Practical Manual

SOIL CHEMISTRY

APS-503, 3(2+1)

For
M.Sc. (Ag.) Soil Science

2023

Department of Soil Science

College of Agriculture
Rani Lakshmi Bai Central Agricultural University
Jhansi-284003
 Practical manual

SOIL CHEMISTRY

APS-503, 3(2+1)

M.Sc. (Ag.) Soil Science

DR. BHARAT LAL

DR. SUSHEEL KUMAR SINGH
DR. ARPIT SURYAWANSHI

Department of Soil Science

College of Agriculture
Rani Lakshmi Bai Central Agricultural University
Jhansi-284003
 CONTENT
Ex. No. Practical’s Page No Remarks/sign.

1 Preparation of saturation paste and saturation extract of 1-3

soil

2 Determination of soil pH by using pH meter 4-5

3 Determination of electrical conductivity of soil sample 6-7

4 Determination of carbonate (CO32-) and bicarbonate 8-9

(HCO3-) in soil

5 To determine the extractable calcium and magnesium in 10-11

soil- Versenate (EDTA) method

6 To estimate the available potassium in soil by neutral 12-13

normal ammonium acetate method (Hanway and Heidel,
1952)

7 Determination of exchangeable sodium in soil 14

8 Determination of cation exchange capacity of soil 15-17

9 Determination of AEC in soil 18-19

10 Measurement of redox potential (Eh) in soil 20-21

11 Field measurement of redox potential (Eh) 22-24

12 To determine zero point charge (ZPC) 25-26

13 Determination of variable charge 27-28

14 29-30
Determination of permanent charge

15 Determination of quantity – intensity (Q/I) relation of soil 31-34

potassium
16 Determination of total potential soil acidity by BaCl2 35-36
method
17 Determination of lime requirement of soil 37-38

18 Determination of gypsum requirement of soil 39-40

 Experiment No. 1
Objective: Preparation of saturation paste and saturation extract of soil

Principle
Under laboratory conditions, the soil salinity can be determined by the (i) electrical conductivity (EC, dS
m-1) of soil-water extracts, (ii) proportion and composition of salt species, or (iii) amount of total
dissolved solids (TDS, mg L-1). The most common technique for the measurement of soil salinity is the
laboratory analysis of aqueous extracts of soil samples. Ideally, the salinity of a soil solution measured
at field water- content (usually at field capacity) is the best index because it is the salinity actually
experienced by the plant root. But, it has not been widely used because (i) it varies throughout the
irrigation cycle as the soil water-content changes; and (ii) the methods for obtaining soil solution
samples at typical field water-contents are too laborious, time- consuming, and cost-intensive to be
practical. As a result, soil solution extraction is made at saturation or higher water-contents and the
most commonly used is the aqueous extracts of saturated soil paste.
The soil solution extracts of higher soil: water ratios (e.g. 1:1, 1:2 and 1:5 soil: water) have also been
used because of comparative ease in extraction vis-a-vis soil saturation paste extraction. The strong
linear relationships exist between the ECe and the EC 1:1 and EC 1:5 values (R 2 > 0.93), but the
relationships between extracts of higher soil: water ratios and saturation paste are often imprecise and
inaccurate. The soil: water ratio used for extraction influences the solute partitioning the gas, liquid, and
solid phases of soil. Therefore, the ratio needs to be standardized for a proper interpretion. The extracts
of higher soil: water ratios affect the solubility of less soluble salts such as gypsum (CaSO 4) or lime
(CaCO3). This is particularly true in the case of gypsiferrous soil where the concentrations of Ca 2+ and
SO42- remain nearly constant but that of other ions decreases with increased sample dilution.

Apparatus
• Plastic containers with lid (500-mL)
• Spatula
• Glass rod
• Vacuum-line extraction assembly consisting of a Buechner funnel, vacuum pump,
vacuum flask, filter paper (e.g., Whatman No. 1 and 5), glass tubes or bottles for collecting and
storing the saturation extract
• Electronic balance

Reagents
• 0.1% Sodium hexametaphosphate (NaPO 3)6 : Dissolve 0.1 g of (NaPO3)6 in distilled
water and make up the final volume to 100-mL.

Procedure
(a) Preparation of Saturation Paste
 Collect about 1 kg of homogeneously mixed representative soil sample.
 After air drying, grind the sample and pass through 2-mm sieve.
 Weigh about 500 g of air-dried soil of < 2.0 mm size in a plastic container fitted with a lid.

1
  Add distilled or deionized water in increments to moisten the soil and stir the sample with a
spatula.
 Continue to add incremental volumes of water while stirring the sample with a spatula until the
soil is water saturated.
 Allow the mixture to stand for about 2 h to permit the soil to imbibe the water and dissolve
readily-soluble salts. Add more water if required to obtain a uniformly saturated soil paste.
 Check the preparation of saturation paste using the following criteria. At saturation, (i) the soil
paste glistens, (ii) the saturated paste flows slightly when the container is tipped, (iii) the paste
slides smoothly off the spatula, and (iv) if a depression is made on the soil surface, no water
collects in it and the depression fills slowly by flow of the paste under gravity. It may be ensured
that there are no dry clumps of soil and the soil paste is homogeneous.
 Close the container with lid and allow the sample to stand (preferably overnight) in cooled
conditions, preferably at 4 °C. Next morning take out the saturated paste from the cold storage
and let the paste attain the room temperature.
 Recheck the paste for saturation criteria. Free water should not collect on soil surface, nor
should the paste stiffen markedly or loose its glisten. If the paste is too wet or stiffen, add
accordingly more dry soil or water to make soil saturation paste. Stir the soil paste with a
spatula to ensure that it is homogeneous.

(b) Extraction of Saturation Paste

 Transfer the saturated soil paste to a Buechner funnel fitted with a filter paper Whatman No. 1
and apply vacuum. The soil paste should evenly and completely cover the filter paper. As
moisture is drawn from the soil paste, cracks may develop in the soil cake. If cracks are not
checked in time, air passing through the cracks may prematurely dry the cake at the cracks or
air travelling through the extraction flask may evaporate the extract. To overcome it, the
extraction process should be monitored regularly and cracks if developed in the soil cake
should be smoothened immediately with the spatula.
 Collect the saturation extract sufficiently enough for analysis in a bottle or tube.
 Colouration of the extract imparted by the dissolved organic matter does not affect its quality for
the chemical analysis or conductance measurement. But, if the initial filtrate is turbid either
discard it or filter it again. In such cases, a higher retentive filter paper such as Whatman No. 5
may be used.

The samples containing a substantial amount of clay may require a longer time for extraction or need
the transfer of the unextracted soil paste to another extraction funnel because the soil paste at the
bottom of the soil cake dries, sealing the extract above from getting through.

Precautions
 The soil sample should not be oven-dried before extraction. Heating the soil at 105 °C may
convert a part of the gypsum (CaSO4.2H2O) to plaster of paris CaSO4.1/2H2O) which is more
soluble in water.

2
 Special precaution should be taken in the preparation of saturation paste of peat and muck
soils, very fine or very coarse textured soils. The peat and muck soils should not be dried
following collection as their saturation water content changes upon dryness.
 In the case of fine textured soils, enough water should be added immediately with minimum
mixing to bring the sample near saturation. This also minimizes the formation of clumps of soil
during stirring.
 The coarse textured soils should not be over-wetted. The presence of free water on the surface
of soil paste indicates over-wetting. Even a small amount of free water may cause appreciable
error in the saturation paste water content.
 One drop of 0.1% sodium hexametaphosphate (NaPO 3)6, solution per 25 mL of extract may be
added before storage to prevent precipitation of CaCO 3.
 A few drops of thymol (C10H14O) can be added to the paste to minimize microbial activity on the
saturated extract during equilibration.

3
 Experiment No. 2
Objective: Determination of soil pH by using pH meter.
The pH Measurement specifies the degree of relative acidity or alkalinity of an aqueous solution at a
given temperature. Theoretically, pH is derived from the word “Pondus Hydrogenii” which means
“Potential Hydrogen” or power of hydrogen ion, pH is represented in the form of an equation as the
negative logarithm of the hydrogen ion concentration. The pH value which is a measure of the
hydrogen (or hydroxyl) ion activity of the soil water system indicates whether the soil is acidic,
neutral or alkaline in reaction. The pH value of the solution surrounding the soil particles
fluctuates in the natural state because of changing soil: solution relationships brought about by
climate, cultivation, crop growth and other factors.

Working principle of pH meter: The pH Meter measures the voltage of an electro chemical cell and
based on the temperature sensor determines the pH of a solution. The overall potential or the voltage is
the algebraic sum of the potentials of the measuring electrode, reference electrode and the liquid
Junction. The reference electrode provides a stable voltage as it has a fixed concentration of potassium
chloride solution which is a neutral solution. On the contrary, the potential of the measuring electrode
depends only on the pH of the solution. The potential difference (voltage) between a glass membrane
of measuring electrode and a reference electrode which is dipped in the sample solution to be tested is
measured. When the two electrodes are dipped in the sample solution, ion-exchange process occurs,
where in some of the hydrogen ions move towards the outer surface of the measuring electrode and
replaces some of the metal ions inside it. The potential difference of glass electrode and sample
solutions will be recorded in the galvanometer.

Suspension effect in soil pH measurement

In the pH measurement the reference and indicator electrodes are immerse d in a hetero-
geneous soil suspension comprising of dispersed solid particles in an aqueous solution. If
the solid particles are allowed to settle down, the pH can be measured in the supernatant
liquid or in the sediment. Placement of electrode pair in the supernatant normally gives a
higher pH reading than placement of the same in the sediment. The difference in soil pH
reading is called the ‘Suspension effect’.In practice a measured quantity of soil is shaken
with a convenient volume of water or salt solution under consistent conditions and the pH of the
suspension is determined electronically on a direct-reading pH meter, using a glass electrode
with a saturated KCl–calomel reference electrode. Usually soil : water ratio of 1 : 2.5 or 1 : 1 is
used for routine analysis.

