The Origin of Negative Charges in Soil :- Isomorphous Substitution- Major source of negative charges in 2:1 layer clays and called permanent or pH independent charge. Silicon in the tetrahedral sheet is subjected to replacement by ions of similar size, usually by Al3+ and produce one net negative charge. Al in the octahedral sheet maybe replaced by Mg2+ , without disturbing the crystal structure and produce one net negative charge. The difference in dimension of substituted ions was reported to be NO more than 15%, and the valency between those substituted ions should not differ more than one unit. Such a substitution takes place following the Goldschmidt’s laws of crystal chemistry. 2 ) The Dissociation of Exposed Hydroxyl Groups :- The appearance of OH groups on crystal edges or on exposed planes, can also give rise to negative charges. Especially at high pH, the hydrogen of these hydroxyls dissociate slightly, and the surface of clay is left with the negative charges of the oxygen ions. This type of negative charge is called variable charge or pH-dependent charge. The magnitude of the variable charge varies with pH and type of colloids. It is an important type of charge for 1:1 layer, iron and aluminum oxide clays and organic colloids. The Origin of Positive Charges in Soil :- Soil colloids may also exhibit positive charges as well as negative charges. The positive charges make possible anion exchange reaction and are very important in phosphate retention. These charges are arise from the protonation or addition of H+ ions to hydroxyl groups. The mechanism depends on pH and the valence of the metal ions. It is usually important in Al and Fe oxide clays, but it is of less important in Si-oxides. At a soil pH<ZPC, the surface of the mineral is positively charged and has the capacity to attract anions. If an Al3+ ion substitutes for one of the Mg2+ ions, a positive charge results. Such positive charges are characteristic of trioctahedral sheets of chlorites. Goldschmidt’s : Law 1- Isomorphous replacement of one cation by another, without incurring any change in the order of the crystal pattern, is permitted provided that the radii of the cation replaced and the cation substituting it agree with in 15%. Isomorphous replacement is responsible for permanent net negative charge. 2. H.V. Helmholtz- German Scientist: The concept of Zero point charge (ZPC) was first proposed by Hermann Von Helmholtz, in the late 19th century. He also proposed ZPC term first time. ZPC is the pH at which Charge on surface of mineral particle is Zero. 3. K. H. Tan: In 1979, K.H. Tan proposed the concept of variable charge soils, which are soils that contain both permanent and variable surface charges. He demonstrated that the permanent charge is due to the presence of isomorphic substitution, which leads to a net negative charge on soil particles. The term “variable charge” is used to describe organic and inorganic soi

1. Topic- Five Scientists Who Have
Contributed on Charge
Development of Soil
Course No.: SSC-503
Course Name: Soil Chemistry
Presented by- Arvind Yadav
M.Sc. (Ag.) 1st Sem. 2nd Year
ID. No- 21412SAC022.
Enrollment number- 435652
Presented to – Prof. Surendra Singh
Prof. Nirmal De
Dr. Amitava Rakshit

2. Introduction
The Origin of Negative Charges in Soil :-
1) Isomorphous Substitution-
• Major source of negative charges in 2:1 layer clays and called permanent or pH independent charge.
• Silicon in the tetrahedral sheet is subjected to replacement by ions of similar size, usually by Al3+ and
produce one net negative charge.
• Al in the octahedral sheet maybe replaced by Mg2+ , without disturbing the crystal structure and produce
one net negative charge.
• The difference in dimension of substituted ions was reported to be NO more than 15%, and the valency
between those substituted ions should not differ more than one unit.
• Such a substitution takes place following the Goldschmidt’s laws of crystal chemistry.
2 ) The Dissociation of Exposed Hydroxyl Groups :-
• The appearance of OH groups on crystal edges or on exposed planes, can also give rise to negative charges.
• Especially at high pH, the hydrogen of these hydroxyls dissociate slightly, and the surface of clay is left with
the negative charges of the oxygen ions.
• This type of negative charge is called variable charge or pH-dependent charge.
• The magnitude of the variable charge varies with pH and type of colloids.
• It is an important type of charge for 1:1 layer, iron and aluminum oxide clays and organic colloids.

3. The Origin of Positive Charges in Soil :-
• Soil colloids may also exhibit positive charges as well as negative charges.
• The positive charges make possible anion exchange reaction and are very important in phosphate
retention.
• These charges are arise from the protonation or addition of H+ ions to hydroxyl groups. The mechanism
depends on pH and the valence of the metal ions.
• It is usually important in Al and Fe oxide clays, but it is of less important in Si-oxides.
• At a soil pH<ZPC, the surface of the mineral is positively charged and has the capacity to attract anions.
• If an Al3+ ion substitutes for one of the Mg2+ ions, a positive charge results. Such positive charges are
characteristic of trioctahedral sheets of chlorites.

4. 1. Goldschmidt’s :
• Law 1- Isomorphous replacement of one cation by another, without incurring any
change in the order of the crystal pattern, is permitted provided that the radii of the
cation replaced and the cation substituting it agree with in 15%.
• Isomorphous replacement is responsible for permanent net negative charge.
2. H.V. Helmholtz- German Scientist:
• The concept of Zero point charge (ZPC) was first proposed by Hermann Von
Helmholtz, in the late 19th century.
• He also proposed ZPC term first time.
• ZPC is the pH at which Charge on surface of mineral particle is Zero.
3. K. H. Tan:
• In 1979, K.H. Tan proposed the concept of variable charge soils, which are soils that contain both
permanent and variable surface charges.
• He demonstrated that the permanent charge is due to the presence of isomorphic substitution, which leads
to a net negative charge on soil particles.
• The term “variable charge” is used to describe organic and inorganic soil constituents with reactive surface
groups whose charge varies with pH and ionic concentration and composition of the soil solution.
Scientists Who Have Contributed on Charge
Development of Soil

5. 4. J. M. Bigham:
• In 1991, J.M. Bigham proposed the mechanism of pH-dependent surface charge, which is due to the
protonation and deprotonation of functional groups on soil particles. He showed that the pH-dependent
charge affects soil nutrient availability and pollutant adsorption.
Protonation S-OH + H+ SOH2+ (Development of anion exchange capacity)
Deprotonation S-OH S-O- + H+ ( Development of cation exchange capacity)
5. Thompson and Way (1850) –
Thompson and Way made more systematically some quantitative measurements with soils and ammonium
sulphate on the basis of which they were in position to formulate the following generalizations:
I. Soil, when leached with a solution of ammonium sulphate on a filter paper, release predominantly Ca
and other cations like Mg and K in small amounts, but retain ammonium.
II. Total amount of ammonium retain is equivalent to that of calcium and other cations so released.
III. Retained ammonium helps in predicting negative charge on the clay colloids.