PARTICLE SIZE ANALYSIS OF SOIL BY HYDROMETER METHOD

Particle size analysis of soil refers to the process of determining the amounts of individual soil

separates, which are below 2 mm in diameter. Results are expressed in percentage of sand, silt and clay

on oven dry basis. This analysis is used to know the texture of soil.

Sand, silt and clay are the mechanical components of soil, and this is why, determining the amounts of

individual soil separates is also known as mechanical analysis of soil.

Mechanical analysis of soil consists essentially of two distinct operations, namely, (i) Dispersion of

soil and (ii) Fractionation of the sample i.e. grading the soil particles into different size groups.

Dispersion of soil has to be made to ensure the separation of primary particles from each other as

aggregation by cementing agents, results in the formation of particles of larger sizes from those of

smaller dimensions. Cementing materials / cementing agents / chelating agents are those, which are

responsible for aggregate / structure formation of soil. Cementing materials of soil are:-

i. organic matter

ii. carbonates of calcium and magnesium

iii. oxides and hydroxides of Fe and Al

iv. organic exudates

v. microbial gum

vi. clay to clay cementation, and other flocculating agents.

Treatments that affect the destruction of cementing agents are effective measures to break down the

secondary particles (aggregates) into primary particles. The process of separating into primary particles

from secondary particles (soil aggregates) is known as pretreatment. The process of resolving (to

separate or be separated into components) to primary particles is also termed as “pretreatment”. In

other words, treatment done for the destruction of cementing agents is known as pretreatment.

Reagents used in pretreatment are:

Hydrogen peroxide (H2O2): Hydrogen peroxide is used to remove organic matter from soil. The

reaction may be shown as follows:

Organic matter + H2O2 = Destruction of organic matter + evolution of CO2.

Hydrochloric acid (HCl): Hydrochloric acid is used to remove carbonates of calcium and magnesium.

The reaction may be shown as follows:

HCl + CaCO3 = CaCl2 + H2O + CO2. Subsequently, the primary particles are to be kept in dispersed condition during fractionation of the

sample. This may be achieved by adding a suitable peptizing / deflocculating / dispersing agent to the

soil suspension. The following dispersing agents can be used to study the particle size analysis of soil

and they (agents) produce almost same yield (sand, silt and clay fractions) in a soil:

i. Calgon or sodium hexametaphosphate [(NaPO3)6]-5%.

ii. Sodium hydroxide (NaOH)-1N

iii. Sodium oxalate (Na2C2O4)-1N

iv. Sodium carbonate (Na2CO3)-1N

v. Lithium carbonate (Li2CO3)-1N

According to the international Society of Soil Science, sodium hydroxide is used, and according to the

Soil Division of United States Department of Agriculture, sodium hexametaphosphate is used.

Fractionation of soil particles into different size-groups is based on principles of sedimentation. When

a mixture of particles of different sizes, after thorough mixing, is allowed to settle in a fluid, the rate of

settling of different particles depends on the effective diameter of the particles, other variables such as

viscosity of fluid, temperature etc. remaining constant.

The analysis can be accomplished by the two methods -

i. Hydrometer method and ii. Pipette sample method. Hydrometer method (Bouyoucos, 1926) has

gained much popularity, as the method is easy, simple and rapid one. But the method is less accurate

than the pipette sampling method.

Apparatus required (For hydrometer method):

i. Dispersion cup, ii. Electric stirrer, iii. Hydrometer, iv. Sedimentation

cylinder, v. Measuring cylinder, vi. Thermometer, etc.

Chemicals required:

5% calgon solution [Sodium hexametaphosphate, (NaPO3)6]

Procedure:

1. Screen the air-dry soil through 2mm sieve and make the moisture correction during

weighing to express the result on oven dry basis.

2. Take 50gs of oven dry soil into a dispersion cup and add some water to cover the soil

and add also 100mL of 5% calgon solution in it. Keep it for 15 minutes for digestion

(preferable to keep it overnight).

3. Add water into the dispersion cup to about 1 inch below to the brim. Stir the soil with

electric stirrer for about 10 minutes.

4. Transfer the whole contents of the cup into the sedimentation cylinder without any loss

and add water up to the mark of the cylinder.

5. Place a cork on the mouth of the cylinder and invert the cylinder several times until the

whole soil mass appears in the suspension. Set the cylinder upright and insert the

hydrometer into the suspension at 30 seconds of sedimentation. Record the hydrometer

reading at 40 seconds of sedimentation (in USDA system). Take the similar

measurement (second hydrometer reading) at 2 hours of sedimentation. Record the

temperature by thermometer just after taking the reading in both the times.

6. Make the correction of hydrometer reading. As the hydrometer is calibrated at 670

F

(19.40

C), add 0.2 (0.3) for each degree above 670

F (19.40

C) with and subtract 0.2(0.3)

for each degree below 670 F (19.4

0 C) from hydrometer reading.

Calculation: Corrected 40 seconds hydrometer reading

% (Silt +Clay) = …………………………………………………… × 100

Weight of oven dry soil

Corrected 2 hours hydrometer reading

% Clay = …………………………………………………… × 100

Weight of oven dry soil

% Sand = 100 - % (Silt +Clay)

% Silt = % (Silt +Clay) -% Clay

Result: By plotting the values of %sand, silt and clay on the textural triangle, the textural class was:

DETERMINATION OF MOISTURE PERCENTAGE IN A FIELD MOIST SOIL

The moisture percentage in a field moist soil may be stated as the amount of water held by the field

soil in natural moist condition and is expressed as a percentage of the oven-dry soil.

Apparatus required:

i. Aluminum dish, ii. A chemical balance, iii. An electric oven, iv. Desiccator- containing

some desiccating agent i.e. CaCl2.

Procedure:

i. Transfer a suitable amount (about 10g) of the field moist soil into a clean, dry and

previously weighed aluminum dish along with the cover in duplicate.

ii. Then take the weight of the dish with the moist soil.

iii. Now remove the lid and place the dish containing the field moist soil in a well-ventilated

oven at 1050C.

iv. Dry the soil for 16-24 hours or until a constant weight is obtained.

v. Then cover the dish with its lid, cool in a desiccator and weigh as oven dry soil.

Calculation:

i. The weight of the empty dish with lid = W g.

ii. The weight of the dish with lid + weight of the moist soil = W1g.

iii. The weight of the dish with lid + weight of the oven dry soil = W2g.

iv. The amount of moisture present in the moist soil =( W1 - W2) g.

v. The amount of oven dry soil = (W2 – W) g.