Reagents and Apparatus

 Standard buffer solutions
At least, two buffer solutions must be used one at low pH range and the other at high pH
range. The buffer solution most commonly used for low pH values is a 0.05 M solution of potas -
sium hydrogen pthalate which has a pH of 4.001 at 20°C. The pH of this buffer varies with
temperature from 4.000 at 15°C to 4.020 at 35°C. The reagent must be of highest purity and
the water used for its solutions should be double distilled water. The buffer can be stored in
well sealed Pyrex or polythene bottles but preferably should be freshly prepared every 2

4
 weeks. For high pH buffer solution a 0.01M solution of borax is convenient and at 20°C has
a pH of 9.22. This buffer if protected from atmospheric carbon dioxide can be kept for about a
month. Three to four drops of toluene addition prevents growth of mould.
Standard certified buffer tablets/capsule are also available for a whole range of pH-
values and these are most convenient. One tablet or capsule dissolved in a specified amount of
water, usually 100 cc provides a solution of known pH to within about 0.02 of a unit. Buffer
tablets or capsules of pH 4.0, 7.0 and 9.2 are mostly used now a days for instrument
calibration.

 Glass electrode pH meter with calomel reference electrode and salt bridge.
 50 ml or 100 ml beakers, short stirring rods and distilled water wash bottle.

Procedure
 Weigh 20 g soil sample in a 100 ml beaker and add 50 ml of distilled water ( soil: water
ratio of 1:2.5).
 Stir the suspension at regular intervals for 30 minutes.
 Measure the pH with the glass electrode stirring the suspension well just before
immersing the electrode.
 Switch on the pH meter at least 15 minutes before for allowing it to warm up and
standardize the glass electrode using standard buffers.
 Adjust the temperature compensation knob to the temperature of the test solution.
 Rinse the electrode with distilled water after each determination and remove water
from the surface with a piece of blotting paper.
 Check the standardization process after every ten determinations.
 Record the pH value of the samples
Applications of pH measurement
 pH measurement is very crucial parameters for soil evaluation.
 Major crops require pH 6.5 to 7.5 for their optimum growth.
 pH controls the availability of various nutrient in soil.
 It is also used in food industry especially for dairy products like cheese, curds, yogurts, etc.
 pH also identify the salt affected soil.
 pH level monitoring is essential in water quality for irrigation.
Classification of soil pH
Soil Class pH
Extremely acid <4.5
Very strongly acid 4.5 – 5.0
Strongly acid 5.1 – 5.5
Moderately acid 5.6 – 6.0
Slightly acid 6.1 - 6.5
Neutral 6.6 – 7.3
Slightly alkaline 7.4 – 7.8
Moderately alkaline 7.9 – 8.4
Strongly alkaline 8.5 – 9.0
Very strongly alkaline >9.0
5
 Experiment No. 3
Objective: Determination of electrical conductivity of soil sample
Principle

The conductivity of a soil is precisely the specific conductivity at 25°C of a water extract obtained at a
definite soil : water ratio. The electrical conductivity is measure on an electrical conductivity bridge and
is normally reported in mmhos cm–1. A fairly quantitative estimate of the soluble salt content of solutions
extracted from the soils can be made from their electrical conductance. Soil extracts obtained using
high water to soil ratios are as less accurate measure of the solute content of the soil since more salts
may be removed than are ever present in the soil, at field moisture contents. Usually soil : water ratio of
1 : 2.5 or 1.5 is used for routine measurement. Thus the soil: water ratio employed must be specified
with the analysis
The cell constant ‘k’ of a conductance cell is determined by measurement of the electrical conductance
‘C’ of a standard KCl solution usually 0.01M KCl solution and use of equation
k = L/C
where L is the known specific electrical conductance mmhos cm -1
C is the conductance of the standard solution measured in a given cell (mmhos).
The measured conductance ‘C’ of a test solution in mmhos multipled by the cell constant (k) gives the
specific conductance, L mmhos/cm of the test solution. i.e. L = kC
For soil classification purpose the conductivity of saturation extracts of soils is required. However,
extraction of solution from a saturated paste is very difficult process. As an approximation, the
conductivity of the water extracts from a 1: 2.5 soil:water suspension is determined and the conductivity
of the saturation extract is calculated as EC (saturation extract) = E.C (1 : 2.5 extract) × 250/saturation
percentage.
This does not hold good for soil containing gypsum for which saturation extract must be obtained.
Reagents and apparatus
 0.01M Potassium chloride prepared from AR grade salt and double distilled water [0.74569 g
KCl in 1000 ml or prepare fresh from a stock of 0.2 (M) KCl solution (14.92 g of the salt in 1000
ml)].
 Conductivity bridge with dip type or pipette type conductivity cells.
 Beakers – 50 ml or 100 ml.
Procedure
 Weigh 20 g air dry soil, into a 100 ml beaker and add 50 ml distilled water.
 Stir at regular intervals for 1 hour.
 Allow to settle for 30 minutes and filter the supernatant through a dry Whatman No. 42 filter
paper into a dry beaker.
 Measure the temperature of the soil extract, for future correction.
 Measure the conductivity of the extract with the conductivity bridge. The specific conductivity is
obtained by multiplying the electrical conductance with the cell constant. To obtain the
conductivity at the temperature of the extract multiply by the appropriate correction factor (ft)

6
 obtained from below table.
Temperature factors for correcting conductivity data on soil extracts to 25 °C
Temperature Correction Temperature Correction Temperature Correction
°C factor °C factor °C factor
10 1.411 18 1.163 26 0.979
11 1.375 19 1.130 27 0.960
12 1.341 20 1.112 28 0.943
13 1.309 21 1.087 29 0.925
14 1.277 22 1.064 30 0.907
15 1.247 23 1.043 31 0.890
16 1.218 24 1.020 32 0.873
17 1.189 25 1.000 33 0.858

Calculations

Electrical conductivity of the extract at 25°C or (Lmmhos/cm) =

conductivity of the extract as measured (mmhos) × cell constant (cm –1) × temperature correction factor
% of salts in soil = 0.064 x Lmmhos/cm x
For (10 g soil + 25 ml water or 20 g soil + 50 ml water) i.e. 1:2.5 soil : water ratio the above expression

will be, 0.064 x Lmmhos/cm x

= 0.064 x Lmmhos/cm x
= 0.064 x 2.5 x Lmmhos/cm
= 0.16 x Lmmhos/cm
Or 0.32 x Lmmhos/cm for 1:5 soil: solution ratio

7
 Experiment No. 4
Objective: Determination of carbonate (CO32-) and bicarbonate (HCO3-) in soil.

Principle:
When phenolphthalein is used as an indicator, strong alkalis like KOH or NaOH are completely
neutralized whereas weak alkalis like Na 2CO3 or K2CO3 are neutralized to the stage of NaHCO 3 or
KHCO3 according to the equation
Na2CO3 + H2SO4 → NaHSO4 + NaHCO3
The NaHCO3 thus formed requires more H2SO4 to get completely neutralized according to the equation
2NaHCO3 + H2SO4 → Na2SO4 + 2CO2 + 2H2O
It is evident from the above equations that the quantity of H 2SO4 required in both the stages of
neutralization of Na2CO3 is the same. The second stage of neutralization of Na2CO3 (i.e. the
neutralization of NaHCO3) can be indicated by methyl orange which can also indicate complete
neutralization of alkali carbonate or bicarbonate. Thus phenolphthalein and methyl red are used one
after the other during the course of titration in the same solution for evaluating mixtures containing
carbonates and bicarbonates. Methyl orange when used jointly with phenolphthalein after the latter has
decolorized indicates the quantity of acid required for the neutralization of the bicarbonate only.
Reagents
 Phenolpthalein indicator; 0.25% solution in 60% ethylalcohol.
 Methyl orange indicator; 0.5% solution in 95% alcohol.
 Standard H2SO4 ; 0.01 (N)
Procedure
 Weigh 40 g of soil sample in a 500 ml conical flask.
 Add 200 ml double distilled water and shake for one hour in a shaking machine for
equilibration.
 Filter the suspension.
 Pipette out 5 ml of the extract or 5 ml of water sample (containing not more than 1 meq. of
CO32- plus HCO3-) in a porcelain dish and add 2-3 drops of phenolphthalein indicator. Titrate
against 0.01(N)H2SO4 until the pink colour just disappears (indicating phenolphthalein end
point). This end point corresponds to the neutralization of the carbonate to the bicarbonate
stage.
 Record the ml of 0.01(N) H2SO4 required for this process from the burette reading.
 Add 1-2 drops of methyl orange indicator to the colourless solution.
 Titrate it again with 0.01(N) H 2SO4 stirring briskly, until the indicator turns orange indicating
complete neutralization of the bicarbonate present.
 Note the titre value from the burette.

