This document provides information about soil fertility and nutrient management. It discusses key concepts like soil fertility, plant nutrients, integrated plant nutrient management, and how physical properties, water, and problem soils impact nutrient management. Organic matter, soil reactions, fertility evaluation methods, and nutrient interactions are also covered. The document lists various topics to be covered in the syllabus like nutrient functions, deficiency diagnosis, and principles of determining macro and micronutrients. It pays tribute to notable scientists who contributed to the field.

1. Soil Fertility and Nutrient Management
Dr. Pabitra Kumar Mani
Experiential Learning,
NRM Module-1,
ACSS-451(1+1=2)

2. Syllabus: ACSS-451, 2+1
Soil Fertility and Nutrient Management
Theory:
Concepts of soil fertility and productivity
Plant Nutrients- functions, toxicity, deficiency, diagnosis critical
limits, hidden hungers
Integrated plant nutrient Management including micronutrients
 Soil Physical properties and nutrient Management
Soil water and Nutrient Management
 Fertility constraints in problem soils and their managements
Organic matter and soil fertility
Soil reactions and Nutrient management
Soil Fertility evaluation methods
Principles of determination of NPKS and micro nutrients
Nutrient Interactions and Chelates in Nutrient management

3. M N Saha, J C Bose, J C Ghosh./ Snehamoy Dutt, S N Bose, D M Bose, N R Sen, J N Mu
Tribute to
Padmabhusan JN Mukherjee

4. Soren P.L. Sorensen invented
the pH scale in 1909.

5. The Ionization of Water Is Expressed by an Equilibrium Constant
The degree of ionization of water at equilibrium (Eqn 2–1) is small; at 25 °C only
about two of every 109
molecules in pure water are ionized at any instant. The
equilibrium constant for the reversible ionization of water (Eqn 2–1) is
In pure water at 25°C, the concentration of water is 55.5 M (grams of H2O in 1 L
divided by its gram molecular weight: (1,000 g/L)/(18.015 g/mol)) and is essentially
constant in relation to the very low concentrations of H and OH, namely, 1X 10-7
M.
Accordingly, we can substitute 55.5 M in the equilibrium constant expression (Eqn 2–
3) to yield
which, on rearranging, becomes
where Kw designates the product (55.5 M)(Keq), the ion product of water at 25 °C.
The value for Keq, determined by electrical-conductivity measurements of pure
water, is 1.8 X 10 -16
M at 25°C. Substituting this value for Keq in Equation gives the
value of the ion product of water:

6. Thus the product [ H+
][ OH-
] in aqueous solutions at 25°C always equals 1x10 -14
M2
.
When there are exactly equal concentrations of H+
and OH-
, as in pure water, the solution
is said to be at neutral pH. At this pH, the concentration of H+
and OH-
can be calculated
from the ion product of water as follows:

9. [ ][ ]
[ ]HAca
K
−+
=
AcH
[ ] [ ]
[ ]-
Ac
H
HAcKa
=+
[ ]HAc
Aclog
a
pKpH



 −
+=∴
This form of ionisation constant equation is called the
Henderson- Hasselbalch equation.
pH of an weak acid
[ ] 


 −



 += AcHHAc

10. How a pH meter works
When one metal is brought in contact with another, a voltage
difference occurs due to their differences in electron mobility.
 When a metal is brought in contact with a solution of salts or
acids, a similar electric potential is caused, which has led to the invention
of batteries.
 Similarly, an electric potential develops when one liquid is
brought in contact with another one, but a membrane is needed to keep
such liquids apart.
A pH meter measures essentially the electro-chemical potential
between a known liquid inside the glass electrode (membrane) and an
unknown liquid outside. Because the thin glass bulb allows mainly the
agile and small hydrogen ions to interact with the glass, the glass electrode
measures the electro-chemical potential of hydrogen ions or the
potential of hydrogen.
To complete the electrical circuit, also a reference electrode is
needed. Note that the instrument does not measure a current but only an
electrical voltage, yet a small leakage of ions from the reference
electrode is needed, forming a conducting bridge to the glass electrode.

11. Typical electrode system for
measuring pH. (a) Glass
electrode (indicator) and
saturated calomel
electrode (reference)
immersed in a solution of
unknown pH.
(b) Combination probe
consisting of both an
indicator glass electrode
and a silver/silver chloride
reference. A second
silver/silver chloride
electrode serves as the
internal reference for the
glass electrode.
The two electrodes are
arranged concentrically with the internal reference in the center and the
external reference outside. The reference makes contact with the analyte
solution through the glass frit or other suitable porous medium.
Combination probes are the most common configuration of glass electrode
and reference for measuring pH.

