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.
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 is greater, 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 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) 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, which forms readily in soils when CO2 is present;
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.
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.
A more 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 distilled 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 high salt content) b) 25 ml of 1 M KCl (used to mask differences in soil's salt content). Useful if determining exchangable 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). 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
Cation Exchange Capacity (CEC) is the ability of the soil to hold onto nutrients and prevent them from leaching beyond the roots. The more cation exchange capacity a soil has, the more likely the soil will have a higher fertility level. When combined with other measures of soil fertility, CEC is a good indicator of soil quality and productivity.
The cation exchange capacity of a soil is simply a measure of the quantity of sites on soil surfaces that can retain positively charged ions by electrostatic forces. Cations retained electrostatically are easily exchangeable with other cations in the soil solution and are thus readily available for plant uptake. Thus, CEC is important for maintaining adequate quantities of plant available calcium (Ca++), magnesium (Mg++) and potassium (K+) in soils. Other cations include Al+++( when pH < 5.5) , Na+, and H+.
Cation Exchange Capacity can be expressed two ways:
1) the number of cation adsorption sites per unit weight of soil or,
2) the sum total of exchangeable cations that a soil can adsorb.
Soil CEC is normally expressed in units of charge per weight of soil. Two different, but numerically equivalent sets of units are used: meq/100 g (milliequivalents of element per 100 g of dry soil) or cmolc/kg (centimoles of charge per kilogram of dry soil).
The unit of milliequivalents (meq) per 100 g of oven dry soil is used to better reflect it is the charge in the soil that determines how many cations can be attracted. The equivalent weight of an element is the molecular or atomic wt (g) ÷ valence; or charges per formula milliequivalent (meq). One meq wt. of CEC has 6.02 x 10 20 adsorption sites. Cation exchange sites are found primarily on clay and organic matter (OM) surfaces.
| Element | K+ | Na+ | Ca++ | Mg++ |
| Valence | 1 | 1 | 2 | 2 |
| Atomic Wt. g | 39 | 23 | 40 | 24 |
| MEQ Wt. g | .039 | .023 | .020 | .012 |
Normal CEC ranges in soils would be from < 1 meq/100 g, for sandy soils low in OM, to >25 meq/100 g for soils high in certain types of clay or OM. Soil OM will develop a greater CEC at near-neutral pH than under acidic conditions. Additions of an organic material will likely increase a soil's CEC. Soil CEC may also decrease with time through acidification and OM decomposition.
Predicting CEC
CEC estimates can be made based on the texture and amount of organic matter of the soil.
1) Estimation based on texture:
| Texture | Sand | LS to SL | Loam | Clay Loam | Clay |
| CEC MEQ/100g | 0-3 | 3-10 | 10-15 | 10-30 | >30 |
2) Calculation of CEC with % clay and % OM. With this formula assume avg. CEC for % OM=200 meq/100g and the avg. CEC for % clay=50 meq/100g .
Therefore: CEC=(% OM x 200) + (% Clay x 50)
From soil data: soil with 2% (0.02) OM and 10% (0.1) Clay
CEC = (200 x .02 + (50 x .1)=(4 + 5) = 9 meq/100 g