8
Calculations
Weight of soil taken = 40 g
Volume of water added = 200 ml
Let volume of aliquot taken from soil extract or water sample be V ml.
Volume of 0.1 (N) H2SO4 required for the first titration (with phenolphthalein) = t1 ml.
Total volume of H2SO4 required = t2 ml
(phenolphthalein plus methyl red)
Normality of H2SO4 used = 0.01 (N) or N1 (say)
Therefore meq. of H2SO4 used in the first titration = N1 × t1
meq. of H2SO4 used (total) in the successive titration = N1 × t2
Hence meq. of CO32- per 100 g of soil

= (N1 x t 1) x

and mg of CO32- per 100 g soil

= (N1 x t 1) x

Likewise, meq of HCO3- per 100 g soil

= [(t2 – t1) x N1] x

and mg of HCO3- per 100 g soil

= [(t2 – t1) x N1] x x 61

Note: 1 ml of 0.01 N H2SO4 = (0.01 meq H2SO4) = 0.00030 g CO32- = 0.00061 g HCO3-]
Also meq. of CO32- per litre of soil extract or water sample
= (N1 x t 1) x
and, meq of HCO3- per litre of soil extract or water sample

= [(t2 – t1) x N1] x

9
 Experiment No. 5
Objective: To determine the extractable calcium and magnesium in soil- Versenate
(EDTA) method
The Ca in solution is titrated with 0.01N EDTA using first the indicator ammonium purpurate (murexide)
which at the pH of 12.0 change color from red to purple at the end point of titration. Ca + Mg in solution
can be titrated with 0.01N EDTA using Erichrome Black T as indicator at pH 10.0 in the presence of
Ammonium chloride-ammonium hydroxide buffer. At the end point color changes from wine red to blue
or green. The Mg content is determined by the difference.
Reagents required
For Ca determination
 0.01N Versenate (EDTA) solution: Dissolve 2.0 g of EDTA in distilled water and make the
volume in to 1 liter volumetric flask.
 Sodium hydroxide (NaOH) (pH 10): 10 g of AR grade NaOH is dissolved in 90 mL of distilled
water.
 Ammonium purpurate (murexide) indicator: 0.2 g of murexide is mixed with 40 g of powered
potassium sulphate (K2SO4).

For Ca + Mg determination
 0.01N Versenate (EDTA) solution: Dissolve 2.0 g of EDTA in distilled water and make the
volume in to 1 liter volumetric flask.
 Ammonium chloride-ammonium hydroxide buffer (NH4CI-NH4OH) (pH 10): Dissolve 67.5 g
pure Ammonium chloride in 570 mL on conc. Ammonium hydroxide and made to 1 liter and
adjusted to pH 10.
 Eriochrome black T indicator (EBT): Dissolve 0.5 g of EBT (Solochrome black) and 4.5 g of
hydroxylamine hydrochloride AR grade in 100 mL of 95% ethyle alcohol.

Procedure
Ca determination
 Weight 10 g of soil sample in 150 mL conical flask, then add 50 mL of 1 N ammonium acetate
solution and shake for 5 minutes, Filter through Whatman no. 1 filter paper.
 Pipette out 5 mL of soil extract in a porcelain dish (8 cm dia). Add about 25 mL distilled water to
it.
 Add 5 mL of NaOH solution and then 50 g of murexide indicator, stirred well and the solution is
titrated with standard 0.01 N EDTA till the color change from red to purple. The volume of
EDTA used is noted.

Ca + Mg determination
 Same as Ca determination pipette out 5 mL of soil extract in a porcelain dish (8cm dia). Add
about 25 mL distilled water to it.

10
  Add 1 mL of NH4CI-NH4OH buffer solution and then 3-5 drop of EBT indicator, stirred well and
the solution is titrated with standard 0.01 N EDTA till the color change from wine red to blue or
green. The volume of EDTA used is noted.

Mg determination
• The Mg content of the soil is determined from the difference between the contents of Ca + Mg
and Ca.

Calculations:

Ca (me/L) =

Ca (ppm) = Ca (me/L) X equivalent wt. of Ca (20)

Ca+ Mg (me/L) =

Ca+Mg (ppm) = Ca +Mg(me/L) X equivalent wt. of Ca+Mg (32)

Mg (me/L) = Ca+ Mg (me/L) – Ca (me/L)
Mg (ppm) = Ca+ Mg (ppm) – Ca (ppm)

11
 Experiment No. 6

Objective: To estimate the available potassium in soil by neutral normal

ammonium acetate method (Hanway and Heidel, 1952)
Principle

The readily exchangeable plus water soluble K+ is determined in the neutral normal ammonium acetate
extract of the soil. The NH4+ ion provides a sharp and quick separation from the exchange sites while
other cations bring about a gradual replacement of either more or less amount of potassium which
normally increases with the period of contact. Since, NH 4+ holds highly charged layers together just as
K, the release of non-exchangeable K to exchangeable form is retarded during NH 4OAc extraction
[Ammonium ions undergoes equilibrium fixation in the 2 : 1 layer silicates, particularly in the highly
charged vermiculite interlayer spaces, in exactly the same way as K +, by closure of the interlayer space.
The ammonium ions thus fixed undergoes only slow exchange and is reluctant to nitrify, Na+ ions best
replaces NH4+ and K+ from slow exchange position]. Non-exchangeable K also has been found to
contribute appreciably towards potassium availability to crops. The commonly used extractant for such
purposes involve hot 1(N)HCl and boiling 1(N)HNO3. The procedure for determination involves either
prior removal of exchangeable K+ or conditions made sufficiently vigorous to extract both exchangeable
(including water soluble) and a portion of non-exchangeable forms from which the former is subtracted.

Reagents required:
 Neutral normal ammonium acetate solution; Dilute 60 ml glacial acetic acid (99.5%) and 75 ml
concentrated ammonia solution (sp. gr. 0.91, 25% NH 3) to one litre. Mix well, cool and adjust
the pH to 7.0 with dilute acetic acid or ammonia solution.
 Potassium chloride solution : 1000 ppm stock solution; Dissolve 1.907 g of AR grade potassium
chloride (dried at 60°C for 1 hr.) in distilled water and make up the volume to 1 litre.

Procedure for potassium determination in soil

• Weigh 5 g soil sample in a 25 ml conical flask.
• Add 25 ml of neutral normal ammonium acetate (pH = 7) and shake for 25 minutes.
• Filter immediately through a dry filter paper (Whatman No.1).
• Reject first few ml of the filtrate.
• Determine the potassium concentration in the extract flame-photometrically after
necessary setting and calibration of the instrument.
Standard curve for potassium
• From the mother stock solution (1000 ppm K), prepare 2, 5, 10, 15 and 20 ppm K
solutions in 50 ml volumetric flask by proper dilution.
• Construct the standard curve by plotting the flamephotometer readings along Y-axis
and the different concentrations (ppm) along X-axis.

12
Calculations

Available K (Kg /ha) = R – B × = R

Where,
R = ppm of K in the extract, obtained from the standard curve (soil sample reading).
B = reading of blank sample (without soil)

Interpretations
Low Potassium Medium Potassium High Potassium

< 135 Kg/ha 135 - 335 Kg/ha > 335 Kg/ha

13
 Experiment No. 7
Objective: Determination of exchangeable sodium in soil

Principle:
Sodium is readily excited in a flame producing an intense yellow light, the yellow colour is primarily due
to radiation of 589.6 millimicron wavelength popularly known as D-line of sodium. Other less powerful
radiations of different wavelength emitted are effectively blocked by a suitable yellow glass (Na-filter)
allowing only the D-line emission to pass through. Thus, if a solution containing sodium ions is fed as a
fine spray into a flame under controlled and standard instrumental conditions and the emitted light is
passed through a Na-filter, the intensity of the D-line emission can be easily measured photoelectrically
and related to the concentration of the sodium in the test solution. The flamephotometer is calibrated
with a series of standard sodium chloride solution and then used to determine the unknown sodium
concentration of the solution under analysis within the same range.
Procedure
● Analyze directly the ammonium acetate extract for Na+ as same the K+ in the flame photometer.
Standard curve for sodium
 Dissolve accurately weighed 2.542 g NaCl in distilled water and make up the volume to one
litre. This gives 1000 ppm stock solution of Na +.
 From this prepare 1, 2, 3, 4, 5, 6, 7 and 10 ppm Na + by proper dilution.
 Adjust the gas and air pressures of the flamephotometer as per direction given in operation
manual and set to appropriate filter.
 Adjust the flamephotometer reading to zero with blank (0 ppm) and 100 for the maximum (10
ppm).
 Construct the standard curve by plotting the flamephotometer reading along x-axis and
concentrations along y-axis.
 Draw a mean line passing through the origin.
 From this graph obtain the sodium concentration of the sample under analysis in
milliequivalents per litre or in ppm. If there is a dilution of the original sample, multiply by the
dilution factor.
 Check the performance of the flamephotometer at frequent intervals by spraying some
standard solutions and adjusting the sensitivity as necessary.

Calculations

Exchangeable Na+ (ppm) = R x

where R = ppm of Na in the extract as obtained from the standard curve. R must include any dilution
factor, if used.
also, 1 meq./l Na = 23 ppm Na

14
 Experiment No. 8

Objective: Determination of cation exchange capacity of soil.

Principle
When a sample of soil is placed in a solution of a salt, such as ammonium acetate, ammonium ions are
adsorbed by the soil and an equivalent amount of cations is displaced from the soil into the solution.
This reaction is termed as ‘cation exchange’, and the cations displaced from the soil are referred to as
‘exchangeable’. The surface-active constituents of soils that have cation-exchange properties are
collectively termed as ‘exchange complex’ and consists for the most part of various clay minerals and
organic matter. Soil mineral and organic colloidal particles have negative valence charges that holds
dissociable cations and are thus called ‘colloidal electrolytes. The cation exchange capacity
determination involves measuring the total quantity of negative charges per unit weight of the material.
Stated otherwise the total amount of exchangeable cations that a soil can retain is designated as the
cation exchange capacity and is usually expressed as milliequivalents per 100 g soil or [cmol(p+)kg –1].
The determination of CEC is of fundamental importance in soil chemistry research. Adsorption,
desorption and leaching of fertilizers, thermodynamic study of ion exchange; retention and release of
nutrients, agrochemicals, soil pollutants, all depends upon the exchange capacity of the soil; CEC is
also found to be an important parameter for soil classification. The cation exchange capacity is usually
measured by leaching the soil or colloid with neutral normal ammonium acetate. Then the excess salt is
removed by washing with 95% ethanol. The ammonium ion (NH 4+) is then determined by steam
distillation with magnesium oxide in an alkaline medium. The ammonia evolved is adsorbed into a
known quantity of the standard acid containing methyl red indicator and the excess acid back titrated
with a standard alkali.
Reagents required:
 1(N) NH4OAc adjusted to pH = 7; Dilute 60 ml glacial acetic acid (99.5%) and 75 ml
concentrated ammonia solution (sp.gr.0.91, 25% NH3) to 1 litre. Mix well, cool and adjust the
pH of the solution to 7.0 with dilute acetic acid or ammonia solution. Alternatively, weigh 77.08
g NH4OAc and dissolve in one litre distilled water and adjust the pH to 7 carefully with dilute
acetic acid or ammonia solution.
 Ethanol 60%
 Ammonium chloride (AR)
 Magnesium oxide-carbonate free, freshly ignited (ignite at 650°C for 2 hours and cool in a
desiccator over KOH pellets, store in tightly stoppered bottle)
 Standard H2SO4 ; 0.1 (N)
 Standard NaOH ; 0.1 (N)
 Standard oxalic acid – 0.1 (N)
 Methyl red indicator
 NaOH; 45%
 Silver nitrate solution about 0.1 (M) : Dissolve 8.5 g AgNO 3 in 500 ml water. Add 2 ml
concentrated HNO3 and mix well.