Therefore, the percentage of moisture present in the field moist soil on weight basis (w/w)

= 100soildryovenofWeight

moistureofWeight (expressed as a percentage of oven dry soil)

= 100WW

WW

1 1

2

Again, the percentage of moisture present in the field moist soil on volume basis (v/v)

= 100soildryovenofV

mofV

olume

oistureolume

= 100

soilofdesityBulk

soilofWeight

moistureofDeusity

moistureofWeight

= 100soilofWeight1

soilofBDmoistureofWeight

= soilofdensity Bulk 100soilofWeight

moistureofWeight

= Moisture parentage of soil on weight basis × BD of soil

Therefore,

Percentage of soil moisture on volume basis = Moisture parentage of soil on weight basis × bulk

density of soil

Precautions:

i. Weigh the desiccated soil rapidly to avoid any loss or gain of moisture during weighing.

ii. The temperature is to be adjusted at 1050C and drying is done for a period until a constant

weight is obtained.

DETERMINATION OF FIELD CAPACITY OF SOIL

Field capacity: The field capacity of a soil may be defined as the amount of water retained in the soil

against the force of gravity. It is the water content in the soil when the drainage of gravitational water

has become very slow and the water content has become more or less stable. The situation usually

exists one to three days (depending on the soil type) after the soil has been thoroughly wetted by

rainwater or irrigation water. The moisture percentage at field capacity is the maximum available water

in soil. Field capacity of soil is also called capillary capacity.

Field capacity of a soil can be determined by adding water on the soil surface and permitting it to drain

out for 1-3 days (depending on the soil type) with surface evaporation prevented. The soil sample is

then collected by auger for gravimetric measurement of soil water content. As the field capacity varies

with the soil profile and soil structure, so the laboratory determinations are not the reliable indicator of

the values in the field.

Field capacity of a soil may be determined in two ways:

A. Field method

B. Laboratory method

A. Field method:

Procedure:

i. Select a uniform field plot measuring 3m ×3m area.

ii. Remove weeds, pebbles, gravels, stones, etc. with the help of a spade.

iii. Install a water-tight earthen dike around the perimeter of the plot.

iv. Supply sufficient water to completely saturate the soil to the desired depth.

v. Cover the plot with an evaporation barrier such as plastic (or polythene) sheet that is then again

covered with straw or mulching material.

vi. After distribution of soil water in the profile for 48-72 hours, remove the evaporation barrier and

take soil samples to determine water content gravimetrically. To avoid boundary effects, as a

precaution, soil sampling must be confined to the central area of the plot.

vii. Transfer a suitable amount (about 20g) of the moist soil into a clean, dry and previously weighed

aluminium dish along with the cover for gravimetric determination of moisture percentage.

B. Laboratory method

Apparatus required:

i. A small earthen pot with a small opening in the bottom (or A glass cylinder with an

opening in the bottom).

ii. Aluminium dish

iii. Balance

iv. Oven

v. Desiccator

Procedure:

i. Fill up the earthen pot (glass cylinder) with air-dry sieved soil up to (3/4 th of its volume) 1

inch below of its top level after covering its bottom opening with cheesecloth or few

gravels.

ii. Add water into the earthen pot (cylinder) to (thoroughly wet the soil) wet half of the soil

column.

iii. Cover the top of the earthen pot (glass cylinder) with a dish in order to reduce evaporation

from soil surface.

iv. Keep the earthen pot (glass cylinder) for 1-3 days to allow the moisture to reach in

equilibrium condition with soil.

v. Remove about ½ inch of the surface soil and take soil sample for gravimetric determination

of moisture percentage.

Calculation: Same as that of determination of moisture percentage by gravimetric method.

Result:

Precaution:

i. The desiccated soil sample should be weighed rapidly to avoid any loss or gain of

moisture during weighing.

ii. The temperature is to be adjusted at 1050C and drying should be done for a period until

a constant weight is obtained.

DETERMINATION OF MAXIMUM WATER HOLDING CAPACITY OF SOIL

Maximum water holding capacity of soil refers to the amount of water retained in the soil at

saturation. This value varies with the soil texture; a sandy soil has less water holding capacity than a

clayey soil, and is of significance in agronomic practices.

Equipments:

i. Aluminium or brass box with perforated bottom

ii. Filter paper

iii. Knife

iv. Analytical / Chemical balance

v. Glass triangle

vi. Glass dish or Petridish

vii. Oven

viii. Desiccator

Procedure:

I. Cut a filter paper according to the size of the bottom of the perforated brass box.

II. Place the filter paper at the bottom of the brass box.

III. Take the weight of the brass box with filter paper in an analytical balance.

IV. Pack about 30-40g of soil inside the brass box tightly by gentle tapping and adding the soil

in several instalments in small amounts.

V. Place the brass box over glass triangle inside a petridish or suitable glass dish.

VI. Pour water in the petridish so that at least half the height of the brass box remains immersed

under water.

VII. Allow saturating the soil overnight.

VIII. Remove the brass box and place it on a blotting paper for few minutes to strain out the

excess water and wipe the sides of the brass box.

IX. Weigh the brass box again and note the weight.

X. Dry the moist soil in the brass box in oven for 8-12 hours at 100-1100 C.

XI. Cool it in a desiccator and weigh. Repeat heating and desiccating until a constant weight is

obtained.

XII. Cut 5 pieces of filter paper to the size of the bottom of the brass box, saturate them with

water, roll gently with a pencil over the moist filters and then weigh. Dry the filter papers in

an oven at 100-1100c for few hours and cool it in a desiccator. Continue drying and

desiccating till a constant weight is obtained. Calculate the amount of water held by one

filter paper.

Calculation:

I. Weight of the perforated brass box + filter paper = a g.

II. Weight of the brass box + water saturated soil + moist filter paper = b g.

III. Weight of moisture held by one filter paper = c g.

IV. Weight of brass box + dry soil + dry filter paper = d g.

V. Weight of oven dry soil = (d-a) g.

VI. Weight of moisture held by soil = [(b – d) – c] g.

[(b – d) – c]

Therefore, percentage of water holding capacity = --------------------------- x100

(d-a)

Result: The maximum water holding capacity of the collected soil sample was = x %

Precaution:

i. The desiccated soil sample should be weighed rapidly to avoid any loss or gain of moisture

during weighing.

ii. The temperature has to be adjusted at 1050C and drying should be done for a period until a

constant weight is obtained.

Problem: In the determination of MWHC of a soil weight of perforated brass box with filter paper was

found 50g. Weight of perforated brass box, water saturated soil and moist filter paper was found 100g

and moisture held by one filter paper was 5g. Weight of brass box, dry soil and dry filter paper was

found 80g. Find out MWHC of that soil.

