University of Dar es Salaam

CoAF
Department of Crop science and beekeeping technology

PRACTICAL SKILLS

DETERMINATION OF SOIL pH
Background:
SoilpH is the most important and informative measurement to characterize a soil (Thomas 1996).
It is defined as a negative logarithm to base ten of the hydrogen ions concentration. Soil pH has
been redefined in terms of hydrogen ion activity (i.e. Soil pH is the negative logarithm of
hydrogen ion activity). The term soil pH was invented by a Swedish scientist named Sorensen in
1909.The concept and hence definition of soil pH was derived from the ion product of water.
H2O → H+ + OH-
KW = [H+][OH-] = 1x10-14
At 25ºC, H+ and OH- in bracket are activities and when H ions and OH ion activities is equal,
each ion has an activity of (10-14)½ or 10-7.The pH is defined as the negative logarithm of
hydrogen ion activity.
pH = -log [H+]

Thus, the PH of pure water is –log of 1x10-7or 7. Any solution with PH7 is neutral, solution with
PH above 7 is a basic solution and below 7 is an acidic solution.
Method for measurement of Soil pH
Different methods can be used to determine soil pH.Common methods include:
i. Electrometric measurement
ii. Use of microelectrodes
iii. Colorimetric determination of pH
iv. Use of test Kit and
v. The use of pH paper
Choice of the method to use depend on facilities available in the laboratory and the level of
accuracy required. For the purpose of this class you will practice the electrometric measurement
of soil pH in water and 1M KCL or 0.01M CaCL 2 using the pH meter. Since soil pH and
electrical conductivity of the soil are closely related parameters you will also determine electrical
conductivity meter.
Tools and Reagents

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 a) pH meter equipped with glass and reference electrode or combination electrode
b) 50 or 100mL beaker
c) 10mLpipeteor automatic pipet
d) Standard buffers, pH 4 and pH 7
e) Deionized water ( for suspending the soil and rinsing electrodes between readings)
f) 0.01M CaCL2 solution
g) 1M KCL solution
Determination of pH in water
i. Calibrate the pH meter using standard buffer solutions pH 4.0 and pH 7 (two point
calibration) or pH 4.0, pH 7 and 10.0 ( three point calibration ) before pH determination
depending on the expected range of soil pH
ii. Weight out 10g of air dry soil in a 50 0r 100mL beaker
iii. Add 25mL of deionized water to the soil in a beaker and mix well by stirring for one
minutes using a stirring stick or for 30 minutes using a stirring machine (take care to
minimize contamination).
iv. Allow the suspension to stand for one hour after stirring stick or for 10 minutes if stirring
was done on stirring machine.
v. Swirl the suspension in a beaker and insert the electrode into the suspension or
supernatant solution. Read pH and record as pH w

Note: Remember to rinse the electrodes with distilled water between pHreadings to avoid cross-
contamination but blotting is not necessary.
Determination ofpH in 1:0.01CaCl2
The method is used to determine the activity of pH ions in a soil suspension in the presence of
0.01M CaCl2 so as to approximate constant ionic strength in the soils. Determination of pH in
0.01M CaCl2 minimizes interference from variable salt effects and from suspension effects.
Procedures:
i. Repaat the procedure (i-v) as outlined for determination of pH in water, but use
0.01M CaCl2 instead of water to make a suspension
ii. Repeat reading pH and record as pHCaCl2

2.3.3 Determination of pH in 1M KCL

i. Repeat the above procedure (i-v) as outlined for determination of pH in water, but use
1M KCL instead of water to make a suspension
ii. Repeat reading pH and record as pHKCL

Interpretation of Soil pH Values

The value of soil pH determined in water can be used for rough estimation of relative availability
of plant nutrients such as P, Zn, Cu, B, Mn, Fe, and Mo.

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 pH range pH rating General interpretation
<5.5 Low The soil acidic, High possibility of Al toxicity, excess availability of Co,
Cu, Fe, Mn and Zn while Ca, N, K, Mg, Mo, P and S (B if pH is <5.0)
are likely to be deficient.
5.5 – 7.0 Medium Preferred range for most crops but lower end of the range is too acidic
for some crops. Most nutrients available in this range.
7.0 – 8.5 High Availability of B and P will decrease to deficient levels, also liability of
Co, Cu, Fe, Mn and Zn deficiency increase.
>8.5 Very high Alkaline soils, possibility of limited availability of Ca and M. Na may
be high and B may be toxic.