12. The majority of pH
electrodes available
commercially are
combination electrodes that
have
both glass H+
ion
sensitive electrode
and
 additional reference
electrode conveniently
placed in one housing.

13. Glass electrode Reference electrode Combined electrode

14. In principle it should be possible to determine the H+
ion activity or concn. of a soln by
measuring the potential of a Hydrogen electrode inserted in the given soln. The
EMF of a cell, free from liquid junction potential, consisting of a Hydrogen
electrode and a reference electrode, should be given by,
E = E ref – RT/F ln aH+ F= Faraday constant ,96,485
E = E ref + 2.303 RT/F pH R= 8.314 J/mol/°K
∴pH = ( E- Eref )F/2.303 RT T= Kelvin scale
So, by measuring the EMF of the Cell E obtained by combining the H electrode with a
reference electrode of known potential, Eref , the pH of the soln. may be evaluated.
The electric potential at any point is defined as the work done in bringing a unit
charge from infinity to the particular point
Reduced state Oxidised state + n Electron⇋
M = Mn+
+ nE
E(+) = E0 – (RT/F) ln aM
n+
Nernst Equation
Principles of pH Meter

16. For soils containing predominantly negatively charged clays,
dilution of the soil solution by distilled water increases the absolute value
of the surface potential and changes the distribution of H+
ions between
the DDL and the bulk solution.
The proportion of H+
ions in the DDL relative to the bulk solution
increases so that the measured pH, which is the bulk solution pH, is
higher than that of the natural soil.
(H+
)ddl > (H+
) bulk solution
(pH)ddl < (pH)bulk solution
(pH)bulk solution > (pH)ddl

17. where Ci and zi are the concentration and charge, respectively, of ion i in a
mixture of ionic species i = 1 to n. The term ‘concentration’ refers to the mass
of an ionic species per unit volume of solution (e.g. mol/L).
An individual ion experiences weak forces due to its interaction with
water molecules (the formation of a hydration shell), and stronger electrostatic
forces due to its interaction with ions of opposite charge. Effectively, this means
that the ability of the ion to engage in chemical reactions is decreased,
relative to what is expected when it is present at a particular concentration
with no interactions. This effect is accounted for by defining the activity ai of ion i,
which is related to its concentration by the equation
where fi is the activity coefficient of the ion. Values of fi range from 0 to 1. In
very dilute solutions where the interaction effects are negligible, fi
approaches 1, and ai is approximately equal to Ci. There is extensive theory on
the calculation of activity coefficients, but all calculations make use of the ionic
I = 0.0127 x Ecw
Griffin and Jurinak, 1973
Lewis and Randall, 1921
Intensity of the electrical
field due to the ions in soln

18. Peter Debye (1884-1966) Professor of Chemistry at Cornell University
in 1940. He received the Nobel Prize in Chem. in 1936

20. In the pH measurement, the reference and indicator electrodes are immersed in a
heterogeneous soil suspension composed of dispersed solid particles in an
aqueous solution.
If the solid particles are allowed to settle, the pH can be measured in the
supernatant liquid or in the sediment. Placement of the electrode pair in the
supernatant gives a higher pH reading than placement of the electrodes in
the sediment. This difference in soil pH reading is called the suspension effect.
Suspension Effect in Soil pH Measurement
This suspension effect has been interpreted in two ways.
First, according to Marshall,
the higher activity of H+
ions near the clay surface causes the lowest pH.
Second, according to Coleman et al.
the suspension effect is mostly from the electrical charge on the soil which
differentially affects the mobilities of K+
and Cl-
in the calomel electrode, not
the glass electrode. In a strongly negatively-charged soil, the mobility of K+
would be high and that of Cl-
low while in a positively-charged tropical soil the
opposite would occur.

21. The difference in pH(ΔpH) between the sediment and supernatant
produced during soil pH determination is termed the 'suspension effect′
(McLean 1982). If the soil suspension is allowed to settle, the pH as
measured in the suspension liquid is often higher than in the sediment
layer.
The suspension effect is influenced by the
 extent to which electrodes encounter clay and humus particles
the soil CO2 is in equilibrium with atmospheric concentrations, and
 the magnitude of liquid junction effects (Foth and Ellis, 1988).
The suspension effect is minimised by measuring soil pH using
0.01 M CaCl2 or 1 M KCl solutions instead of water (Sumner, 1994).
In addition, the use of CaCl2 or KCl solutions has the advantages of
(i) decreasing the effect of the junction potential of the calomel reference
electrode,
(ii) equalising the salt content of soils, and

