Study

1. pH
• The most important characteristic of the soil
solution is its reaction. The soil reaction describes
the degree of acidity or alkalinity. The acidity or
alkalinity is expressed in terms of concentration of H
ions or OH ions.
• pH values are used to express the concentration of
H or OH ion.
• Sorenson (1909) devised the present pH scale.

2. • Water is a neutral substance with equal concentration of
H and OH ions. The ionization constant of water is 10-14.
• That is at neutral, the concentration of products of (H+)
and (OH-) is 10-14 g/l.
• The pH scale ranges from 0-14.
Conc.H+ x Conc OH- = Kw = 10-14
At neutral the concentration of H+ = 10-7 or = 0.0000001
g/l.
The reciprocal of this is 1/0.0000001 = log 10 10000000
the log of which is 7.

3. • Thus the pH can be defined as the logarithm of
reciprocal of H+ ion concentration or the negative
logarithm of H ion concentration (hydrogen ion
activity).
• It is also called as soil reaction.
• If the concentration of H ion in a solution is
0.000001 g/l (10-6) the pH is 6.0. Then at pH 6 the
concentration of H ion is 10 times more as
compared to that at pH 7.

4. • Since the product of concentration of H+ and OH- ions is
always equal to 10-14 we can know the concentration of
OH- ions if we know the pH value.
• A pH of 6.0 would indicate the concentration of OH-
ions as 10-8. Since the soil acidity is mainly due to H+ ions
a pH 7.0 indicates neutrality as the concentration of H+
and OH- are equal.
• Above pH 7.0 the concentration of OH- is more and it
indicates alkalinity.

6. • pH measurement
• Principle:
• A glass electrode in contact with H+ ions of the solution
acquires an electric potential which depends on the
concentration of H+ ions. This is measured potentiometrically
against some reference electrode, which is usually a calomel
electrode.
• The potential difference between glass electrode (indicator)
and calomel electrode is (reference) expressed in pH units.

7. • Two electrodes are used in the determination of pH. One is
reference electrode, which provides a standard voltage. The
reference electrode is usually a saturated calomel electrode
which has two layers
(1) Saturated solution of KCl and
(2) mixture of solid HgCl2 and Hg.

9. • Electrical conductivity (EC)
• The electrical conductivity (EC) measurement gives the
total amount of soluble salts present in the soil and is
expressed as milli mhos cm-1 or dS m-1.
Principle
• As the amount of the soluble salts in a solution
increases the electrical conductivity also increases. This
electrical conductivity is measured in terms of the
resistance offered to the flow of current using a
conductivity bridge.

10. Conductivity meter

11. • It is known that solutions offer some resistance to the
passage of electric current through them, depending upon
the concentration of salts present.
• Hence EC is measured in terms of electrical resistance
between parallel electrodes immersed in the soil suspension
of water.
• In such a system, the solution between the electrodes
becomes the electrical conductor to which the physical laws
relating to resistance are applicable.
• Modern conductivity meters are calibrated to read directly
the electrical conductance with given cell.

12. • The electrical resistance “R” is directly proportional to the
distance “L” between the electrodes and inversely proportional
to the cross sectional area “A” of the conductor.
Hence R = L/ A or R= r x L/ A
Where r = proportionality constant known as electrical resistivity
If L = 1 cm and A = 1 cm2 then R = r.
Where, r is called specific resistivity. Hence specific resistance is
the resistance of a conductor 1 cm in length and 1 cm2 in area.
• Higher the salt content, higher the passage of current and
lesser the resistance to the flow of the current.
• Hence the reciprocal of specific resistivity is called as specific
conductivity. Therefore specific conductivity is defined as the
conductivity of a solution enclosed in a cell whose electrodes
are exactly 1 cm and possess a surface area of 1 cm2.

13. Oxidation-reduction potential or
Redox potential
• Redox potential (Eh) is otherwise called oxidation
reduction reaction.
• Oxidation is the addition of oxygen or removal of
hydrogen. Reduction is the addition of hydrogen or
removal of oxygen.

14. • In oxidation process, energy is released, electron is
donated, or loss of electrons occurs.
• In reduction energy is accepted or acceptance of
electrons or gain of electrons. This is also called as
electron acceptance process.
• The compound, which donates electron or hydrogen, is
called hydrogen donor or electron donor. The compound,
which that accepts electron is called electron acceptor.

15. • A substance is said to be oxidized when it looses it’s
electron or when it act as a hydrogen donor and a
substance is said to be reduced when it gains electron or
when it act as hydrogen acceptor.
• In any chemical reaction every oxidation or loss of
electron is accompanied by reduction or gain of
electron. This system is called electron transfer system.

16. • Redox potential is defined as “the capacity or
potentiality of a compound either to oxidize or
to reduce a substance or compound and this is
expressed by electron unit called mV.

17. • In soil system, soil organic matter provides the energy
for release of electron.
Higher oxidation – positive (+) volt
Higher reduction – negative (-) volt
• The redox potential is denoted by the symbol Eh
Where E denotes energy
h = Poise (stability of the system)
• Eh is one of the physicochemical characters of soil
used to determine whether the soil is submerged or
aerated.
• When the soil is oxidized or aerated then the Eh is +ve.
• Soil is in reduced condition, then Eh is –ve.