Soil Formation:

Weathering of rocks: Physical


Soil Organic Matter

Surface Charges in soils

Permanent Charge

In clays and other minerals different charged ions in crystal structure are substituted during the formation of the mineral.

(Al +3 for Si +4 ; Mg +2 for Al +3, Fe +2 for Al +3)

pH Dependent Charge

At low pH, soil charge becomes more positive.

At high pH, negative charge increases.

Overall charge is usually negative.

Soil Organic Matter usually has a negative charge due to presence of carboxyl, phenolic groups:

Charge on soil particles affects:

     mobility of inorganics in soil

     mobility of organics in soil

     soil buffering of pH changes


Range of composition of humic material:


45-55% Carbon

30-45% Oxygen

3-6% Hydrogen

1-5% Nitrogen

Significant amount of humus exists as colloidal particles

Generally carries a negative charge and has very high surface area.

Sorption Properties of Soil Organic Material

Heavy metals


An element in the soil may exist in three forms :

Fixed, adsorbed, and dissolved.

Cation Sorption:

Like layer silicates, humic substances also have a surface negative charge, because of dissocation of and OH groups

Sorbed ions can be exchanged, depending on relative concentrations


Total amount of sorbed cations is defined as the cation exchange capacity CEC -- CEC is mostly defined by the clay and SOM content of soil.

The amount exchanged depends on the distribution coefficient:

Kd = Cs / Cw (Conc in soil /Conc in water)

These Kd vary with ion and with different types of soils:

The most strongly sorbed is the ion with the smallest hydrated radius and the highest charge: For example: Ba+2 < Ca+2 < Mg+2 and Cs+1 < K+1 <Na+1 < Li+2


Complex formation in aqueous phase : Formation of a complex will tend to make the cation more leachable.

Anion Sorption

Most soil particles are negatively charged, but some anions are also bound:

Metal Oxides--

M-O-H in equilibrium with soil water:

At higher pH values MOH + OH- MO- + H2O

At lower pH MOH + H+ MOH2+


The edges of Aluminosilicate mineral grains have some atoms which are not fully coordinated, so they can also behave in the same manner as MOH.


Soil Organic Matter (SOM) has some functional groups which form positive sites: e.g. R-N-H3+

Anion exchange capacity of soil materials is much lower than the CEC--on order of 20 x less.


Ion exchange is stiochiometric and reversible.


Extent of fixation, sorption of positive and negative ions, complex formation and the kinetics of the sorption reactions all affect the the rate of movement of an element through soil.

     Higher surface area decreases the movement of elements

     Higher pH will decrease cation mobility

     Anion mobility is decreased in high clay soil, and in high metal oxide content soils.

     Higher pH will increase anion mobility


Acid base reactions in soil


Sources of soil acids:


     Organic acids from microbial oxidation of plant material

     Oxidation of H2S from anerobic decomposition of biomass

     Microbial oxidation of NH4+ from fertilizer

     Phosphate fertilizers

     Acid cat clay from weathering of FeS

     Acid rain

     Dry acid deposition


Soil pH--Soil is a buffer

Materials responsible for soil buffering


     aluminosilicate minerals

     oxides and hydroxide minerals



soil pH Ranges

Soil Type


Soil Type


Very strongly acidic

4.5 - 5



Strongly Acidic

5 - 5.5

Mildly Alkaline

7.4 - 7.9

Moderately Acidic

5.5 - 6

Moderately Alkaline

7.9 - 8.4

Mildly Acidic

6 - 6.5

Strongly Alkaline

8.4 - 9


Salt affected soils


Sorption of Organic Compounds in Soil


q       Molecular Size

q       Hydrophobicity

q       Molecular Charge

q       Hydrogen Bonding

q       Coordination


Soil Solution

In capillaries and in films--not easy to sample


Dissolved ions:


Importance of soil to agriculture

Pesticides: Federal review policies for agricultural pesticides


Delaney Clause as related to pesticides:


Plant nutrients











Wastes and pollutants


Soil Erosion

Biochemistry--Genetic engineering and the green revolution


Contamination of Food by Agricultural Practice