Unit 12 Chapter 1 - Soil pH
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.
Definition of soil pH.
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 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.
Causes for Acid Soils
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.
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. 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.
Soil pH Determination
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 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)
or 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).
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.