Note:

Weight of the brass box + water saturated soil + moist filter paper = b g

Weight of the brass box + oven dry soil + oven dry filter paper = d g

________________________________________________________________

By subtracting,

Weight of moisture held by soil + Weight of moisture held by one filter paper = (b-d) g.

But, Weight of moisture held by one filter paper = c g.

So, Weight of moisture held by soil = [(b – d) – c] g.

DETERMINATION OF SOIL MOISTURE BY TENSIOMETER METHOD

Introduction:

The attractive force with which water is held by the soil particles is called soil water tension.

Tensiometer measures the soil water tension. Soil moisture contents may also be attained against a

corresponding reading through a calibration curve. Different calibration curves should be made for

different types of soils. A typical tensiometer consists of a porous ceramic cup, an water tight

connecting glass/plastic tube leading to a pressure sensing device and a covered opening for refilling

the tube with water.

Tensiometers are of two types depending on the type of pressure sensing device:

1. Gauge type tensiometer.

2. Mercury manometer type tensiometer.

1. Gauge type tensiometer: In this type of tensiometer, the watertight connecting tube is led to a

Bourdon gauge. Tensiometer gauge scales read centibars.

2. Mercury manometer type tensiometer: Mercury manometer type tensiometers are more

sensitive and accurate than the gauge type. The porous cup when placed in the soil permits free

diffusion of water and solution but restricts the movement of soil particles. When tensiometer is

placed in a relatively dry soil, water in the tensiometer would be absorbed by the soil through

the porous cup. As a result vacuum would be created at the end of the manometer tube, which

will cause the rise of mercury in the mercury manometer tube. The height of the mercury

column in the manometer would indicate the dryness or wetness in the soil. Drier the soil,

higher the mercury column in the manometer tube. Mercury would not rise in the saturated soil.

Gauge sensitivity: It is the pressure change per unit volume of water transfer from the tensiometer.

Generally, tensiometers are more sensitive in the higher soil moisture ranges and less sensitive in the

lower soil moisture. Consequently, the range of tensiometer is limited (0-0.85 bar tension) and will

decrease with increasing altitude and temperature.

Equipments:

1. Tensiometer (Gauge type)

2. Balance

3. Bucket/Earthen pot

4. Aluminium dishes

5. Oven

Procedure:

Step-I (Preparation of calibration curve):

i. Remove caps of 4 tensiometers and fill up the glass tubes with water with the help of a

wash bottle and then recap/ replace the caps on their positions.

ii. Fill up 4 buckets/ earthen pots (each of about 4-5 litre volume) with soil nearly 3-5 cm.

below to the brim.

iii. Add different quantities of water (250ml., 500ml., 750ml. and 1000ml.) to the 4 pots.

iv. Insert/ fix a tensiometer in each earthen pot.

v. Allow reaching the soil in equilibrium with water (generally within 24 hours) and note the

tensiometer reading in each case.

vi. Collect soil samples around the porous cup from each pot separately.

vii. Determine moisture contents gravimetrically.

viii. Plot the tensiometer readings on the ordinate and the respective moisture contents on the

abscissa of a graph paper and thus prepare a calibration curve/standard curve.

Step-II (Determination of soil moisture content in the test soil):

i. Fix a tensiometer in the field soil from which moisture content is to be determined.

ii. Record the tensiometer reading at equilibrium.

iii. Plot the tensiometer reading on the calibration curve and observe the moisture content.

iv. Find out the moisture percentage of the field soil gravimetrically also to check the accuracy

of the tensiometer.

Advantages of tensiometer:

1. The advantage of tensiometer method over the other method is that it can be kept in the

field for entire growing period of the crop.

1. The tensiometer reading can be taken as and where necessary.

2. The tensiometers are relatively cheap and handy equipments.

3. Tensiometers of different sizes are available and they may be placed at different depths.

Disadvantages:

1. The main disadvantage of this method is that it does not give accurate readings above

0.85 bar tensions.

2. Above 0.85 bar tension soil air enter into the tensiometer through the porous cup, thus

filling the vacuum and restricting the rise of the mercury in the manometer tube and

gives erroneous reading.

3. Tensiometers do not give accurate reading due to the presence of air bubble in the

system.

4. Readings at different depths cannot be taken by one tensiometer at a time at a place.

5. Always close connection between porous cup and the soil can not be kept.

Experimental data:

A. DATA FOR THE PREPARATION OF STANDARD CURVE:

Sl. no. Amount of

water added

to the soil of

bucket

Tensiometer

reading(cb)

Determination of soil moisture percentage

Wt. of

empty Al.

dish (g.)

Wt. of Al.

dish + moist

soil(g.)

Wt. of Al.

dish + oven

dry soil(g.)

Moisture

percentage

I. Bucket-1 250ml. 60 19.70 36.50 36.0 3.06

II. Bucket-2 500ml. 50 19.85 35.0 34.20 5.57

III. Bucket-3 750ml. 17 13.6 32.6 28.3 24.2

IV. Bucket-4 1000ml. 2 11.5 53.5 41.3 40.9

Fig. Data computation for the preparation of calibration of curve

B. DATA FOR THE UNKNOWN FIELD SOIL SAMPLE:

Serial No. Tensiometer

reading (cb)

Moisture

percentage

from

calibration

curve

Moisture percentage from gravimetric method

Wt. of the

empty Al.

dish

Wt. of Al.

dish +

moist soil

Wt. of Al.

dish +

oven dry

soil.

Percentage

of soil

moisture

1. Relatively

dry soil

39 15 15.1 54.8 48.6 18.5

2. Relatively

moist soil

28

21 21 45.3 38.9 24.7

Result: From the calibration curve the moisture percentage of relatively dry soil and relatively wet soil

were found 15 and 21, respectively. The gravimetric method has shown the moisture percentage 18.5

and 24.7, respectively, which indicates that the moisture percentage determined by both the method is

more or less accurate.

Precaution:

1. Tensiometer reading should be taken carefully.

2. Very dry soil in the field should be avoided.

3. Soil and water should be mixed uniformly.

4. The working condition of the tensiometer must be checked before fixing in the soil.

5. Fixing must be gentle to save the life of tensiometer.

DETERMINATION OF SOIL WATER INFILTRATION BY DOUBLE RING INFILTROMETRER

METHOD

INFILTRATION: Infiltration is the entry of water into the soil through the soil surface.

The entry of water from the surface to the soil is referred to soil water infiltration. It is a surface

characteristic and expressed in terms of inches/hr. or cm/hr. Quantitatively, infiltration rate is the

volume of water entering into the soil per unit area per unit time, when the soil is subject to a shallow

depth of ponding at the surface.