15
Procedure
 Transfer without loss 10 g of air dry soil sample accurately weighed in a 250 ml beaker and add
50 ml of neutral normal ammonium acetate solution.
 Stir occasionally for an hour cover with watch glass and leave overnight.
 Filter the contents through Whatman No. 44 filter paper receiving the filtrate in a 250 ml
volumetric flask.
 Transfer the soil completely on to the filter paper and continue to leach the soil with 1(N)
NH4OAc (using 20 ml at a time), allowing the leachate to drain out completely before adding a
fresh aliquot.
 Continue the process, until the flask is full to the mark.
 Preserve this for estimation of exchangeable bases (Na +, K+, Ca++ and Mg++). The recidue left
on the filter paper is intended for determination of cation exchange capacity of the soils.
 Wash the recidue left on the filter paper with 60% alcohol to remove excess ammonium
acetate. To ensure this add a pinch of solid NH 4Cl to the recidue on the filter paper and wash
with alcohol till the filtrate is free from chloride (as tested with silver nitrate solution, the filtrate
is perfectly clear when free from chloride). If the washing is to be interrupted such as for the
night, attach a rubber tube to the tail of the funnel and pinch it tight with a clip when there is
solution above the level of soil in the filter paper. i.e. in no case the soil should dry otherwise
loss of ammonia may occur.
 Remove the soil with the filter paper into a 800 ml distillation flask and add about 200 ml of
water and about 3 g MgO (one spoonful approximately).
 Add few glass beads and little liquid paraffin so as to avoid bumping and frothing during
distillation.
 Pour 100 ml of 45% sodium hydroxide and immediately connect the distillation flask to the
condenser and distill ammonia in a known excess of 0.1(N) H 2SO4 (say 25 ml) to which a few
drops of methyl red indicator is added. (Continue distillation to collect about 150 ml distillate).
 Back titrate the excess of acid with 0.1(N) NaOH. Standardize NaOH versus oxalic acid and
H2SO4 versus standard NaOH.
 Perform a blank distillation without the soil on a similar volume of liquid.
Calculations
CEC is normally expressed in milliequivalents of the cation per 100 g soil, presently as c mol/(pt) kg –1.
Milliequivalent means the equivalent weight expressed in milligrams. For instance 20 g of Ca 2+
represents 1 equivalent or 1000 milliequavalent Ca 2+. Likewise 18 mg NH4 + would represent 1
milliequivalent (meq) of NH4+.

Since 1000 ml of 1(N) acid or alkali = 1.0 g equivalent of any cation.

It follows that 1000 ml of 1(N) acid or alkali = 1000 milliequivalents of any cation.
Therefore, 1 ml 1(N) acid or alkali = 1 milliequivalents of any cation

Cation exchange capacity =

16
Where,
V1 = ml of standard acid taken initially for ammonia absorption

N1 = normality of standard acid

V2 = ml of standard base used in back titrating of excess acid
N2 = normality of standard base
w = weight of sample in g

17
 Experiment No. 9

Objective: Determination of AEC in soil

Anion exchange capacity (AEC) is defined as the quantity of phosphate bound at pH 4 or 5.7. Many
anions are often involved in anion exchange reactions viz. PO 43-, SO42-, NO3- , Cl– etc. However,
phosphate is usually very suitable for AEC estimation. Under low pH and high concentration, anions
may be adsorbed and exchanged on soil colloids. The adsorption usually occurs on surfaces having a
positive charge, viz. iron and aluminium hydroxides. The anion retention is related to the nature of
anions and that of the soil surface together with amphoteric properties of organic colloids as well as iron
and aluminium hydroxides. In highly acidic soil conditions phosphorus acid anions are retained directly
on the surface of colloidal particles from the soil solution by adsorption phenomena. The mechanism of
anion exchange may be illustrated as follows:
By the addition of a proton (H+ ion) to the –OH group linked to a sesquioxide clay particle (R) i.e. Al 2O3,
Fe2O3, etc.
R – OH + HOH → R – OH2OH

R – OH + HCl → R – OH2Cl
By the addition of a proton to the functional groups of the organic fraction in acid soil
R – COOH + H+ → R – COOH + + Cl– → RCOOH Cl
The study of AEC of soils helps in understanding the retention and release mechanism of important
plant nutrient anions, viz. sulphate, phosphate, nitrate particularly in light textured soils of humid tropics.

Principle
The method involves initial leaching of the soil with a solution of barium chloride- triethanolamine
buffered at pH 8.1, followed by calcium saturation. The Ca-saturated soil is equilibrated with standard
phosphoric acid solution and the quantity of phosphorus adsorbed is evaluated. From this adsorbed
phosphorus plus phosphorus extracted initially the AEC of the soil is calculated using the formula.

AEC (meq./100 g soil) = [(extractable P + adsorbed P)] expressed as meq./100 g soil.

Reagents required:
 Calcium chloride solution; Dissolve 50 g CaCl2.2H2O in 100 ml of distilled water and adjust to
pH = 8.0 with saturated Ca(OH)2 solution.
 Triethanolamine solution; Dilute 90ml of triethanolamine to 100ml and adjust the pH to 8.1 with
HCl. Dilute to 200ml and mix equal volume of distilled water containing 100g of BaCl 2.2H2O.
 Phosphoric acid solution [0.01 (M) in H3PO4]
 Bray’s (I) Reagent for P extraction [0.025(N) NH 4F in 0.03 (N) HCl].
 Dikman and Bray’s reagent for colour development KH 2PO4. stock solution of P for standard
curve construction.
 Ethanol – 95%

18
Procedure
 Weigh 10 g soil and leach with 100 ml of triethanolamine and wash 6 times with 95% ethanol.
 Leach the soil with 100 ml of CaCl2 solution and wash again.
 Dry the calcium saturated soil at 45°C and weigh into a centrifuge tube sufficient to give a CEC
of about 0.2 meq.
 Add 20ml phosphoric acid solution and shake for half an hour and let stand for 24 hours. Again
shake for half an hour. Centrifuge and take 1 ml aliquot for P-estimation.
 In a separate soil sample, extract ‘P’ with Bray’s reagent and determine ‘P’ colorimetrically
using chloromolybdic acid reagent.
Calculations
Weight of soil taken = 10 g
Volume of phosphoric acid solution added = 20 ml.
Volume of aliquot taken = 1 ml.
Let this 1 ml is made upto V ml. and concentration of P from standard curve = C ppm.
Hence first dilution =
Second dilution =
Total dilution = 2V times
Therefore, concentration of P in solution phase = (2 x C x V ) ppm
Thus P adsorbed = [ P added (ppm) – 2 C.V] = X ppm =

Also , extractable P = Y ppm =

So , AEC (meq/100 g soil ) =

19
 Experiment No. 10

Objective: Measurement of redox potential (Eh) in soil

The reduction-oxidation status of a soil (redox potential) is used to characterize the degree of reduction
or oxidation which is mainly related to the biological processes occurring in flooded soils under reduced
conditions. The redox potential decreases as a consequence of soil flooding, followed by
disappearance of soil O2. The aerated soils have redox potentials in the range of +400 to +700 mV.
However, the seasonally flooded soils have a wider range of Eh as a result of prevalence of both
oxidized (+400 to +700 mV) and reduced (as low as -250 to -300 mV) systems. The narrow range and
poor reproducibility of Eh in well-drained (aerated) soils limit their practical applicability for
characterizing aeration. The poor reproducibility is caused primarily by the lack of poising of the
reduction-oxidation systems dominated by O2 . The measurement of redox potential is mostly done for
the flooded soils only.

Principle
The Eh is measured by using platinum electrodes, as platinum is chemically inert but is an electrically
conducting material. When a platinum electrode is introduced in the system, it registers the electron
potential of the system which is measured in terms of voltage, produced across the cell, as a result of
introducing a standard electrode of known potential in the soil-water system, nearby the platinum
electrode. The voltage recorded in the pH meter is known as the redox potential. The electrode
potentials are converted to the soil Eh by addition of 246 mV to account for the standard calomel
electrode, relative to the standard hydrogen electrode. The redox potential for the reaction is given by:

Oxidized state + e- → Reduced state

At equilibrium, it is governed by the activity ratio of the reduced (Red) and oxidized (OX) couples,
according to following (Nernst equation):

Eh = Eo +

where, Eo is the standard electrode potential; n is the number of electrons involved in the reaction; R is
the gas constant; T is the absolute temperature and F is the Faraday constant; RT/F = 0.0593 at 25 °C.
If the activities of the oxidized and the reduced species are unity, the ratio becomes unity and
consequently, Eh = Eo. Therefore, the standard redox (electrode) potential is defined as the Eh of the
system at which the activities of the oxidized and the reduced species are unity.

Apparatus
 Platinum electrode,
 pH meter with a saturated calomel electrode,
 Beaker.

20
Reagents required:
(i) pH 4.0 buffer solution of quinhydron in 0.1 M potassium hydrogen phthalate;
(ii) 0.01 M CaCl2, solution;
(iii) 0.01 N HCl;
(iv) 10% aqua regia.

