Soil Fertility and Nutrient Management

Dr. Pabitra Kumar Mani

Experiential Learning,

NRM Module-1,

ACSS-451(1+1=2)

Syllabus: ACSS-451, 2+1Soil Fertility and Nutrient Management

Theory: Concepts of soil fertility and productivityPlant Nutrients- functions, toxicity, deficiency, diagnosis critical limits, hidden hungersIntegrated plant nutrient Management including micronutrients

Soil Physical properties and nutrient ManagementSoil water and Nutrient Management

Fertility constraints in problem soils and their managementsOrganic matter and soil fertilitySoil reactions and Nutrient managementSoil Fertility evaluation methodsPrinciples of determination of NPKS and micro nutrientsNutrient Interactions and Chelates in Nutrient management

M N Saha, J C Bose, J C Ghosh./ Snehamoy Dutt, S N Bose, D M Bose, N R Sen, J N Mukherjee, N C Nag

Tribute to Padmabhusan JN Mukherjee

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

The Ionization of Water Is Expressed by an Equilibrium ConstantThe 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:

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:

HAcaK

AcH

-AcH HAcKa

HAc

AclogapKpH

This form of ionisation constant equation is called the Henderson- Hasselbalch equation.

pH of an weak acid

AcH HAc

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.

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.

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.

Glass electrode Reference electrode Combined electrode

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

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

This effect is especially noticeable in saline soils.

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 equationwhere 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 strength I, such as in the Debye–Huckel limiting law (Atkins, 1982)

I = 0.0127 x EcwGriffin and Jurinak, 1973

Lewis and Randall, 1921

Intensity of the electrical field due to the ions in soln

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

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.

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 particlesthe 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 (iii) Preventing dispersion of the soil (Tan, 1995).

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

Possible pH Ranges Under Natural Soil Conditions

black walnut: 6.0-8.0

Most desirable

carrot: 5.5-7.0cucumber: 5.5-7.0

spinach: 6.0-7.5tomato: 5.5-7.5

white pine: 4.5-6.0

Verystrong Strong Moderate Slight Slight Moderate Strong

Verystrong

NeutralAcid Basic

3 4 5 6 7 8 9 10 111 2 12 13 14

Most agricultural soilsExtreme pH range for most mineral soils

cranberry:4.2-5.0

apple: 5.0-6.5

Soil pH & Nutrient Availability

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

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 MoO42-

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

- becomesdominant, and at very low pH values the un-ionized acid H2MoO4 and the cation MoO2

2+ are the principal species present (Krauskopf, 1972).

The MoO42- 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-

Soil pH

0

100

4.0 5.0 6.0 7.0 8.0

Plant benefit

Plant Injury

Toxicity DeficiencyRel

ativ

e pl

ant y

ield

(%)

(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.

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

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.

Structure and properties of diffuse double layerBecause 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.

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

Parallel-Pate Condenser Model: The Helmholtz-Perrin TheoryThe 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 parabolaLimitations: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

The ionic distribution at a negatively charged clay surface.

Gouy- Chapman ModelAssumptions: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 =n0iexp[ -zie0Ψx/κT]

n0i =no of ions in bulk soln

ni =no of ions at a distance x from the surfaceΨx = electrical potentialΚ = Boltzmann constantZi = 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

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 layeraccording to Stern's theory

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.

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.

Reference Electrodes

2.) Silver-Silver Chloride Reference Electrode

Convenient- Common problem is porous plug becomes clogged

Eo = +0.222 V

Activity of Cl- not 1E(sat,KCl) = +0.197 V

Reduction potential

Electrodes and Potentiometry pH Electrodes

2.) Glass Membrane Irregular structure of silicate lattice

Cations (Na+) bind oxygen in SiO4 structure

pH Electrodes

2.) Glass Membrane Two surfaces of glass “swell” as they absorb water

- Surfaces are in contact with [H+]

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

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

If a difference of a H+ concnexists between the inner buffer and the outer soln, a potential difference develops between the inner and outer sides of the membrane glasswhich is proportional to the difference in pH between the inner buffer and the outer solution. Tobe 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 thecharacteristic slope of the glass electrode. Therefore the gel layer is of critical importanceto the electrode performance.

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 25oC:

Na+ : 5.19 × 10 –8 m2/sV K+ : 7.62 × 10 –8 Cl– : 7.91× 10 –8