Several mathematical relationships have been proposed for infiltration study. One of the most widely

used equations is that of Kostiakov (1932) equation:

Ic = atb .......................................(I) Where, Ic = cumulative infiltration

a =Intercept

b = slope

t = time

Determination of “a” and “b” is made by converting equation (I) to its logarithmic form.

logIc = logatb

=log a + log tb

=log a + b log t

logIc=log a + b log t ........................(II)

Equation (I) again can be differentiated with respect to time (t) to obtain the rate of infiltration.

d(Ic) d(atb)

Rate of infiltration = = = abtb-1

d(t) d(t)

So, Rate of infiltration = a.b.tb-1

.......................(III)

Methods of determining infiltration:

Several methods can be used to determine the infiltration characteristics of a soil under field

conditions:

1. Cylinder (ring) infiltrometer method

2. Field plot method by ponding.

Cylinder (ring) infiltrometer method is a metallic cylinder, which is driven into the soil. Primarily it

measures the vertical rate of water movement into the soil surface. Ring infiltrometer infiltration data

are obtained by measuring the depth of water inside the ring at time intervals.

The soil water infiltration rate depends on some factors that are given as follows:

i. Soil moisture content at the time of irrigation

ii. Soil texture

iii. Soil structure

iv. Surface strata (plough pan, clay layers, etc.)

v. Surface soil compaction

vi. Surface sealing (sedimentation, erosion, dispersion, etc.)

vii. Soil cracking

viii. Crops and surface mulches

ix. Soil and water salt ion concentration

x. Soil and water temperature

Methods (Ring infiltrometer method): The following 2 methods can be used to measure the infiltration

rate: i. Single ring infiltrometer method

ii. Double ring infiltrometer method – Double ring infiltrometer is preferred to single ring method

as it reduces seepage of water outside the ring (central ring) from where (inner ring) data is to be

collected.

Equipments (Double ring infiltrometer method):

1. Double ring infiltrometer- Inner ring is 20-30 cm (30cm) in diameter and 30-40 (30cm) cm

in height, and the outer ring is 60-70 cm (60cm) in diameter 30-40 (30cm) cm in height

2. Scale for measuring the changes in water level in the central ring

1. Bucket

2. Stop watch

3. Shovel

4. A plastic sheet or other waterproof membrane

5. Source of water

6. Driving hammer

Procedure:

i. A suitable site is selected avoiding areas that have been affected by unusual animal or

machinery traffic. The central ring is driven into the soil up to about 15 cm depth with the

help of a hammer in such a way that no water gets out of the ring. Also outer buffer ring is

put into the soil up to about 10 cm depth by the same way. A plastic sheet is placed on the

soil of the inner ring. The plastic sheet should be in contact with the soil at the bottom of

the inner ring and extended up to the wall of the inner infiltrometer (ring) at least 15 cm in

height.

ii. The outer infiltrometer is then filled up with water to a depth roughly up to equal depth

desired in the inner ring. This level of water should be maintained throughout the period of

observation (mainly in case of outer ring).

iii. The inner infiltrometer is then filled up to 15 cm (this level is variable) with water. Meter

scale is then placed vertically in the soil inside the inner ring and the plastic membrane is

then quickly and carefully removed from the soil level of the inner ring and the time is

recorded simultaneously. Height of water decreases is recorded after 5 minutes interval.

The observations are repeated for 5 to 6 times with time intervals of 5 minutes for each

reading. Cumulative time and declined head is then calculated and cumulative infiltration is

found out from ordinary graph paper.

Outer ring

Inner ring

Scale

Soil level

Fig. Double ring infiltrometer

iv. Then the cumulative time (minute) and cumulative head decline (inches) is plotted in a log

log paper, and intercept „a‟ and slope „b‟ is obtained from log log paper.

Experimental data:

Number of

observations

Time interval

(minute)

Cumulative

time(minute)

Head decline

(inches)

Cumulative head

decline (inches)

1 5 5 1.2 1.2

2 5 10 0.8 2.0

3 5 15 0.7 2.7

4 5 20 0.6 3.3

5 5 25 0.6 3.9

Find out the infiltration rate in cm/sec. from the data given below:

i. Intercept a = 0.13cm

ii. Slope, b = 0.5

iii. Time, t = 10 minutes = 6oo seconds.

Solution:

We know Ic = atb = infiltration in time t

d( Ic) d(atb)

So, Infiltration rate, = = a.b.tb-1

= 0.13 × 0.5 × (600)0.5-1

d(t) d(t)

= 0.13 × 0.5 × (600)-0.5

= 0.13 × 0.5 × 0.0408

= 2.65 × 10-3

cm/sec

DETERMINATION OF SATURATED HYDRAULIC CONDUCTIVITY OF SOIL BY

CONSTANT HEAD METHOD

Hydraulic conductivity of soil:

When water, whose source is either rainfall, or irrigation, is applied to soil, it enters the soil pores

replacing the air contained in them. If sufficient quantity of water is available, the entire pore space

may be filled with water and the excess water would move downward by a physical process known as

saturated flow.

The flow of water through a porous material like sand was first studied by a French scientist named

Henry Darcy as early as 1856 and deduced his famous formula named as Darcy‟s law. It states that the

quantity of water passing through a porous medium is directly proportional to its cross sectional area,

hydraulic head difference and time, and inversely proportional to its length. The Darcy‟s law may be

written as follows:

Q α L

tLHA )(

So, Q = K L

tLHA )(

Where, Q = Volume of water passing through the porous medium, cm3

A = Cross sectional area, cm2

H + L = Hydraulic head difference, cm

T = time (second, hour, day etc.)

H = cylindrical sample holder

L = Length of the soil column/profile

K = The proportionality constant K in the Darcy‟s law, which is known as hydraulic

conductivity, cm/sec

The hydraulic conductivity is defined as or measure of the ease with which water can be transmitted

through a porous material. In other words, the hydraulic conductivity measures the ability of the soil to

conduct water through it and may be expressed as the proportionality factor K in the Darcy‟s equation:

K = dHAt

LQ

Where, dH = Head decline (=H + L)

K = dH

L

At

Q

Methods of measuring hydraulic conductivity:

The hydraulic conductivity of saturated soils to water is determined by 2 laboratory methods.

i. The constant head method

ii. The falling head method

DESCRIPTION OF THE CONSTANT HEAD METHOD:

Equipments required:

i. Core sampler

ii. Beaker

iii. Rubber tube

iv. Funnel

v. Scale

vi. Spade or Shovel or Shavol

vii. Conical flask

viii. Stand or clamp etc.

ix. Cloth

Procedure:

i. First collect a natural undisturbed soil sample by core sampler.

ii. Cover the lower end of the soil core with a cloth.

iii. Allow the soil sample to soak for 16 hours or more to saturate.

iv. Then connect an empty cylindrical sample holder having inlet and outlet to the top of the

soil sample tightly and place a small piece of blotting paper on the top of the sample.

v. Then add water slowly into the upper cylinder with the help of a rubber tube that connects

the inlet of the upper cylinder. Also connect a rubber tube with the outlet of the upper

cylinder to discharge the excess of water keeping the constant head in the upper cylinder,

where the delivery outlet of the upper cylinder again flows into the water source if water is

scarce.

vi. Maintain a constant head of water level at the top of the upper cylinder.

vii. Collect the percolated water in a beaker per unit time and record the volume of percolated

per unit time.

viii. Measure the length of the soil sample and head difference. Collect water at least thrice.