Procedure
 Attach a Pt-electrode to the glass electrode (plus terminal) of a pH meter having a millivolt
scale and a saturated calomel electrode is connected to the negative terminal (to make a Pt-
electrode, fuse both ends of the length of a Pt-wire into a glass tube, so that a loop extends
beyond 3-4 cm).
 Add a few drops of mercury (Hg) inside the tube to make the electrical contact between the
wire and the glass.
 Rinse the Pt-electrode, before each reading (not the reference electrode) in a 0.01 N HCI,
followed by 10% H2O2 and then rinse thoroughly with distilled water. Clean in aqua regia after
a few hours of use.
 Adjust the potentiometer to read +210 mV when the electrodes are dipped in a buffer solution
of pH 4.0.
 Add 30 mL of 0.01 M CaCl2, to 10 g soil in a beaker. Stir until the soil and the solution are well
mixed. Let it stand for 20 to 30 min with occasional swirling.
 Insert the electrode so that the reference electrode is in the upper half of the supernatant
solution and the Pt-electrode is near the bottom of the suspension.
 Swirl for a few seconds, let it stand for at least 5 min without touching the cup, read · Eo, in
mV.
 Measure the pH of the same suspension.
Observations
Sample No.

 Observed redox potential, E (mV), of suspension:

 pH of the suspension.
 Eh (mV)
Calculations
Eh= (Eo+246) + 60 (R-7)
where, Eh, is the value of the redox potential at pH = 7.0; Eo, is the observed potential in mV; 246 is
the constant potential of the calomel half cell; 60 is the mV increase per pH at 25 °C and R is the
pH of water sample or the suspension.

21
 Experiment No. 11

Objective: Field measurement of redox potential (Eh)

The field measurements of Eh are widely used to estimate the intensity of oxidation or reduction of
soils. This method is semi-quantitative but is used for predicting a number of important biogeochemical
processes occurring in the soil such as activities of aerobic microorganisms at more oxidizing end to
predictions of sulphate reduction and methane formation on the reducing end of the redox scale. The
status of N, Mn and Fe systems can also be predicted from the properly measured redox potentials and
pH values. The field measurements of redox potential can detect the aerobic and anaerobic conditions
more rapidly than by any other measurement.

Procedure
The redox potential measurements are made in the field using a portable pH/millivolt meter and a
saturated calomel or silver/silver-chloride reference electrode. The method is as under:

 Push the reference electrode into the wet or moist soil to a short distance to ensure a good
electrical contact.
 If the soil is relatively dry, break up a small volume of soil with a knife and add some water to
form a paste and then install the reference electrode into it so as to provide a good contact with
the soil solution.
 If the soil is dry or highly weathered, a dilute salt solution (i.e. 5 g KCl in 100 mL H2O) can be
used to moisten the reference electrode hole and prevent the establishment of a junction
potential between the reference electrode and the soil.
 Although the position of reference electrode in respect to the redox electrode may not be so
critical, it is recommended to place the reference electrode within 2 m of the surface position of
the redox electrode.
 Some reference electrodes may be submerged without problems, but it is preferred not to
submerge the entire reference electrode where water is deep.
 If water is deeper than the length of the reference electrode, submerge only the working end of
the reference electrode in the flood water.
 If the flooding is periodic, immerse the above-ground electrode lead, a thin film of salt or clay
may coat the entire electrode lead after the water is receded. This coating may provide an
additional electrical contact between the alligator clip attached to the top of the electrode and
the soil surface, bypassing the platinum wire. To overcome this problem, the above-ground
insulation on the electrode lead should be cleaned with a moist tissue and dried. It is very
important to have a good electrical contact between the alligator clip and lead of the electrode.
To ensure proper contact, gently scrap the exposed copper lead with a knife.
 A drift in the meter reading may be recorded when it is first connected to the electrodes in the
field. If the electrodes are functioning properly, the rate of drift decreases rapidly within a
couple of minutes. Now, the drift rate is low enough to record Ec reading (Ec is the direct meter
reading using a calomel reference electrode).
 Well-poised soil systems tend to give minimal drift. This is typical of many reduced soils. The
oxidized soils and surface waters tend to be poorly poised and often exhibit considerable drift
for longer periods.
22
  Sometimes, it is necessary to wait for this drift to slow to a low rate before recording a value
from the meter and often it is not necessary to wait. For example, if a meter reading is drifting
upward after the Eh (corrected Ec reading) is above 450 mV, the system is clearly oxidized
regarding all redox components of interest and likely to contain dissolved O 2 in the soil solution.
Thus, for interpretation purposes, it is not necessary to wait longer for the drift to stop.
 If the Eh reading is in the range of +100 mV and drifting upward, or -100 mV and drifting
downward, it is advisable to wait for the drift to attain a lower level before recording a reading.
This is necessary as several redox active components (Fe, Mn, NO 3) are transformed in the
redox range of +100 to +350 mV. Thus, it is necessary to know the soil redox potential more
precisely.
 Likewise, if a downward drift is noted from around -100 mV, it is useful to know if a soil is
sufficiently reducing (about -150 mV) for reduction of sulphate (SO 42-) to sulphide. If the
electrodes are temporarily installed, it is adviseable to leave them overnight before taking a
reading.
 Generally, electrochemists use a standard hydrogen electrode (SHE) as the reference
electrode for the measurements of electrode potential. In an electrochemical cell, the
contribution of SHE has been defined as zero millivolts.
 For field or routine laboratory work, the use of SHE is not convenient. The reference electrodes
such as the calomel or silver/silver-chloride are commonly used.
 To convert a redox potential meter reading using a calomel reference electrode filled with a
saturated KCl solution (Ec) to a value that would be obtained had the SHE been used as a
reference electrode (Eh), the meter readings are adjusted by adding 245 mV to the Ec value.
 If a silver/silver-chloride reference electrode is used, the correction added to obtain an Eh value
would be +199 mV. Though the correction factor is temperature dependent, corrections for field
temperatures are generally not made as the error involved from this source is relatively small
compared to other inherent errors in the system.

Interpretation
Redox potential (mV) (Corrected to pH 7.0) Status of soil
-300 to -100 Highly reduced
-100 to + 100 Reduced
+100 to + 400 Moderately reduced
+400 to +700 Oxidized

If Eh=0 mV, it means that the system is:

(a) Devoid of O2, and NO3-,
(b) Fe and Mn compounds are in a reduced state, and
(c) Sulphate is stable with no sulphide injury and methane production.

23
Precautions
 The reduced conditions prevail when the soil is flooded or waterlogged for a certain period
of time. The physicochemical environment of soil changes with the biological and chemical
reduction of inorganic soil components. The oxidized components are reduced by
facultative anaerobes only on depletion of O 2.
 The reduced NO3- and higher oxides of Mn occur at high redox potential, i.e. Eh between
+200 and +300 mV. The reduction of hydrated ferric oxide occurs at intermediate redox
potential of Eh between +100 and -100 mV, whereas the reduction of sulphate occurs at
low redox potential, from -300 to -100 mV.
 The relative proportion of oxidized components in soil generally control its redox potential.
The presence of nitrates along with higher oxides of Mn tend to maintain Eh above +200
mV, while the presence of a significant amount of hydrated ferric oxide maintains the
anaerobic conditions. When ferric oxide content of soil becomes very low, as in the case of
degraded rice soils, silica and hydrated ferric oxide are depleted. Under such conditions,
the reduction of sulphates leads to the formation of H 2S which is toxic to rice plants.
 The reduction of higher oxides of Mn and Fe increases the concentration of soluble Mn and
Fe in soil, leading to its higher uptake by the rice plant to the toxic level in some soils.
There is an optimum Fe/Mn ratio for a higher yield of rice. A very high Fe/Mn ratio is not
conducive for obtaining a high yield.
 Under moderately reduced conditions, the recovery of added nitrogen to rice is low
because of the losses of nitrogen by denitrification. However, in some soils, in which
reduced condition causes a significant reduction in crop yield, addition of nitrates is
recommended not as a N-supplement but for preventing the Eh becoming low.

24
 Experiment No. 12

Objective: To determine zero point charge (ZPC)

Zero point charge (ZPC) is the value of pH at which the total net charge is nil or zero. This is also
termed as point of zero charge (PZC). At this point as the inter particle forces are inactivated, the
particles flocculate and do not move when an electric field is applied (i.e. electrophoretic mobility is nil).
ZPC is important in formation of soil aggregates and the retention of ions. If the soil pH is raised above
ZPC value, the charge becomes negative for which the cation exchange capacity is increased. If the
soil pH is decreased below ZPC value, the charge becomes positive for which the anion exchange
capacity appears. Measurement of ZPC helps in predicting the response of the soil to modifications in
environmental conditions by cultivation, use of fertilizers etc.
The method developed by Block and de Bruyn (1970) and experimented by the Hendershot and
Laukulich (1979) is discussed here.

Principle
The method is based on the indirect measurement of point of zero salt effect (PZSE) by potentiometric
titration of the net absorption of H+ and OH - at different pH and different ionic force. As certain colloids
are amphoteric in nature, the curves intersect at a given pH where the adsorption of protons is
independent of the ionic force. This point is named as PZSE.

Materials required
A.Equipment and other materials
0.5 mm diameter sieve, 50 ml beakers (9 nos.), bar magnetic agitator, pipette, measuring cylinder,
balance, pH meter in which an automatic titrimeter along with recording tape is connected to a
combined electrode.

B. Reagent:
I. NaCl (0.2 M): 11.688 gm NaCl is dissolved in distilled water and the volume is made up to 1
litre.
II. NaCl (0.05 M): 2.922 gm NaCl is dissolved in distilled water and the volume is made up to 1
litre.
III. NaCl (0.01 M): 0.5844 gm NaCl is dissolved in distilled water and the volume is made up to 1
litre. (iv) NaCl (0.001 M): 100 ml of 0.01 M NaCl is diluted to 1 litre with distilled water.
IV. HCl (0.1 M): 8.6 ml concentrated HCl is diluted to 1 litre by distilled water.
V. NaOH (0.1 M): 4 gm NaOH is dissolved in distilled water and the volume is made up to 1 litre.

