
solution, they will produce hydroxyl ions (OH-). This will raise the pH of the
soil.
The "pH of the soil" refers to the concentration of hydrogen ions
in the soil solution--not on the exchange
complex. We will discuss later how this will affect soil pH. See
Chemical Reactions.
One method to determine soil pH is by using a pH indicator dye. This picture shows the indicator dye pH kit called Poly D. It is easy to use and gives a suitable pH value for most soils. The indicator dye is added to the soil in the spot plate until it is saturated. The solution is stirred using a small spatula. The solution will change color depending on the soil pH. The solution color is compared to a color card that has been calibrated to various pH readings. (Be sure to clean the spot plates when you are through.)
Soil pH test kits you get from a garden center for under $15 may give variable results. It is always best to compare the kits with an electronic pH meter.
The most accurate determination can be made using a pH meter and glass electrode. The electrical conductance of the solution is measured using the meter. The conductance is correlated in the machine to pH values which are read directly.
Methodology: There are three main internationally accepted methods available for measuring soil pH. All of them rely on shaking (or stirring) soil with a solution for 1-2 hours and then determining the pH of the resultant soil slurry.
1. Weigh out 5 g of soil into labelled 50 ml plastic (polypropylene) tubes
2. Add one of the following 3 solutions
a)25 ml of de-ionized water. (This is the simplest method and normally OK for most soils. It doesn't remove H+ from the exchange sites and is not very good for soils with a high salt content)
or b) 25 ml of 1 M KCl (used to mask differences in soil's salt content). Useful if determining exchangeable cations as both cations and pH can be done on the same sample. It does displace H+ from the soil's cation exchange sites, so the results are usually slightly lower than obtained with methods (a) and (c).
or c) 25 mL of 0.01 M CaCl2. This is an intermediate between methods (a) and (c) and masks small differences in the soil's salt content.
3. Shake for 1 h at room temperature (25°C)
4. Let the soil settle for a few minutes (e.g. 3 min) and measure the pH after a two point (pH 4 and pH 7) calibration of the pH meter
5. Normally 2 replicates are performed for each soil sample
6. Field moist soil (store at 5°C) should preferably be used .
The sample you measure in the lab will have been pre-shaken and the meter calibrated. You will just need to measure the pH of the solution.
Descriptive terms commonly associated with certain
ranges in soil pH are:
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;
lime=12
Chapter 2 Cation Exchange Capacity
© Terence H. Cooper & Regents of the University of Minnesota, 2007. The University of Minnesota is an equal opportunity educator and employer.