Nearly equal amount of water should be collected each time.

ix. Then calculate the hydraulic conductivity of water by applying the equation.

Factors affecting hydraulic conductivity:

I. In case of properties of fluid:

i. Viscosity of fluid

ii. Density of the fluid

iii. Interaction of fluid with the porous medium

II. In case of properties of soil:

i. Soil porosity

ii. Soil texture

iii. Soil structure

iv. Particle density

v. Bulk density

Example: In a constant head method of measuring hydraulic conductivity of soil, 20 cc of water was

collected in 30 minutes time from a soil core of length 10 cm, diameter 7 cm, with a hydraulic head

difference of 52.5 cm. Calculate the hydraulic conductivity of the soil.

Diameter of the core = 7 cm

Radius of the core = 3.5 cm (7/2 cm =3.5 cm)

The cross sectional area of the core = A = π r2 = 3.14 × (3.5)

2 = 3.14 × 12.25 = 38.50 cm

2.

Q = 20 cc.

t = 30 min.

L = 10 cm.

dH = hydraulic head difference = 52.5 cm.

Now, K = dHAt

LQ

Where, dH = Head decline = hydraulic head difference (=H + L)

= cmcm

cmcm

5.52min3050.38

10202

3

= 5.523050.38

1020

cm/min.

= 5.523050.38

24601020

cm/day.

= 4.75 cm/day.

Result: Hydraulic conductivity of soil = 4.75 cm/day.

DETERMINATION OF FREE CARBONATES PRESENT IN SOIL BY RAPID TITRATION

METHOD

Free calcium carbonate (and magnesium carbonate) is present in all calcareous soils. Carbonates of

sodium as well as small quantities of carbonates of other metals are present in alkaline soils. The

carbonates can be determined by decomposing them with acid and the amount of carbondioxide

evolved may be determined either gravimetrically or volumetrically. For quick estimation, the

carbonates may be reacted upon by a known amount of standard acid solution and the acid required to

decompose the carbonates may be determined volumetrically.

Test for the presence of carbonates:

Add few drops of concentrated HC1 to the soil sample. If carbonates are present, effervescence will

occur.

Effervescence

CaCO3 + 2HCl ========= CaCl2 + H2O + CO2

Equipments:

(1) Electric balance, (2) Clock glass, (3) Burette, (4) 20 ml pipette, Funnel, (6) 250 ml conical flask,

(7) Glass Rod, (8) Retort stand, (9) Clamp, (10) Tall 400 ml Beaker and (11) Filter paper etc.

Chemicals:

(1) N NaOH solution,

(2) N HCl solution and,

(3) Bromthymol Blue indicator solution.

Procedure:

1. Weigh 5 grams of finely powdered soil and transfer it to a tall 400 ml beaker.

2. Add 100 ml N HC1 solution and cover the beaker with a clock glass.

3. Stir occasionally with a glass rod during a period of one hour.

4. Transfer 20 ml of supernatant solution with the help of a 20 ml pipette. (If the supernatant

solution is turbid, filter the extract through a dry filter paper on a dry funnel and collecting the

filtrate in a dry beaker and then transfer 20 ml of the filtrate)

5. Add 6 to 8 drops of bromthymol blue indicator.

6. Titrate the hydrochloric acid extract against the N NaOH solution.

7. At the end point the colour of the solution will change to yellowish green (A precipitate forms

before the end point).

8. Record the amount of sodium hydroxide required to neutralize the acid extract of soil.

9. Run a blank experiment using all materials except the soil.

Calculation:

1 ml of NHC1 ≡ 0.05 g of CaCO3

The amount of NHCl required to neutralize the carbonates = (B-T) × f ml

Where, B = ml of NaOH solution required to neutralize 20 ml of HC1 employed.

T = ml of NaOH solution required to neutralize 20 ml of HC1 extract of soil.

f = Normality factor of NaOH solution.

(B-T) × f × 0.05 × 100 × 100

Hence percentage of CaCO3 present in soil =

20 × 5

= (B-T) × f × 5.

Again, from Blank titration we can get the actual strength of normal (2ndary standard) NaOH

V1S1 = V2S2

So, S1 = f = 1

22SV

V Where, V1 = Volume of approximately normal NaOH= 20.8 mL

S1 = Actual strength of N NaOH = ?

V2 = Volume of N HCl = 20 mL

S2 = Strength of N HCl =1N

Hence percentage of CaCO3 present in soil = (B-T) × f × 5.

DETERMINATION OF FREE CARBONATES PRESENT IN SOIL BY RAPID TITRATION

METHOD Personal copy

Reference: A Text Book of Soil Chemical Analysis by P. R. Hesse. pp. 45-46

Calcium and magnesium carbonates are not usually found in soils if the pH is less than 7 but if the pH

exceeds this value carbonates should always be tested for. If no sodium carbonate is present the pH of

a calcareous soil will seldom exceed 8.5 although, as discussed by Russel(1961), higher pH values can

occur under certain conditions.

Free calcium carbonate (and magnesium carbonate) is present in all calcareous soils. Carbonates of

sodium as well as small quantities of carbonates of other metals are present in alkaline soils. The

carbonates can be determined by decomposing them with acid and the amount of carbondioxide

evolved may be determined either gravimetrically or volumetrically. For quick estimation, the

carbonates may be reacted upon by a known amount of standard acid solution and the acid required to

decompose the carbonates may be determined volumetrically.

Test for the presence of carbonates:

Add few drops of concentrated HC1 to the soil sample. If carbonates are present, effervescence will

occur.

Effervescence

CaCO3 + 2HCl ========= CaCl2 + H2O + CO2

Equipments:

(1) Electric balance, (2) Clock glass, (3) Burette, (4) 20 ml pipette, Funnel, (6) 250 ml conical flask,

(7) Glass Rod, (8) Retort stand, (9) Clamp, (10) Tall 400 ml Beaker and (11) Filter paper etc.

Chemicals:

(1) N NaOH solution,

(2) N HCl solution and,

(3) Bromthymol Blue indicator solution.