Procedure
 2 gm of oven dry soil sieved through 0.5 mm diameter sieve (or equivalent amount of air dry
soil) is taken in 50 ml of 8 no. beakers numbered from 1-8.
 40 ml of 0.001 M NaCl is added to 1 and 2 no. beakers and each beaker is agitated with a bar
magnet for 5 minutes without stopping.
 After that, the pH is measured. This pH corresponds to zero point titration (ZPT).
25
  In beaker no. 1 using an automatic titrimeter connected to a combined electrode, the titration is
started with 0.1 M HCI solution by regulating the additions to 1 drop in every two minutes and
the pH value is recorded. The titration is continued until pH value becomes 3.0. The
approximate time taken for this titration is 2 hours.
 In beaker no. 2 using an automatic titrimeter connected to a combined electrode, titration is
started with 0.1 M NaOH solution by regulating 1 drop in every two minutes and the pH value is
recorded. The titration is continued until pH value becomes 9.5-10.0.
 40 ml of NaCl in beaker no. 3 and 4, 40 ml of 0.05 ml NaCl in beaker no. 5 and 6, 40 ml of 0.2
M NaCl in beaker No. 7 and 8 is added.
 Then after agitation with a bar magnet for 5 minutes pH is measured, titration is continued with
0.1 M HCl until pH becomes 3 and with 0.1 M NaOH until pH becomes 9.5 to 10.0 as stated
above.
 A blank (reagents without soil) test is carried out in beaker no. 9 to correct the result it
necessary.

Observation and Calculation

a. Weight of oven dry soil = 2 gm
b. Volume of 0.001 M NaCl added =40 gm
c. pH value after agitation = ZPT =
d. Figure of the recorded tape
e. pHo =
f. ZPC =
g. Specific adsorptions

Remarks
The pHo value corresponds to a point of maximum chemical stability. The measurement can be done
on untreated soil as well as on samples that have been subjected to pre-treatment by (a) saturation of
soil by 1 M NaCl (b) destruction of organic matter by sodium hypochloride (c) elimination of oxides and
hydroxides. The results are concluded as follows:

On untreated soil
I. If the pH measured in water is higher than that measured in KCl (pH н20 > pHkcl), ZPC is located
below ZPT.
II. If the pH measured in water is lower than that measured in KCl ((pH H20 < pHkcl) ZPC is above
ZPT.

On pre-treated soils
I. Destruction of organic matter by hypochlorite shows the depression effect of organic matter on
ZPC as compared to the untreated soil samples saturated in Na +.
II. Higher the content of oxides and hydroxides, higher is the ZPC.
III. The presence of silica and organic matter results in relatively low values of pH o.

26
 Experiment No. 13

Objective: Determination of variable charge

The variable charges or net proton charges originate from iron and aluminium oxides, alumina silicates,
organic matter, edge charges and charges of surface functional groups. This charges can be nil,
positive or negative. The variable charges are sometimes measured by titration with an indifferent
electrolyte at different ionic forces. The pH at which the curves intersect is the point of zero salt effect
(PZSE), this being a particular case in which variable charges become invariable. At this point, the
concentration of salts has no effect on the pH, but it is not necessarily equal to ZPC. As soil pH
determines the magnitude of the net charge, the variable charges can be roughly estimated during
measurement of pHн20 and pHKCL. The intersection of the two curves gives pH o. The variable charge is
given by the difference in density of charges between soil pH н20 and pHo.
The method described here was developed by Uehara and Gillman (1981). As variable charge is given
by the difference in density of charges between soil pH н20 and pHo, (i.e. the intersection point of the
titration curves with variable concentrations), it is determined by drawing a graph between volumes of
0.1 M HCl / 0.1 M NaOH added against pH of the suspension. The graph enables the increase in the
density of negative charges (i.e. variable charges) between soil pH н20 and pHo.

Principle
A given weight of soil sieved through 0.5 mm diameter sieve is taken in 15 numbers of 50 ml numbered
(1 - 15) beakers and the soil is thoroughly mixed with 10 ml of distilled water. Then increasing quantities
of 0.1 M HCl solution is added to 1 - 7 numbered beakers and increasing quantities of 0.1 M NaOH
solution is added to 9 - 15 numbered beakers making the final volume to 20 ml with distilled water.
Beaker number 8 is control. The beakers are allowed to stand for 4 days agitating occasionally. After 4
days of equilibrium the pHH20 is measured. The variable charge is estimated from the difference in
density of charges between pH н20 and pHo (which is the intersection point of the titration curves with
variable concentration) by drawing a graph between the volume of 0.1M HCl/0.1 M NaOH added
against pH of the suspension. If necessary, correction is made with the blank value.

Materials required
A. Equipment and other materials:
0.5 mm diameter sieve, 50 ml beakers (15 numbers), pipette, measuring cylinder, polythene sheet,
balance, pH meter, rubber bands, glass rod.

B. Reagents:
I. HCl solution (0.1 M): 8.6 ml conc. HCI is diluted to 1 litre by distilled water.
II. NaOH solution (0.1 M): 4 gm NaOH is dissolved in distilled water and the volume is
made up to 1 litre.
III. Buffer for pH measurement.

27
Procedure
 4 gm of oven dry soil sieved through 0.5 mm diameter sieve (or equivalent amount of air dry
soil) is taken in 50 ml of 15 beakers numbered from 1 to 15.
 10 ml of distilled water is added to the beakers and the soil is mixed thoroughly with distilled
water. Increasing quantities (ie. 0.50, 1.0, 1.5, 2.0, 2.5, 3.0 and 4.0 ml) of 0.1 M HCl solution
are added to 1-7 numbered beakers and increasing quantities e. 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and
4.0 ml) of 0.1 M NaOH solution are added to 9 - 15 numbered beakers. Beaker number 8 is
control.
 The volume is finally made up to 20 ml with distilled water. Each beaker is covered with
polyethylene sheet bound by rubber band.
 The beakers are allowed to stand for 4 days agitating occasionally.
 The pH of all the beakers are measured and recorded.
 Then 1 ml of 2.1 M KCl solution is added in each beaker and allowed to stand for 3 hours with
intermittent stirring (on addition of 1 ml of 2.1 M KCl to 20 ml soil suspension, the final
concentration of the KCI soil suspension will be 0.1 M).
 After 3 hours, the pH of 0.1 M KCl suspension of each beaker is measured and recorded.
 A graph is drawn between volumes (ml) of 0.1 M HCI / 0.1M NaOH added against pH of the
suspensions of H2O and 0.1 M KCl.

Observations and Calculation

a) Weight of oven dry soil = 4 gm
b) Volume of water added = 10 ml
c) Volume of 0.1 M HCl / 0.1 NaOH added = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 4.0 ml of 0.1 M HCI in
1 to 7 numbered beakers and 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 4.0 ml of 0.1 M NaOH in 9 to 15
numbered beakers. Beaker number 8 is control.
d) The final volume of all beaker is made with distilled water = 20 ml.
e) Volume of 2.1 M KCl added to each beaker = 1 ml
f) Soil pH recorded

Beaker number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
pH H2O
pH 0.1 MKCl

g) A figure is drawn between the volume (ml) of 0.1 M HCI / 0.1 M NaOH added against pH of the
suspension of H2O and of 0.1 M KCl. The intersection of the two curves i.e. pH o =
h) Variable charge = Charge density at pH H2O - Charge density of pHo

28
 Experiment No. 14
Objective: Determination of permanent charge
Permanent charges originate from the lattice structure of silicate clays due to isomorphous substitution
by ions of different valence. In most of the cases permanent charge is negative. These charges are
usually high in smectite type (2:1) clays and low in kaolinite type (1:1) clays and hydrous oxides. The
method developed by Uehara and Gillman (1981) is used for determination of permanent charge in a
soil.

Principle
The determination of permanent charge in a soil is based on the absorption of ions at pHo. At this point
the absorption of cations and anions on surfaces are equal resulting nil density of the charge. Any
excess adsorption of cations or anions at pH o, constitutes a measure of the permanent negative charge
and permanent positive charge respectively. For elimination of the specifically adsorbed ions and to
render the medium homoionic, a pre-treatment of soil is carried out.

A. Equipment and other materials Required:

0.5 mm diameter sieve, 50 ml beaker (15), pipette, measuring cylinder, polythene sheet, balance, pH
meter, rubber bands, glass rod, 1000 ml conical flask, centrifuge.
B. Reagents:
I. KCl (1M): 149.12 gm KCl dissolved in distilled water and the volume is made up to 2 litre.
II. KCI (0.2M): 14.912 gm KCl dissolved in distilled water and the volume is made up to 1 litre.
III. KCI (0.1M): 0.7456 gm KCl dissolved in distilled water and the volume is made up to 1 litre.
IV. KCl (0.002M): 200 gm of 0.01M KCl is diluted to 1 litre by distilled water.
V. HCl (0.1M): 8.6 ml conc. HCI is diluted to 1 litre by distilled water.
VI. KOH (0.1M): 5.6 gm KOH is dissolved in distilled water and the volume is made up to 1 litre.
VII. NaOH (0.1M): 4 gm NaOH is dissolved in distilled water and the volume is made up to 1 litre.
VIII. NH4NO3 (0.5M): 40 gm NH4NO3 is dissolved in distilled water and the volume is made up to 1
litre.
Procedure
A. Preliminary treatment of soil
 This treatment is needed to eliminate the specific adsorbed ions such as SO 42-. For this 100 gm
air dry soil already passed through 0.5 mm diameter sieve is taken in a 1000 ml conical flask.
 500 ml of IM KCl solution is added into the conical flask and it is shaken for few minutes in a
reciprocating shaker.
 The pH of the suspension is adjusted to 7.5 with 0.1M KOH solution. The material of the flask is
shaken for one hour in a reciprocating shaker.
 After shaking, it is allowed to stand for 1 hour and the supernatant is discarded.
 Again 500 ml of 1M KCl is added, the pH is adjusted to 7.5 with 0.1 M KOH solution, shaken for
1 hour, allowed to stand for 1 hour and the supernatant is discarded. This process is again
repeated.
 The soil is washed with distilled water repeatedly until the conductivity of the liquid phase is
equal to that of 0.0002 M KCl solution. This soil is dried in the air.