22. A relatively simple method to determine whether the net charge of the soil
colloids is negative, zero, or positive, is the analysis of soil pH in 1 N KCl and
in water. The difference between the two pH values is called ΔpH,
Δ pH = pH(KCI) –pH(H2O)
The value for Δ pH can be positive, zero, or negative, depending on the
net surface charge .
A negative Δ pH indicates the presence of negatively charged clay
colloids and the soil pH is above ZPC.
A positive value means the presence of a positively charged clay
colloid and soil pH is below the ZPC.
The ZPC is reached when Δ pH equals zero.
This is generally true if only KCl is present in the system, since KCl is
considered to be an indifferent electrolyte

23. Possible pH Ranges Under Natural Soil Conditions
black walnut: 6.0-8.0
Most desirable
carrot: 5.5-7.0
cucumber: 5.5-7.0
spinach: 6.0-7.5
tomato: 5.5-7.5
white pine: 4.5-6.0
Very
strong
Strong Moderate Slight Slight Moderate Strong
Very
strong
Neutral
Acid Basic
3 4 5 6 7 8 9 10 111 2 12 13 14
Most agricultural soils
Extreme pH range for most mineral soils
cranberry:4.2-5.0
apple: 5.0-6.5

24. Soil pH & Nutrient Availability

27. Schematic representation of a hypothetical soil zone and the
various chemical processes that occur in a soil solution.

28. Soil pH is one of the most important factors affecting the uptake of Mo
by plants. The beneficial effects of liming acid soils to increase Mo
solubility are obvious. The concentration of MoO4
2-
, which is the form
most readily available to plants, increases 100-fold for each unit
increase in pH (Lindsay, 1972).
With increasing pH, the amount of the soluble MoO4
2-
species in
equilibrium with soil Mo is much greater than those for HMoO4
-
and
H2MoO4. At a pH of 5 or 6, the ion HMoO4
-
becomes
dominant, and at very low pH values the un-ionized acid H2MoO4 and
the cation MoO2
2+
are the principal species present (Krauskopf, 1972).
The MoO4
2-
anion exists in an exchangeable form in the soil. Thus, the
fact that Mo availability to plants increases with increasing pH may
possibly be explained by an anion exchange of the type 2OH-
↔
MoO4
2-

29. Soil pH
0
100
4.0 5.0 6.0 7.0 8.0
Plant benefit
Plant Injury
Toxicity Deficiency
Relativeplantyield(%) (Weil and Kroontje,1984)
General relationship of plant health and soil pH. Some nutrients can reach toxic
levels at low pHs, while deficiencies can occur at high pHs.

30. Factors Influencing pH
Organic Matter
Soil Parent Material (carbonates, etc.)
Temperature and Precipitation
Agricultural Practices
Organic matter affects the buffering capacity of the soil
(more SOM-resistance to change pH)
Soil parent materials such as basalt have higher pH levels than those of parent
materials from granite.
High temperature and moisture leach bases from soil .
Agricultural practices tend to lower pHAgricultural practices tend to lower pH
•crops removing nutrients
• leaching nutrients through soil;
•decomposition of organic materials;
• fertilization, particularly ammonium fertilizers;
•Liming materials raise soil pH

31. Uptake of NH4
+
cations causes the roots to release equivalent positive charges
in the form of H+
cations, which lower the pH.
When a NO3
-
anion is taken up, the roots release a HCO3
-
which raises the pH.

32. Structure and properties of diffuse double layer
Because of the thermal motion, the exchangeable ions are distributed
within a certain space, form a diffuse layer or ion swarm. The structure
of which for a given particle is determined by the
(i)Surface Charge density
(ii)Kind of counter ions
(iii)Temperature
(iv)Concentration of electrolytes (salts) in the solution.
The exchangeable ions are surrounded by water molecules and may
thus be considered as forming a solution which is often called a
micellar solution or inner solution as distinct from the outer solution
of the free electrolytes, the so called inter-micellar solution.