Procedure:

10. Weigh 5 grams of finely powdered soil and transfer it to a tall 400 ml beaker.

11. Add 100 ml N HC1 solution and cover the beaker with a clock glass.

12. Stir occasionally with a glass rod during a period of one hour.

13. Transfer 20 ml of supernatant solution with the help of a 20 ml pipette. (If the supernatant

solution is turbid, filter the extract through a dry filter paper on a dry funnel and collecting the

filtrate in a dry beaker and then transfer 20 ml of the filtrate)

14. Add 6 to 8 drops of bromthymol blue indicator.

15. Titrate the hydrochloric acid extract against the N NaOH solution.

16. At the end point the colour of the solution will change to yellowish green (A precipitate forms

before the end point).

17. Record the amount of sodium hydroxide required to neutralize the acid extract of soil.

18. Run a blank experiment using all materials except the soil.

Calculation:

1 ml of NHC1 ≡ 0.05 g of CaCO3

The amount of NHC1 required to neutralize the carbonates = (B-T) × f ml

Where, B = ml of NaOH solution required to neutralize 20 ml of HC1 employed= 20.8 mL.

T = ml of NaOH solution required to neutralize 20 ml of HC1 extract of soil.

f = Normality factor of NaOH solution(Actual strength of normal NaOH).

(B-T) × f × 0.05 × 100 × 100

Hence percentage of CaCO3 present in soil =

20 × 5

= (B-T) × f × 5.

1L NHCl ≡ 1 g eq. wt. of CaCO3

or 1L NHCl ≡ ggg2

100

2

481240

2

3161240

CaCO3

or 1L NHCl ≡ 50g CaCO3

or 1mL NHCl ≡ 50mgCaCO3 ≡ 0.05g CaCO3

So, (B-T)×f mL NHCl ≡ 0.05×(B-T)×f g CaCO3

Now, 20mL filtrate contains (B-T)×f×0.05g CaCO3

So, 100 mL ,, ,, = 320

10005.0T)-(BgCaCO

f

Now, 100 mL filtrate ≡ 20g soil

So, 20 g soil contains = 320

10005.0T)-(BgCaCO

f

So, 100 g soil contains = 3520

1001000.05fT)-(BgCaCO

= (B-T)×f×5 g CaCO3

Again, from Blank titration we can get the actual strength of normal (2ndary standard) NaOH

V1S1 = V2S2

So, S1 = f = 1

22SV

V Where, V1 = Volume of approximately normal NaOH= 20.8 mL

S1 = Actual strength of N NaOH = ?

V2 = Volume of N HCl = 20 mL

S2 = Strength of N HCl =1N

So, S1 = f = 8.20

120 = 0.961N

For soil sample A:

Initial burette reading = 20.8

Final burette reading = 40.8

Difference in burette reading = 20

For soil sample B:

Initial burette reading = 0

Final burette reading = 19.3

Difference in burette reading = 19.3

So, For Soil sample A: 100 g soil contains = (B-T)×f×5 g CaCO3 = (20.8 - 20) × 0.961 × 5 = 3.84 g.

So, For Soil sample B: 100 g soil contains = (B-T)×f×5 g CaCO3 = (20.8-19.3) × 0.961 × 5 = 7.2 g.

Result:

Soil sample A contains = 3.84 % CaCO3.

Soil sample A contains = 7.2 % CaCO3.

DETERMINATION OF CARBONATE AND BICARBONATE BY DIFFERENTIAL

TITRATION METHOD

[Ref. The nature and properties of soils by Brady and Weil]

The primary source of carbonates and bicarbonates in soils is carbonic acid (H2CO3) that forms when

CO2 from microbial and root respiration reacts with water.

CO2 + H2O H2CO3 H+

+ HCO3-……. causing pH about 3.8

Because of this reaction, a solution that is in equilibrium with the CO2 in the soil atmosphere has a pH

of about 4.6. Generally in arid and semiarid soils base forming cations dominate the exchange complex

of soil causing the higher pH levels of soil due to which abundant OH- ions react with the H2CO3 to

form first the bicarbonate (HCO3-) and then the carbonate ion.

H2CO3 + OH- HCO3

- + H2O………………. causing pH about 8.3

HCO3- + OH

- CO3

2- + H2O ……………… causing pH about 10.0

A bicarbonate dominated soil solution in equilibrium with atmospheric CO2 will have a pH of about

8.3, while pH values of 10 or more occur in solutions dominated by soluble carbonates. At this

condition due to higher concentration of Na, K, Ca, Mg, etc. basic cations H2CO3 may not exist (be

formed) rather carbonates of Na, K, Ca, Mg, etc. are formed. Soluble carbonates are not likely to occur

in a soil if the pH is less than 9.5. Soluble carbonates are the carbonates of Na and K.

Other possible sources of carbonate in soil are calcareous parent material and lime whereas the sources

of bicarbonate are decomposition of organic matter and reduction of amorphous ferric compound and

sulphate ion under strongly reduced soil condition.

SO42-

+ 2CH2O + 2OH- S

2- + 2HCO3

- + 2H2O

(CH2O indicates general formula of organic matter)

Fe(OH)3 + e- Fe

2+ + 3OH-

(Fe2+

indicates the reduced state of iron)

CO2 + OH- HCO3

-

(This CO2 comes from decomposition of organic matter and at higher pH it produces bicarbonate)

The estimation of carbonate and bicarbonate in a mixture is carried out by titrating with a standard acid

first to the bicarbonate stage (pH approximately 8.3) using phenolphthalein as the indicator. The

titration is then continued to the carbonic acid stage (pH 3.8) with methyl orange as the indicator.

These reactions may be represented as follows:

2Na2CO3 + H2SO4 ====== 2NaHCO3 + Na2SO4

2NaHCO3 + H2SO4====== 2H2CO3 + Na2SO4

Apparatus required:

1. Erlenweyer flask

2. Burette

3. Pipette

4. Filter paper (Whatman No. 42.)

5. Mechanical shaker.

Reagent required:

1. 0.05N H2SO4 solution: Take 27.1 ml of con. H2SO4 (98%) in one liter volumetric flask and make the

volume up to the mark with distilled water. This is 1N H2SO4. Then dilute a requisite amount of this

solution by 20 times to get 0.05N H2SO4 solution.

2. Phenolphthalein indicator solution (1%): dissolve 1g of methyl orange indicator in 100ml ethanol.

3. Methyl orange indicator solution (0.1%): Dissolve 0.1g methyl orange indicator in 100 ml distilled

water.