29
 B. Measurement:
 4 gm previously treated soil (equivalent to soil dried at 105°C) is taken in 50 ml of 15 numbered
beakers, numbered from 1 to 15 and the soil is thoroughly mixed with 10 ml of distilled water.
 Then increasing quantities of 0.1M HCl solution is added to 1 - 7 numbered beakers and
increasing quantities of 0.1 M NaOH is added to 9-15 numbered beakers making the final
volume to 20 ml with distilled water. Beaker number 8 is control.
 The beakers are allowed to stand for 4 days agitating occasionally. After 4 days of equilibrium
the pHH2o is measured.
 Then 1 ml of 2.1M KCI is added in each beaker and pH KCl is measured.
 The pHo, is noted at the intersection point of titration curves with variable concentration by
drawing a graph between volume of 0.1M HCI / 0.1M NaOH added against pH of the
suspension
 If necessary, correction is made with the blank value [Saturation of K and elimination of the
specifically adsorbed ions can modify the pH o value as determined during estimation of variable
charge].
 The soil residue is recovered. The beakers with the pH closest to pH o are used for
determination of the permanent charges. The residue soil is washed with 20 ml of 0.2M KCl
solution and transferred in a 50 ml centrifuge tube.
 The tube is agitated for 1 hour and the supernatant is discarded.
 Then 20 ml of 0.01M KCl is added, agitated for few minutes and the pH is adjusted to the value
closest to pH by using 0.1M HCl or NaOH solution.
 The tube is allowed to stand for 1 hour. After 1 hour o establishment of equilibrium it is
centrifuged and the supernatant is drained out for analysis of K + and CI-.
 The centrifuge tube containing the soil residue is weighed immediately after draining out of the
supernatant to determine the value of 0.01M KCl solution retained by the soil. Let the value is V
ml. The adsorbed K + and CI- in the soil residue is removed by washing the residue soil five
times with 20 ml of 0.5M NH4NO3 solution through filtering.

Observations and Calculations:

I. Weight of soil taken = 4 gm

II. Volume of KCl (0.2M and 0.01M) added = 20 ml
III. K+ in the supernatant = A meq (Let)
IV. Cl- in the supernatant = B meq (Let)
V. Volume of 0.01M KCl retained = V (Let)
VI. Adsorbed K+ extracted into 100 ml of 0.05M NH 4NO3= C meq (Let)
VII. Adsorbed Cl- extracted into 100 ml of 0.05M NH4NO3 = D meq (Let)
VIII. K adsorbed in soil [mmol(+) / kg] = (100 C-VB) x 100/4= X (Let)
IX. Cl adsorbed in soil[mmol(+) / kg] = (100 D-VB) x 100/4 = Y (Let)
X. Permanent charge = Adsorbed K+ - Adsorbed Cl- =
= (X-Y) [mmol(+) / kg] or meq/100 gm soil.

Remarks: The sum of permanent charges and variable charges roughly corresponds to the cation
exchange capacity of the soil measured by normal neutral ammonium acetate method.

30
 Experiment No. 15
Objective: Determination of quantity – intensity (Q/I) relation of soil potassium
The Q/I relation of liable K relates the quantity (Q) of liable or immediately exchangeable K ion to its
intensity (I) factor which is best measured as the chemical potential or the activity ratio of the potassium
ion in the soil solution. Beckett (1964) gave the quantity - Intensity (Q/I) relation to obtain a clear picture
of the potassium status of a soil. The potassium intensity in soil solution thus depends on the nature of
the liable pool, the rate of release of fixed potassium and the rate of diffusion and transport of
potassium ions in soil solution. Beckett estimated the immediate Q/I relationships for a number of soils
and observed them to be linear over the activity ratios usually encountered. The form of immediate Q/I
relationships regulates the uptake of potassium for short periods and also regulates the total potassium
uptake. The potential buffering capacity of soil K, which is an intrinsic soil property, and which is not
commonly affected by manuring and cropping can be known from Q/I relation of soil potassium.

Principle
Under controlled temperature soil samples are shaken with aliquots contain varying concentrations of
potassium (K) in calcium chloride (CaCl2) solutions of known strength in a given soil: solution ratio for a
given length of time (usually for 12 hours). After equilibrating for 12 hours the solutions are filtered and
potassium (K), sodium (Na), calcium (Ca), magnesium (Mg) and aluminium (Al) content in the filtrate
are determined. For each suspension, the amount (± AK in meq/100 gm of soil) by which the
exchangeable potassium content of the soil had changed is calculated from the difference between the
potassium concentration of the initial and the resultant solutions. The activity ratios, ak/√a(Ca+Mg) of
the resultant solutions were calculated using the Debye-Huckel equation modified by Guggenheim's
(1950) expression for activity coefficients. The weighted average slope of the Q/I relationship curve is a
measure of the potassium buffering capacity of soil potassium to maintain the potassium potential in
soil solution.

Materials required:
Flame photometer, horizontal shaker, volumetric flasks (50 ml), conical flasks (100 ml).
Reagents:
I. Equilibrating solution of K (10 m mole/l) in 0.01 M CaCl2 solution:
(a) Calcium chloride (CaCl2.2H2O) solution (0.01 M): 1.47 gm of CaCl2.2H2O is dissolved in distilled
water and diluted to 1 litre by distilled water.
(b) KCl (10 m mole K/l): 744.5 mg of KCl is dissolved in 0.01 M CaCl 2 solution and the volume is made
up to 1 litre by 0.01 M CaCl2 solution. This is a stock solution. From this solution various graded
solutions like 0, 0.1, 0.2, 0.4, 1.0, 2.0, 3.0, 4.0 m mole K/l in 0.01 M CaCl 2 solution are prepared.

II. EDTA and other solutions required for estimation of Ca + Mg

Procedure
 5gm of oven dry soil sieved through 20 mm diameter sieved (or equivalent amount of air dry
soil) is taken in 100 ml of 8 conical flasks numbered from 1 to 8. 50 ml of each of the various
graded potassium solutions like 0, 0.1, 0.2, 0.4, 1.0, 2.0, 3.0 and 4.0 m mol K/l is added to 1 to

31
 8 number conical flasks respectively and the flasks are shaken for half an hour on a horizontal
shaker and allowed them to equilibrate for 12 hours.
 The suspension is filtered and the filtrate of the equilibrium solution is analysed for K by flame
photometer method and Ca + Mg by versenate titration method.
 In another set of experiment after addition of 50 ml of each of the various graded potassium
solutions as stated above, it is thoroughly mixed with soil and then filtered and the filtrate is
analysed for K by flame photometer and Ca + Mg by versenate titration method.
 The quality of loss (-AK) or gain (+AK) of K is calculated from the decrease or increase in
concentration of K in the equilibrium solution, compared to the original or initial solution.
 A figure is drawn between + AK (on the y-axis) against the activity ratio (on the x-axis) [activity
ratio is calculated by multiplying the concentration ratio by activity coefficient].
 The linear portion of the curve is extrapolated to the ordinate by drawing a tangent, from the
point of abscissa where AK = 0.
 The curvilinear portion of the graph is also extrapolated to the ordinate.
 The slope of the linear portion of the curve is also measured.

Observation and calculation

Parameters Concentration of added K (m mol /l) in 0.1 M CaCl2
0 0.1 0.2 0.4 1.0 2.0 3.0 4.0
1 2 3 4 5 6 7 8
i ii i ii i ii i ii i ii i ii i ii i ii
(a) K Concentration
(b) Ca+Mg conc.
(c) Ionic strength
(d) Act. Coeff. Of K
(e) Act. Coeff. Of Ca and or
Mg
(f) Activity of K
(g) Activity of Ca+ Mg
(h) Activity ratio of K
(i) Gain or loss of K

Parameters from Q /I plot:

I. The concentration ratio (C.R. in mol/lit) is calculated by the following equation:

C.R.=

32
 Where = Potassium content of equilibrium solution in mol /1.

(Ca+Mg)E= Calcium plus magnesium content of equilibrium solution in mol/1.

II. The activity ratio (A.R.) is calculated by multiplying the concentration ratio (C.R.) by activity
coefficient.

A.R. = C.R. X

III. Equilibrium activity ratio (AR, K in mol/lit): It is obtained from the intersecting point of the Q/I
curve at the x-axis. Here K is neither gain nor loss (i.e. AK= 0). It is a measure of the
intensity factor, I and is responsible for K nutrition of crops.
I. Non-specific K (Ko in c mol (+)/kg): It is obtained by drawing a tangent from the point of ARe
K axis where AK= 0, i.e., by extrapolation of the linear part of the Q/I graph to the ordinate.
It is that K held on non-specific planer sites of soil clays.
II. Specific K (Kx in c mol (P+)/kg): It is obtained by subtraction of the non specific K from the
liable K i.e., Kx = K1- Ko. It is that K associated with specific sites.
III. Labile K (KL in c mol (P+)/kg): The curvilinear portion of the Q/I graph, extrapolated to meet
the y-axis, gives the labile K which is a loosely held K.
IV. Potential buffering capacity (PBC K in c mol/kg. mol -1/2 I1/2): This is calculated as the linear
slop of the Q/I curve i.e., AQ/ AI.
V. Potassium potential [Ko x PBCK in (c mol (+)/kg) mol/kg. mol mol -1/2 I1/2]: It is the product of
K held on non specific planer sites and the potential buffering capacity.