33. Schematic representation of ion and potential distribution in the double
layer according to the theories of Helmholtz, Gouy and Stern,
ψi = Total potential ; ψδ
= zeta potential, x = distance from particle
surface, σ = surface charge density; δ= thickness of the Stern layer

34. Parallel-Pate Condenser Model: The Helmholtz-Perrin Theory
The electrified interface consists of two
sheets of opposite charge, one on the
electrode and the others on the
solution. Hence the term double layer.
The charge densities on the two sheets
are equal in magnitude but opposite in
sign, exactly as in a parallel-plate
capacitor.
The Helmholtz –Perrin model would be
quite satisfactory for electro capillary
curves (ecc) which are perfect parabola
Limitations:
Slight assymetry of parabolic curve
 The dependence of ecc on the nature of the
anions present in the electrolyte.
Differential capacity changes with potential
H. L.F. von
Helmholtz
ecc

35. The ionic distribution at a negatively charged clay
surface.
Gouy- Chapman Model
Assumptions:
The surface is assumed to
be flat, or infinite extent and
uniformly charged.
Ions are assumed to be
point charges distributed as
per Boltzmann distribution
ni =n0
iexp[ -zie0Ψx/κT]
n0
i =no of ions in bulk soln
ni =no of ions at a distance x
from the surface
Ψx = electrical potential
Κ = Boltzmann constant
Zi = valence, e0= charge of electron
Solvent will influence the dl
only through its uniforrn
dielectric constant
The potential decays
exponentially into the soln;
deep enough inside the solution,
x→α, the potential becomes zero

37. Stern Model:
In the stern model the double
layer is divided into two parts
with a compact layer adjacent
to the surface in which the
potential changes linearly from
Ψ0 toΨδ , as an Helmholtz
classical molecular condenser
type double layer.
 The remainder of the model
comprises a diffuse Gouy-
Chapman layer in which the
potential drops from Ψδ to Ψα
O. Stern (NL) 1943
Schematic representation of the
structure of the electric double layer
according to Stern's theory

38. Grahame Model:
Grahame (1947) refined the
Stern Model by splitting the
Stern layer into two to allow
consideration of two types of
strongly adsorbed ion or ions.
Nearest the solid surface
Grahamme recognised an
Inner Helmholtz plane (IHP)
in which the adsorbed ions
lose some of their water of
hydration and an outer
Helmholtz plane (OHP)
supposed to contains normally
hydrated counter ions close
to the colloid surface.

42. To summarize, the double layer consists of three constituents:
a) A inner layer (inner Helmholtz layer) in which the potential changes
linearly with the distance. It comprises the absorbed Water molecules
and sometimes the specifically adsorbed anions.
b) An outer Helmholtz layer. It comprises hydrated (solvated) cations.
The potential varies linearly with the distance.
c) An outer diffuse layer, also called the Guoy-Chapman layer, which
contains excess cations or anions distributed in a diffuse layer. The
potential varies exponentially with the distance, F.

43. Reference Electrodes
2.) Silver-Silver Chloride Reference Electrode
 Convenient
- Common problem is porous plug becomes clogged
Eo
= +0.222 V
Activity of Cl-
not 1E(sat,KCl) = +0.197 V
Reduction potential

45. Electrodes and Potentiometry
pH Electrodes
2.) Glass Membrane
 Irregular structure of silicate lattice
Cations (Na+
) bind
oxygen in SiO4 structure

46. pH Electrodes
2.) Glass Membrane
 Two surfaces of glass “swell” as they absorb water
- Surfaces are in contact with [H+
]

47. pH Electrodes
2.) Glass Membrane
 H+
diffuse into glass membrane and replace Na+
in hydrated gel
region
- Ion-exchange equilibrium
- Selective for H+
because H+
is only ion that binds
significantly to the hydrated gel layer
H(0.05916)constant pE β−=
Charge is slowly carried
by migration of Na+
across glass membrane
Potential is determined
by external [H+
]
Constant and b are measured when electrode is calibrated with solution of known pH

48. If a difference of a H+
concn
exists between the inner buffer and
the outer soln, a potential difference
develops between the inner and
outer sides of the membrane glass
which is proportional to the
difference in pH between the inner
buffer and the outer solution. To
be able to measure the membrane
potential, the membrane itself has
to be conductive. This is achieved
by the mobility of the alkaline ions
in the membrane glass (Li+ ions in
most glasses today or Na+ ions in
older membrane glasses).
The thickness and composition of
the gel layer determine the
response time and the
characteristic slope of the glass
electrode. Therefore the gel layer
is of critical importance
to the electrode performance.

51. Junction Potential
1.) Occurs Whenever Dissimilar Electrolyte Solutions are in Contact
 Develops at solution interface (salt bridge)
 Small potential (few milli volts)
 Junction potential puts a fundamental limitation on the
accuracy of direct potentiometric measurements
- Don’t know contribution to the measured voltage
Again, an electric potential is generated by a separation of charge
Different ion mobility results in
separation in charge
Mobilties of ions in water at 25o
C:
Na+
: 5.19 × 10 –8
m2
/sV K+
: 7.62 × 10 –8
Cl–
: 7.91× 10 –8