Procedure:

1. Take 20 g soil into a 250 ml Erlenmeyer flask.

2. Add 100 ml of distilled water to it.

3. Shake for 30 minutes.

4. Filter through a Whatman No. 42 filter paper.

5. Transfer 50 ml of the filtrate into a 100 ml conical flask by means of a pipette.

6. Add 3 drops of Phenolphthalein indicator to it. If the solution is colorless, no carbonate is present

but if pink colour is seen titrate slowly and carefully with 0.05N H2SO4 till the pink color just

disappears. This indicates the bicarbonate end point.

7. Note the volume of the acid required, say „A‟ ml of 0.05N H2SO4 is required.

8. Add 2 drops of methyl orange indicator to the flask and continue the titration with 0.05N H2SO4 till

the indicator just changes to light rose red. Let the second portion of the titre value be „B‟ ml of 0.05N

H2SO4. This indicates the carbonic acid end point.

Calculation:

Molecular weight of CO32-

= 12+48 = 60; so, the gram equivalent weight of CO32-

= 60/2 g = 30 g

Molecular weight of HCO3- =1+12+48=61; so, equivalent weight of HCO3

- = 61g.

For CO32-

estimation:

1L N H2SO4 =1LN CO32-

solution = 30g CO3

2-

Or 1ml N H2SO4 = 30 mg CO32-

or 1 ml 0.05N H2SO4 =30× 0.05 mg CO32-

or „A‟ ml 0.05N H2SO4 = A×30× 0.05mg CO32-

Now, in 50 mL soil-water extract the amount of CO32-

= A×30× 0.05mg CO32-

100 mL ,, ,, ,, ,, ,, ,, =A×30× 0.05×100/50 mg CO32-

=A×30× 0.05×2 mg CO32-

So, in 1000 mL ,, ,, ,, ,, ,, ,, = A×30× 0.05×2×10 mg CO32-

=A×30 mg CO32-

So, the solution CO32-

content = A×30 mg L-1

Again,

100 mL soil water extract ≡ 20 g soil

Now, 20g soil contain = A×30× 0.05 ×2 mg CO32-

1000g ,, ,, = A×30× 0.05×2 ×1000/20 mg CO32-

= A×30×5 mg CO32-

So, the soil CO32-

content = A×30×5 mg kg-1

For HCO3- estimation:

1L N H2SO4 = 1LN HCO3- = 61g HCO3

-

Or, 1ml N H2SO4 = 61mg HCO3-

Or, 1ml 0.05 N H2SO4 = 61×0.05 mg HCO3-

Or, (B-A)ml N H2SO4 =(B-A) × 61×0.05mg HCO3-

Now, in 50ml soil-water extract the amount of HCO3-= (B-A)×61×0.05 mg.

Or, in 100 ml soil-water extract the amount of HCO3-= (B-A)×61×0.05×100/50mg HCO3

-

= (B-A) × 61×0.1 mg HCO3-

Or, in 1000 ml soil-water extract the amount of HCO3- = (B-A) × 61×0.1×10 mg HCO3

-

= (B-A) × 61 mg HCO3-

So, in soil water extract the amount of HCO3- = (B-A) × 61mg L

-1.

Now, In 100 ml soil water extract amount of HCO3- ≡ 20 g soil

So, In 1000 ml soil water extract ≡ 200 g soil

200 g soil contains (B-A) × 61 mg HCO3-

1000 g ,, ,, = (B-A) × 61×1000/200 mg HCO3-

= (B-A) ×61×5 mg HCO3-.

So, in soil the amount of HCO3- = (B-A) × 61×5 mg Kg

-1.

DETERMINATION OF CARBONATE AND BICARBONATE BY DIFFERENTIAL TITRATION

METHOD

The primary source of carbonates and bicarbonates in soils is carbonic acid (H2CO3) that forms when

CO2 from microbial and root respiration reacts with water.

CO2 + H2O H2CO3 H+ + HCO3

-

Because of this reaction, a solution that is in equilibrium with the CO2 in the soil atmosphere has a pH

of about 4.6. In soils with higher pH levels the abundant OH- react with the H2CO3 to form first the

bicarbonate (HCO3-) and then the carbonate ion.

H2CO3 + OH- HCO3- + H2O

HCO3- + OH- CO3

2- + H2O

A bicarbonate dominated soil solution in equilibrium with atmospheric CO2 will have a pH of about

8.3, while pH values of 10 or more occur in solutions dominated by soluble carbonates.

Other possible sources of carbonate in soil are calcareous parent material and lime whereas the sources

of bicarbonate are decomposition of organic matter and reduction of amorphous ferric compound and

sulphate ion under strongly reduced soil condition.

SO42-

+ 2CH2O + 2OH- S

2- + 2HCO3

- + 2H2O

Fe(OH)3 + e- Fe2+

+ 3OH-

CO2 + OH- HCO3

-

The estimation of carbonate and bicarbonate in a mixture is carried out by titrating with a standard acid

first to the bicarbonate stage (pH approximately 8.3) as the indicator. The titration is then continued to

the carbonic acid stage ( pH 3.8) with methyl orange as the indicator. These reactions may be

represented as follows:

Apparatus required

1.Erlenweyer flask

2.Burette

3.Pipette

4. Filter paper (Whatman No. 42.)

5.Mechanical shaker.

Reagent required: 1.0.05N H2SO4 solution: Take 27.1 ml of con. H2SO4 (98%) in one liter volumetric

flask and make the volume up to the mark with distilled water. This is 1N H2SO4. Then dilute a

requisite amount of this solution by 20 times to get 0.05N H2SO4 solution.

ELECTRICAL CONDUCTIVITY

According to Ohm's law the current flowing in an electric conductor is directly proportional to the

electromotive force and inversely proportional to the resistance in the conductor.

R

VI (1)

Where, I = flowing current (amp)

V= Electromotive force (volt)

R = Resistance (ohm)

The equation (1) can be written as

I

VR (2)

The resistance of a homogeneous material under given conditions depend on the quantity and shape of

the sample and its intrinsic property. If a material is of uniform cross section with an area of A square

centimetre and length of l centimetre, the longitudinal resistance is given by the equation

A

lR (3)

As R A

l So, R =

A

l

where is specific resistance. The specific resistance is the intrinsic property and it is the resistance

per centimetre with a uniform cross sectional area of 1 cm2.

