Organic Matter
1. Aluminosilcates are composed of two fundamental
units: silica tetrahedra and aluminum octahedra to
form sheet-like structures.
2. Cation substitutions can take place in either the
tetrahedral sheet or the octahedral sheet which
result in negative charge on the mineral.
3. The total number of negative sites on clay minerals
is represented by the Cation Exchange Capacity (CEC).
4. Positively charged cations are attracted to the negative
sites on clay minerals which can function as storage
for important plant-essential nutrients.
5. Preference for cations at mineral surfaces is dictated
by charge, concentration in solution and size of the cation.
Flocculation means to bring particles together
Dispersion pushes particles apart.
- charge
Na+
Al3+
Al3+Na+ Na+
3+
Na+ Al Na+
Na+
Na+
Na+ Na
+
- charge
- charge
2000 B.C.
Small, higher-charged cations tend to
flocculate clay particles.
Ca2+, Mg2+, Al3+
Large cations with low charge tend to
disperse clay particles
Na+, K+
Iron oxides originate from iron-bearing
primary or secondary minerals.
Reduced iron (Fe2+) occurs in low-oxygen
environments and results in a greyish color.
Oxidized iron (Fe3+) occurs in high oxygen
environments and results in orange-red colors.
Water restricts diffusion of oxygen in soils
Grey colors interspersed with orange-red colors
often indicates the presence of water tables in soils.
Iron and Aluminum Oxides
Can possess negative, positive, zero charge
Potential interaction with cations and anions
Cl-, F-, Br-, SO42-, NO3-, CO32-, PO43Anion Exchange
Silicate Clays vs. Al/Fe oxides
Silicate clays possess negative charge
due to isomorphous substitution during
the formation of the mineral.
Cation Exchange
Iron and aluminum oxides are products
of weathering and can possess negative
positive or zero charge. Charge derives from
Interaction with hydrogen ions in solution.
Cation or Anion Exchange
Soil Organic Matter and
Organic Colloids
Organic matter
plant debris or litter in various stages of decomposition
and includes the living organisms in the soil
Soil Organic Matter
Accumulation of partially disintegrated and decomposed
plant and animal residues as well as living biomass .
Decomposition principally by soil microorganisms
Transitory soil constituent (hours to 100s of years)
Requires continual addition to maintain O.M. levels.
1 – 5% (by weight) in a typical, well-drained mineral soil
Soil Organic Matter
Aids in soil aggregation, structure
Increases water-holding capacity/porosity
Can increase infiltration rates.
Principal source of essential plant nutrients
Energy source for soil microorganisms
Categories
Soil Organic Matter:
Natural C-containing organic materials living or dead
Microbial Biomass:
It is the living population of soil microrganisms.
Litter:
It comprises the dead plant and animal debris
on the soil surface.
Macroorganic Matter:
Organic fragments from any source which
are > 250µm (generally less decomposed than humus).
Humus:
Material remaining in soils after decompostion of
macroorganic matter.
Organic Carbon:
The carbon content is commonly used to characterize
the amount of organic matter in soils.
Organic matter = 1.724 x percent organic carbon
or, organic matter is 58% organic carbon
Composition
Plant Materials
Carbon (42%)
Hydrogen (8%)
Oxygen (42%)
Nitrogen, Sulfur, Phosphorus, Potassium
Composition
plants
compounds
elements
Soils and Global Carbon
Carbon
Soils contain more that 4x more
Carbon than all vegetation combined.
Amounts
2400 pentagrams (1015 g) in soil
700 pentagrams as soil carbonates
Storage of earth carbon
Decomposition
Compounds
Sugars, starches
Crude proteins
Hemicellulose
Cellulose
Fats, waxes
Lignins, phenols
Rapid Decomposition
Slow Decomposition
Decomposition
The biochemical breakdown of mineral
and organic materials.
Majority of breakdown results in
Carbon dioxide, water, energy and heat
Essential elements (N, P, S) are released
This is called “mineralization”
Some of the substrate carbon is incorporated into
the cells of microorganisms: called “immobilization”
Highly resistant compounds are formed which
remain in the soil for long periods: “humification”
Humus
amorphous, colloidal, organic substances
(possessing no plant cellular organization)
Highly resistant to breakdown
Can be highly reactive due to carbon content, surface area, and charge
Humic Substances
a series of high-molecular-weight amorphous compounds
Fulvic Acids
decay products of higher plants and microbial residue.
Humic Acids
products of fulvic acids and other decay products
Impacts of SOM on Soil
Chemical Properties
Cation Exchange
Soil Acidity
Absorption of Organic Compounds
Soil Organic Matter, Acidity, and Reactivity
Acid
Any substance which increases the
hydrogen ion concentration in solution.