Evaluation of the parameters from Q/I plot

Parameters Concentration of added K (mol /l) in 0.1 M CaCl2

0 0.1 0.2 0.4 1.0 2.0 3.0 4.0
1 2 3 4 5 6 7 8
i ii i ii i ii i ii i ii i ii i ii i ii
(a) C.R. in mol/lit
(b) A.R.
(c) AReK [in moles/lit]
(d) Ko [in c mol (+)/kg]
(e) Kx [in c mol (+)/kg]
(f) PBCk [in c mol (+)/kg. mol-
1/2 I1/2 ]

(g) K-potential [(c mol (+)/kg

mol-1/2 I1/2 ]

33
Interpretation:
(h) Higher the value of AReK, greater is the supply of K to crops.
(i) Generally, higher the value of PBC k, greater is the capacity of soil to maintain K concentration
for longer periods, although it often leads to low K intensity. A soil with a low value of PBC k
needs frequent application of fertilizer as the soil fails to maintain K supply for a considerable
period.
(j) The deficiency and sufficiency or toxicity of potassium may be known from the value of K
potential.
K potential (kcal/mol) Rating
-4.0 to -3.5 Deficient in available K
-3.0 to -2.5 Adequate supply of available K
< -2.0 K-toxicity

34
 Experiment No. 16

Objective: Determination of total potential soil acidity by BaCl2 method

Principle
The total potential soil acidity includes all the acidity components including the weakly acidic R–COOH
and R–OH groups on soil organic matter and partially neutralized Al-hydroxy polymers that may be
present even in soils of pH > 7. The soil is leached with 0.5 (N) BaCl 2 and 0.055 (N) triethanolamine
buffered at pH 8-8.2. This pH corresponds very nearly to the pH of complete neutralization of soil
hydroxy-Al compounds. Barium ion effectively displaces the H + and Al3+ ions and also causes
hydrolysis of adsorbed Al-ions and dissociation of acidic R–COOH and R–OH groups, present in soil
organic matter, which are neutralized by triethanolamine, which is a weak base. The leachate is titrated
with a standard acid using bromocresol green- methyl red mixed indicator. A blank titration is also
performed on the same volume of leachate.
Reagents
 1(N) BaCl2 solution
 0.2 (N) HCl
 Buffer extractant of 0.5(N) BaCl2 plus triethaholamine (pH 8-8.2); Dilute 25 ml of
triethanolamine (sp.gr.1.126, normality 8) to 250 ml with water and adjust the pH to 8.0-8.2 with
HCl (approx.90 ml 1(N) HCl is required for this partial neutralization process). Dilute the
solution to 500 ml and then mix with 500 ml of 1(N) BaCl 2 solution. The final solution must be
kept free of CO2.
 Bromocresol green-methyl red indicator; Dissolve 0.5 g bromocresol green and 0.1 g methyl
red in 100 ml of 95% ethanol and adjust the pH to 4.5.
Procedure
 Weigh 10 g soil and take it in a 250 ml conical flask.
 Add 100 ml of extracting buffer solution and shake for half an hour and keep for overnight.
 Transfer the contents to a buchner funnel fitted with a Whatman No. 42 filter paper and carry
out gently suction filtration.
 Rinse the conical flask with additional extracting solution so that no soil particle is left over in
the flask.
 Now, transfer the leachate to a 250 ml volumetric flask and make up the volume with the
extracting solution.
 Pour the leachate into a conical flask and add a few drops of mixed indicator into it. Titrate with
0.2 (N) HCl until the end point colouration (pink) is obtained.
 Perform a blank keeping all conditions identical excepting soil.

35
Calculations
Let Volume of 0.2(N) HCl required for blank titration = B ml
Volume of 0.2(N) HCl required for sample titration = S ml.
Hence, meq. of total potential acidity = (B – S) × 0.2
Now 10 g soil has total potential acidity = [(B – S) × 0.2)] meq.

So 100g soil has total potential acidity =

Thus, total potential acidity [2 (B-S)] meq/100g

36
 Experiment No. 17
Objective: Determination of lime requirement of soil.
For satisfactory plant growth, the soil should have a pH between 6.5 to 7.5, though cer- tain plants can
grow well at low pH like tea and also at high pH like sugarbeet. In India acid soils are located mostly in
eastern, southern and south central parts. Also some soils at higher elevations in north India are acidic.
For sustained agricultural production and higher yields, through efficient soil management practices, it
is essential to lime and acid soil, as it has consid- erable influence on soil environment, besides
correcting soil acidity.
Principle
In this method the soil is equilibrated with a pH 7.5 buffer solution, whereby the reserve H + is brought
into solution, which results in the depression of pH of the buffer solution, a note of which is made and
interpreted in terms of lime required to raise the pH to a desired value.

Reagents
Extractant buffer; Dissolve 1.8 g paranitrophenol, 3 g potassium chromate, 2 g calcium acetate, 53.1 g
calcium chloride dihydrate (CaCl2.2H2O) and 2.5 ml triethanolamine in 1 litre of distilled water. Adjust
the pH to 7.5 with NaOH.
Procedure
 Determine the pH of the soil sample in 1 : 2.5 soil:water ratio.
 For this weigh 10 g soil and add 25 ml distilled water.
 Shake intermittently for half an hour and record the soil pH. If the pH exceeds 6.0 then this
method is not applicable.
 If the measured pH is 6.0 or low then proceed as follows:
 Weigh 5 g soil in a 50 ml beaker.
 Add to it 5 ml of distilled water and 10 ml buffer solution.
 Stir continuously for 10 minutes or intermittently for 20 minutes.
 Determine the soil pH with the pH meter.
 Lime requirement is determined on the basis of soil-buffer pH ready reckoner given below.
The values in this table are given in tons of pure CaCO 3 per acre required to bring the soil to the
indicated pH and thus are required to be converted to their equivalents in the form of agricultural lime to
be used. The figures are multiplied by a factor of 2.43 to express in tons per hectre.

37
pH of soil- Lime required to bring the soil to
buffer indicated (tons/acre of pure
suspension pH 6.0 CaCO3) pH 6.8
pH 6.4
6.7 1.0 1.2 1.4
6.6 1.4 1.7 1.9
6.5 1.8 2.2 2.5
6.4 2.3 2.7 3.1
6.3 2.7 3.2 3.7
6.2 3.1 3.7 4.2
6.1 3.5 4.2 4.8
6.0 3.9 4.7 5.4
5.9 4.4 5.2 6.0
5.8 4.8 5.7 6.5
5.7 5.2 6.2 7.0
5.6 5.6 6.7 7.7
5.5 6.0 7.2 8.3
5.4 6.5 7.7 8.9
5.3 6.9 8.2 9.4
5.2 7.4 8.6 10.0
5.1 7.8 9.1 10.6
5.0 8.2 10.1 11.2
4.9 8.6 10.6 11.8
4.8 9.1 12.4

38
 Experiment No. 18

Objective: Determination of gypsum requirement of soil

Principle
Presence of large amount of sodium as high as 15% or more in the exchange complex results in high
soil pH (> 8.0) for sodic (alkali) and saline-sodic soils which causes nutritional imbalances, depletion of
soil organic matter, deterioration of soil physical health and also affects the soil biotic community
Gypsum (CaSO4 .2H2O) is commonly used for soil amendment under such situation. A fixed weight of
soil is equilibrated with a known amount of Ca solution, and the amount of Ca left in solution is
determined by EDTA-titration. The difference between the amount of Ca added and Ca left in solution,
gives the amount of Ca exchanged. Practically it has been observed that gypsum addition of about 1/3
of the value obtained by this method is satisfactory in most cases.
Reagents
 Ammonium chloride–Ammonium hydroxide buffer ; Dissolve 67.5 g of NH 4Cl in 570 ml of
NH4OH (sp.gr.0.86), and dilute to 1 litre.
 Saturated CaSO4 solution; Shake about 5 g CaSO4 .2H2O with 1 litre of distilled water for 15
minutes on a mechanical shaker and filter.
 Eriochrome Black T (EBT) indicator; Dissolve 0.5 g of EBT and 4.5 g of hydroxylamine
hydrochloride in 100 ml of 95% ethanol.
 Standard EDTA solution 0.01N; Dissolve 2 g of disodium dihydrogen-ethylene-diamine- tetra
acetate and 0.05 g of MgCl2.6H2O in water and dilute to 1 litre. Standardise against standard
Ca-solution.
Procedure
 Weigh 5 g of soil sample in a 250 ml conical flask, and add 100 ml of saturated CaSO 4 solution.
 Shake for 5 minutes on a mechanical shaker and filter.
 Pipette out 5 ml of the extract into a 100 ml conical flask and dilute to about 25 ml with distilled
water.
 Add 0.5 ml of the NH4Cl-NH4OH buffer and 3-4 drops of the EBT indicator.
 Titrate with the standard EDTA solution until the colour changes from wine red to blue.
 Titrate in a similar way 5 ml of the saturated CaSO 4 separately.
Calculations
Weight of soil taken =5g
Total volume of extract = 100 ml
Let volume of EDTA used for titration of x ml of gypsum solution be A ml (say) and volume of EDTA
used for titration of y ml of sample aliquot be B ml (say)
Therefore, meq. of Ca/l in gypsum solution

39
Meq. of Ca/l in sample solution

Total meq. of Ca remained in soil after addition of 100 ml gypsum

=

Now 5 g soil contains 0.1 × (P – Q) meq

Hence 100 g soil contains

= [ 2 X (P- Q] meq/100 g

Thus 1 kg soil requires [20 × 20 × (P – Q)] mg Ca

= 400 × (P – Q) mg Ca.
2.24 million kg soil requires

= 896 (P -Q ) kg Ca
Now 40 kg Ca is obtained from 172 kg gypsum

So, [896 × (P – Q)] kg Ca is obtained from

= [ 3.85 (P-Q) ] tons of gypsum

Thus gypsum requirement of the soil = [3.85 × (P – Q)] tons/ha.

40