It is worth to mention that there are two types of conductors. One is electronic conductor, metal, which

transfer electricity by vibration of atoms and another is electrolytic conductor where transfer of electric

current is caused by the movement of ions. With the increase in temperature metal conductor decrease

resistance and similarly ions in electrolytic conductor moves faster. In electrolytic conductor instead of

resistance, conductivity is measured. Logically, the reciprocal of resistance is termed conductance and

the reciprocal of specific resistance is termed specific conductance. The conductance is usually

denoted by the Greek letter lambda (Λ) and the specific conductance by kappa (Κ). However, the

reciprocal of equation (3) is given by

l

Al

R

1

Therefore, A

lA

or, A

lA

The unit of is -cm (ohm-cm), since is the reciprocal of so the unit of is 11cm (= ohm-

1 cm

-1 = mho cm

-1 and in plural mhos cm

-1). The inverse of ohm ( ) is mho. And it is interesting that

the mho, unit of conductance, has been named just reversing the order of the word ohm. However, now

a day, the international unit of conductance is decisimens per meter (dS m-1

).

Sub units of siemen:

1 siemen (S) = 10 decisimens (dS)

1 decisiemen (dS) = 10 centisiemens (cS)

1 centisiemen (cS) = 10 millisiemens (mS)

1 millisiemen (mS) = 10 microsiemens (S)

1 microsiemen (S) = 10 nanosiemens (nS)

So,

1 siemen(S)=10 decisimens(dS) =102 centisiemens(cS) =10

3 millisiemens(mS) = 10

6

microsiemens(S) = 109 nanosiemens (nS)

Sub units of meter:

1 m =103 mm

1 mm = 103

micrometer (m) ( m = , micron)

1 m = 103 nm

1 nm = 103 pc (pc = picometer)

1Angstrom (A0) = 10

-10 meter.

1 m = 1010

A0

Prove that 1 mmho cm-1

= 1 decisiemen m-1

Q 3. Prove that 1mmhos cm-1

= 1dSm-1

1 mmhos cm-1

= 1 mScm-1

= mS × 1cm

ms100

dS

DETERMINATION OF ELECTRICAL CONDUCTIVITY OF SOIL BY CONDUCTIVITY

METER METHOD

ELECTRICAL CONDUCTIVITY

Electrical conductivity is the phenomenon of a transfer of electricity carried by charged particles (ions

or colloids) under the force of an applied electric field. The EC is normally expressed as siemens/meter

(S/m) or deci siemens/meter (dS/m) or millisiemens/cm (mS/cm) or micro siemens/centimetre

(S/cm).

Electrical conductivity is generally measured to know the salinity of soils. It also gives an idea about

the charged materials present in a soil.

FACTORS AFFECTING EC:

1. Nature of clay and its interaction with charged ions.

2. Amount of soluble salts.

3. Amount of water.

4. Temperature.

5. Soil pH.

APPARATUS REQUIRED:

1. Conductivity meter.

2. Conductivity flow cell/beaker.

3. Glass rod.

REAGENT REQUIRED:

Standard 0.01 M KCL Solution: Dissolve 0.7456 gm of KCL in distilled water. Add water to make one

litre volume at 25°C. This solution has an EC of 1412 S/cm at 25°C ( =1.412 mS/cm ).

PROCEDURE:

1. Take 10 g soil in a 100 mL beaker/ conical flask.

2. Add 50 ml water to the soil (soil water ratio being 1:5)

3. Stir the suspension for 15 minutes with a glass rod.

4. Keep the suspension settled for 30 minutes.

5. Determine EC of supernatant in a conductivity meter.

CALCULATION:

Cell constant, K = S/cm)(1412KCL0.01MofvalueECObserved

S/cm)(1412KCLM0.01ofvalueECStandard

= S/cm1350

S/cm1412

(let) = 1.05

EC of soil extract = Observed EC × K

= 123 S/cm × 1.05

= 129.15 S/cm

= 129 S/cm (1:5 soil extract at 25°C temperature)

RESULT: The electrical conductivity of supplied soil sample was 129 S cm-1

.

DETERMINATION OF SOIL pH BY GLASS ELECTRODE pH METER

The most important chemical property of soil as a medium of plant growth is its pH value or

hydrogen ion activities. The activities of ions in soil that enter into the plant nutrition are highly

dependent upon that of H+-ion.

H+- ions are present in the soil in two different forms:

i. Reserve form where the H+-ions are present in the reserved condition as adsorbed by solid

particles.

ii. Active form where the H+ ions are in soil solution.

The pH of the soil is due to the activity of active H-ions of the soil solution. pH may conveniently be

defined as the negative logarithm of hydrogen ion concentration expressed in grams per litre or moles

per litre.

The glass electrode is the most widely used hydrogen ion responsive electrode. The exact fundamental

mechanism of the glass electrode is not fully understood. But it has been suggested that the glass

functions like a semipermeable membrane, permeable only to hydrogen ions. Thus when the electrode

is immersed into a soil suspension an electrical potential is developed which produces an impulse and

accordingly the galvanometric pointer gives direct reading of pH value of the soil suspension. An

asymmetric potential is developed at the glass liquid interface and also there may be some instrumental

error, or error due to voltage fluctuation, etc. But these will not introduce any error, since these are

balanced out during the standardization of the system against buffer solutions of unknown pH value.

Apparatus required:

1. A glass electrode pH meter with calomel reference electrode,

2. 50 ml beakers,

3. Short stirring rods,

4. Spatula,

5. Distilled water,

6. wash bottle, etc.

Reagents required: Standard buffer solutions of pH 4.0 and 7.0

Procedure:

i. To a 10 gm of soil in a 50 ml. beaker.

ii. Add 25 ml of distilled water and stir the suspension several times during the next

30 minutes.

iii. Let the suspension stand for about 1 hour to allow most of the suspended clay to

settle down from the suspension.

iv. Adjust the position of the electrodes in the clamps of the electrode holder so that

upon lowering the electrodes into the beaker, the glass electrode will be immersed

well into the partly settled suspension, and the calomel electrode will be immersed

just deep enough into the clean supernatant solution to establish a good electrical

contact through the ground glass joint or the fiber capillary hole.

v. Then immerse the electrode into the partly settled soil suspension and measure the

pH. Report the result as “Soil pH measured in water” (soil : water ratio being

1:2.5).

Result: pH of the supplied soil sample was:

Precautions:

1. Since the soil pH measure varies widely with the method of preparation of a given soil, the

details of the preparation procedure must be carefully specified with any soil pH data.

2. Care must be taken not to allow the electrode to remain in the test solution or suspension longer

than necessary, especially if more alkaline than pH 9.0.

3. Immediately after testing, the electrode must be washed off with a strong stream of distilled

water from a wash bottle. If the system was alkaline, the electrode should be dipped for a few

seconds in acid pH buffer of diluted HCl to remove the film of CaCO3 that sometimes forms.

4. For storage, after cleaning the electrode is suspended in distilled water, and the system is

protected from evaporation. Drying out of the electrode is thus avoided.

5. The glass and reference electrode should be thoroughly washed with distilled water after each

measurement and then rinsed with several portion of the next test solution before making the

following measurement.