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.

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.

3. The total number of negative sites on clay minerals is represented by the Cation Exchange Capacity (CEC).

Flocculation means to bring particles togetherDispersion pushes particles apart.

- charge

- charge

- charge

Al3+

Al3+Al3+

2000 B.C.

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Small, higher-charged cations tend to flocculate clay particles.

Large cations with low charge tend to disperse clay particles

Ca2+, Mg2+, Al3+

Na+, K+

Iron oxides originate from iron-bearingprimary or secondary minerals.

Reduced iron (Fe2+) occurs in low-oxygenenvironments 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 colorsoften 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-, PO4

3-

Anion Exchange

Silicate clays possess negative chargedue to isomorphous substitution during

the formation of the mineral.

Iron and aluminum oxides are products of weathering and can possess negative

positive or zero charge. Charge derives fromInteraction with hydrogen ions in solution.

Cation Exchange

Cation or Anion Exchange

Silicate Clays vs. Al/Fe oxides

Soil Organic Matter and Soil Organic Matter and Organic ColloidsOrganic Colloids

Organic matter

plant debris or litter in various stages of decomposition and includes the living organisms in the soil

Accumulation of partially disintegrated and decomposedplant 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 MatterSoil Organic Matter

Increases water-holding capacity/porosity

Can increase infiltration rates.

Aids in soil aggregation, structure

Principal source of essential plant nutrients

Energy source for soil microorganisms

Soil Organic MatterSoil Organic Matter

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 carbonor, organic matter is 58% organic carbon

Categories

Composition

Plant Materials

Carbon (42%)Hydrogen (8%)Oxygen (42%)

Nitrogen, Sulfur, Phosphorus, Potassium

CompositionComposition

plants

elementscompounds

Soils and Global Carbon

CarbonCarbon

Soils contain more that 4x moreCarbon than all vegetation combined.

Amounts Amounts

2400 pentagrams (1015 g) in soil

700 pentagrams as soil carbonates

Storage of earth carbon

Sugars, starchesCrude proteinsHemicelluloseCelluloseFats, waxesLignins, phenols

Compounds

Rapid Decomposition

Slow Decomposition

Decomposition

Majority of breakdown results inCarbon dioxide, water, energy and heat

Essential elements (N, P, S) are releasedThis is called “mineralization”

Highly resistant compounds are formed whichremain in the soil for long periods: “humification”

Decomposition

The biochemical breakdown of mineral and organic materials.

Some of the substrate carbon is incorporated intothe cells of microorganisms: called “immobilization”

HumusHumus

Highly resistant to breakdown

amorphous, colloidal, organic substances

(possessing no plant cellular organization)

Can be highly reactive due to carbon content, surface area, and charge

Humic Substances

a series of high-molecular-weight amorphous compounds

Humic Acids

Fulvic Acids

decay products of higher plants and microbial residue.

products of fulvic acids and other decay products

Impacts of SOM on Soil Impacts of SOM on Soil Chemical PropertiesChemical Properties

Cation ExchangeSoil 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 HClSulfuric Acid H2SO4

Nitric Acid HNO3

Carbonic Acid H2CO3

Acetic Acid HC2H3O2

Ammonium NH4+

HCl H+ + Cl-

HNO3 H+ + NO3-

H2SO4 H+ + HSO4-

Strong Acids

Reaction goes to completion (complete dissociation)

Ammonium NH4+

Carbonic Acid H2CO3

Acetic Acid HC2H3O2

Weak Acids

NH4+ NH3 + H+ (residual NH4

+)

H2CO3 HCO3- + H+ (residual H2CO3)

HC2H3O2 C2H3O2- + H+ (residual HC2H3O2)

Incomplete dissociation

NH4+ NH3 + H+ (residual NH4

+)

H2CO3 HCO3- + H+ (residual H2CO3)

HC2H3O2 C2H3O2- + H+ (residual HC2H3O2)

Incomplete Dissociation

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 waterHigh 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

Orange Juice = 4.2

Beer = 4.3

Vinegar = 3.0

Pure Rain = 5.6

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 (low pH)

Weak Acid

Relevance to Soil Organic Matter

Organic Matter

Carbon (42%)Hydrogen (8%)Oxygen (42%)

Nitrogen, Sulfur, Phosphorus

Accumulation of partially disintegrated and Decomposed Plant and animal residues.

Humus: amorphous, colloidal, organic substances

CarbonOxygenHydrogen

58% carbon

CarbonHydrogenOxygen

Cation ExchangeCation Exchange

COOH

OH

carboxylic

Enolic/phenolic

Acid functional groups

COOH COO- + H+

OH O- + H+

Both are weak acids (incomplete dissociation)

HCl H+ + Cl-

NH4+ NH3 + H+ (residual NH4

+))

Low pH = lots of H+ = less dissociation = low chargeHigh pH = little H+ = more dissociation = high charge

COOH COO- + H+

OH O- + H+

The dissociation of weak acids is inhibited by H+ in solution

Soil solution

Dissociation of HydrogenDissociation of Hydrogen

COOH COO- + H+

OH O- + H+

organic strand-C-C-C-C-

Soil solution

Low pH = lots of H+ = less dissociation = low chargeHigh pH = little H+ = more dissociation = high charge

COOH COO- + H+

OH O- + H+

Both are weak acids (incomplete dissociation)

The dissociation is inhibited by H+ in solution

Cation AdsorptionCation Adsorption

COOH COO- + H+

COO- + K+ COO---K Adsorbed cation

COOH

O-

K+K+

COO-

COO-

COO-

COO-

OHO-

O-

O-

COO-

K+K+

Ca2+

Na+K+

Na+

Na+

Na+

Mg2+

Mg2+Mg2+

Mg2+

Na+

Na+

Na+

Soil Solution

Organic strand

Cation ExchangeCation ExchangeCOOH COO-

OH O-

Functional Groups

H+

H+

H+

H+

- -

-

-

-

Na+

K+

K+

Na+

Na+

K+

K+

K+

Na+

Mg2+Mg2+

Mg2+

Na+

Na+

Na+

Mg2+

Mg2+

K+

K+

K+

Cations and Organic MatterCations and Organic Matter

CEC = 100 – 500 cmol/Kg

Kaolinite 2-5 cmol/kgVermiculite 100-180 cmol/kg

Si

AlSi

Ca2+, Mg2+, Zn2+, Mn2+, K+, NH4

+,Na+, H+, Mn2+

Mineral

organic

Cation ExchangeCation Exchange

Si

AlSi

Total CEC =

Mineral Organic

+

Total Cation Exchange Capacity

pH-Dependent

Mineral Colloids derive charge from substitution Of lower-charged cations for higher charged cationsIn the crystal matrix during mineral formation. The Result is permanent negative charge.

Organic colloids derive their charge from dissociationof hydrogen ions from acidic functional groups on organic matter/humus. The result is pH-dependentcharge

Mineral Colloids – 0 – 180 cmol/kgOrganic Colloids – 100 – 500 cmol/kg

Reactivity of Soil Horizons

Contribution to fertility.