+
H
Common Acids
Hydrochloric Acid
Sulfuric Acid
Nitric Acid
Carbonic Acid
Acetic Acid
Ammonium
HCl
H2SO4
HNO3
H2CO3
HC2H3O2
NH4+
Strong Acids
HCl
H+ + Cl-
HNO3
H+ + NO3-
H2SO4
H+ + HSO4Reaction goes to completion
(complete dissociation)
Weak Acids
Ammonium
Carbonic Acid
Acetic Acid
NH4+
H2CO3
HC2H3O2
NH4+
NH3
+ H+
(residual NH4+)
H2CO3
HCO3-
+ H+
(residual H2CO3)
HC2H3O2
C2H3O2-
+ H+
(residual HC2H3O2)
Incomplete dissociation
Incomplete Dissociation
NH4+
NH3
+ H+
(residual NH4+)
H2CO3
HCO3-
+ H+
(residual H2CO3)
HC2H3O2
C2H3O2-
+ H+
(residual HC2H3O2)
NH4+
NH3 + H+
In pure water, the amount of dissociation is known
Incomplete Dissociation
NH4+
NH3 + H+
In pure water, the amount of dissociation is known
High amounts of NH3 and/or H+ inhibit dissociation
The reaction is inhibited in acid solutions (high (H+))
pH
A measure of the amount of Hydrogen ions in water
- Log (H+)
Low pH = High amount of Hydrogen ions in water
High pH = Low amount of Hydrogen ions in water
Low pH = High amount of Hydrogen ions (acidic)
High pH = Low amount of Hydrogen ions (basic)
Scale: 1 - 14
Battery Acid = < 1
Coca Cola = 2.8
Vinegar = 3.0
Orange Juice = 4.2
Beer = 4.3
Pure Rain = 5.6
Incomplete Dissociation
Weak Acid
NH4+
NH3 + H+
In pure water, the amount of dissociation is known
High amounts of NH3 and/or H+ inhibit dissociation
The reaction is inhibited in acid solutions (low pH)
Relevance to Soil Organic Matter
Organic Matter
Accumulation of partially disintegrated and
Decomposed Plant and animal residues.
Carbon (42%)
Hydrogen (8%)
Oxygen (42%)
Nitrogen, Sulfur, Phosphorus
Humus: amorphous, colloidal, organic substances
58% carbon
Carbon
Oxygen
Hydrogen
Cation Exchange
COOH
OH
carboxylic
Enolic/phenolic
Acid functional groups
Carbon
Hydrogen
Oxygen
COO- + H+
COOH
O- + H +
OH
Both are weak acids (incomplete dissociation)
H+ + Cl-
HCl
NH4+
NH3
+ H+
(residual NH4+))
COOH
COO- + H+
Soil solution
OH
O- + H +
The dissociation of weak acids is inhibited by H+ in solution
Low pH = lots of H+ = less dissociation = low charge
High pH = little H+ = more dissociation = high charge
Dissociation of Hydrogen
Soil solution
COOH
COO- +
H+
organic
strand
-C-C-C-C-
OH
O- +
H+
COOH
OH
COO- + H+
O- + H +
Both are weak acids (incomplete dissociation)
The dissociation is inhibited by H+ in solution
Low pH = lots of H+ = less dissociation = low charge
High pH = little H+ = more dissociation = high charge
Cation Adsorption
COOH
COO- + K+
COO- + H+
COO---K
Adsorbed cation
Cation Exchange
COOH
O-
Soil Solution
Ca2+
COO-
Mg2+
Na+
COOCOO-
K+
K+
Na+
OH
+
O- Na
COO-
Organic strand
Functional Groups
COOH
COOOH
O-
K+
K+
+
O- Na
O- Mg2+
COO-
H+
H+
Mg2+
Na+
H+ Na+
K+
Mg2+
H+
Na+
Na+
Na+
-
-
-
-
K+
K+
Na+
Cations and Organic Matter
K+
Mg2+
Na+
K+
K+
CEC = 100 – 500 cmol/Kg
Mg2+
Mg2+
K+
K+
Na+
K+
Na+
Na+
Mg2+
Mg2+
Kaolinite
2-5 cmol/kg
Vermiculite 100-180 cmol/kg
Cation Exchange
Mineral
organic
Si
Al
Si
Ca2+, Mg2+, Zn2+,
Mn2+, K+, NH4+,
Na+, H+, Mn2+
Total Cation Exchange Capacity
Mineral
Total CEC
=
Si
Al
Si
pH-Dependent
Organic
+
Mineral Colloids derive charge from substitution
Of lower-charged cations for higher charged cations
In the crystal matrix during mineral formation. The
Result is permanent negative charge.
Organic colloids derive their charge from dissociation
of hydrogen ions from acidic functional groups on
organic matter/humus. The result is pH-dependent
charge
Mineral Colloids – 0 – 180 cmol/kg
Organic Colloids – 100 – 500 cmol/kg
Reactivity of Soil Horizons
Contribution to fertility.