Note:The pHw values can’t be used for adequate limiting recommendation because exchangeable
AL which is responsible for exchangeable soil acidity can’t be extracted by water. Determination
of total soil acidity therefore requires extraction of exchangeable AL using normal unbuffered
KCL followed by titration of the extract using standard base to obtain exchangeable acidity.

Soil pH values between 2 and 3 indicate presence of H+ ions but also that the source is
continuing (i.e. free acidic from pyritic minerals that from H 2SO4upon oxidation). This means
that
 Plant will not grow
 Soil minerals are slowly of rapidly being dissolved
 The cost to improve the soil by liming is high
Values of pH 4-5 indicate presence of exchangeable Al3+in mineral and sometime in organic
soils. It means trouble for most crops especially on plant root growth. The problem is not as
serious as with free H+ in the previous case. The problem can corrected with application of CaCo 3
to increase crop yield.
Values of pH 5.5 and above indicate presence of hydroxyl- Al ions, many of them highly
polymerized and completely nonexchangeable. Hydroxyl-Al compounds are well buffered and
do resist the effect of liming materials; they act as sink for H + and OH- and are major source pH-
dependent charge. At this pH range,
 Root growth is suppressed
 Phosphorus and potassium availability is affected.
The pH values 7.6 – 8.3 indicate presence of calcium carbonate in the soil thus, the soils are
calcareous. Soil conditions are good but there are some potential problems especially with Zn
and Fe. Ca is relatively abundant but as pH increases toward 9, CaCO 3become insoluble and
does not buffer the soil any more.
The pH values towards 9 indicate presence of sodium carbonate in the soil. At thispH, CaCO 3 is
longer a problem because it has precipitated and Na 2CO3 is dominant on the exchange complex
with serious consequences. Being a soluble salt, Na swamps the system, and presence of Na
causes dispersion of clay and organic matter, resulting into soils with poor structure called
“black alkali soils”.

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DETERMINATION OF ELECTRICAL CONDUCTIVITY (EC)
Principle
Electrical conductivity (EC) is the measure of the ability of an aqueous solution to carry
electrical current (i.e. Ionic transport in a solution between the anode and cathode). It is normally
considered to be a measurement of dissolved slats in a solution. Since EC depends on the number
of ions in the solution, it is important to know the soil/water ratio used. The higher the water/soil
ratio used is higher the risk that peptization, hydrolysis, cation exchange and mineral dissolution
occurs. In case of routine analysis of EC, 1:1, 1:5 or 1:10 water/soil suspension is normally used.
The EC of an aqueous salt solution increases with increase in temperature at about 2% for every
⁰C, thus EC readings needs to be referred to a standard temperature of 25 ⁰C by adjusting with
temperature correction factor.
Tools and Reagents
i. Conductance meter (EC mater)
ii. Thermometer
iii. Standard 0.001M KCL solution for calibration of the EC meter.
iv. Cleaning solution
Analytical procedure:
i. Calibrate the EC meter using a 0.01M KCL standard solution
ii. Weigh 10.0g of air dry soil using analytical balance into a 100mL plastic bottle
iii. Add 50mL of deionized water using a pipette
iv. Close the bottle and shake on the automatic shaker for 30 minutes or stir for 30 minutes
on an automatic stirrer or use a glass rod to stir the mixture periodically for 30 minutes.
v. Measure the temperature of the extract and set temperature compensation at this
temperature (The reading is then automatically corrected at25⁰C )
vi. Fill the cell with extract or insert/ dip the cell into extract and read the EC
vii. Rinse the cell with extract to be measured between measurements (use distilled water to
rinse the cell and dry with an air- jet if the extract is not enough).
Note: The unit of measurement differs from one EC meter to another depending on the
manufacturer. 1Ms/cm=1mmho/cm, 1cmmho/cm= 1000mmhos/cm and 1dS/m = 1mmhos/cm
Interpretation of EC readings:
EC range Interpretation
0 – 2 mS/cm Salt free soils/ negligible effects of salt on the crop
4 – 8 mS/cm Slightly saline soils, Yields of many crops are restricted at this EC level
8 -15 mS/cm Moderately saline soils, only salt tolerant crops will yield satisfactorily
>15 mS/cm Strongly saline soils, only very tolerant crops will yield satisfactorily

Visit Landon (1991) Booker Tropical Soil Manual: A hand book for soil survey and agricultural
land evaluation in the tropics and subtropics. (pp. 157 – 159 for detailed interpretation)

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DETERMINATION OF EXCHANGEABLE BASES AND CATION EXCHANGE
CAPACITY (CEC) OF THE SOIL
Principle
All soils are capable of retaining ions (cation and anion). These ions (especially cations) are held
in easily exchangeable form thus termed exchangeable ions. The most common exchangeable
cations include Ca2+, Mg2+, K+, and Na+ ; whereas HCO3-, Cl-, and SO42-are the most common
exchangeable anions. The total sum of exchangeable cations that a given soil can retain (adsorb)
is called cation exchange capacity (CEC) of the soil. Soil CEC gives an indication of clay
mineralogy of the soil and its capacity to retain nutrients against leaching losses. CEC is
determined by saturating cations adsorbed on the exchange complex and measuring the amount.
A. DETERMINATION OF EXCHANGEABLE BASES BY AMMONIUM ACETATE
SATURATION METHOD
Procedure
1. Weigh 5g air-dry soil in a 250mL extraction bottle. Add 50mLof 1N NH4O Ac solution
and shake for 30 minutes or more on a shaker. Alternatively you can allow the contents to
stand for 24 hours with occasional shaking by hand.
2. Filter the saturated soil on a Buhner funnel and wash the soil paste two times with
another 50mLof 1N NH4OAc (use 25mLof 1N NH4OAc each times)
3. Pipette 1mL aliquot, add 2mL of Lanthanum oxide followed by 7mL of distilled water
4. Determine the absorbance of the mixture prepared in step 3 by Atomic Absorption
Spectrophotometry method (for Ca2+ and Mg2+) or Emissions of the solution using flame
emission Spectrophotometry method (for Na+ and K+)
5. Prepare a standard curve (plot of concentration against absorbance / emissions) using a
series of concentrations of standard solution prepared from a reagent grade salt
containing each of the bases.
6. Use a standard curve prepared in step 5 translate absorbance/emission readings into
concentration units
Report your results in Cmol (+)/kg soil

B. DETERMINATION OF CATION EXCHANGE CAPACITY (CEC) BY MMONIUM

ACETATE SATURATION METHOD
Procedure
1. Weigh 5g air-dry soil in a 250mL extraction bottle. Add 50mL of 1N NH4O Ac solution
and shake for 30 minutes or more on a shaker. Alternatively you can allow the contents to
stand for 24 hours with occasional shaking by hand.
2. Filter the saturated soil on a Buhner funnel and wash the soil paste two times with
another 50mL of 1N NH4OAc (use 25mL of 1N NH4OAc each times)
3. Wash the soil pad (residue) on the Buchner funnel four times using 50mLportion of
methyl alcohol for every wash

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 4. Transfer the washed soil pad with filter into another plastic bottle. Add 40mL of 4% KCL
and shake on a mechanical shaker for 30 minutes
5. Filter the contents on the Buchner funnel; wash two times with 25mL portion of the KCL
solution. Total volume of the filter and KCl washings should not exceed 100 mL
6. Transfer the filtrate into 200mL Kjeldahl tubes and set on the distillation rack.
7. To the tube, add 50mL of 40% NaOH from the side arm (if using an automated
distillation unit 50mL of NaOH will be automatically added to the tube as programmed).
Distill the contents in the tube and receive the distillate in 50mL of boric acid- mixed
indicator solution in a 250mL Erlenmeyer flask. Continue the distillation until 2/3 of the
content is distilled
8. Titrate the distillate using standard 0.1N H2SO4 solution and record the volume used to
reach titration end point
9. Run a blank containing 100mL KCL solution only
10. Calculate CEC using the following equation:

CEC (meq./100g of soil) = mL H2SO4 x Normality of H2SO4 x 100

Oven dry weight of soil (g)
Report your analysis result resultsin Cmol kg-1 soil

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