Soil colloids are very small _____or

_______ particles with a large surface area

per unit mass.

organic, inorganic

Soil colloids are where most _____  ______ in soils happen.

chemical

reactions

Clays (and other inorganic colloids) are mostly

________  _________ formed from alteration

(chemical transformation) of primary

minerals.

secondary minerals

Effects of Colloids on Soil 

Chemical Properties

– nutrient availability

– pH

– cation exchange capacity

– buffering capacity

Effects of Colloids on Soil Physical Properties

– structure

– stickiness

– shrinking/swelling

– plasticity

– infiltration rate

Colloids with electrical charge are?

– Layered aluminum silicate clays

• Also called phyllosilicates or layer silicates

• Largest and most broadly important group of soil colloids

– Iron and Al oxide colloids

• Important in Ultisols, Oxisols, some Andisols

– Organic colloids

The most common soil clays are crystalline,  

layered __________ silicates.

aluminum

Silica tetrahedra form _________sheets

tetrahedral

Alumina _______ form octahedral sheets

 These sheets stack in specific orders (repeating units of

T‐O‐T or T‐O) to form layers

octahedra

Tetrahedral and octahedral sheets are connected

by sharing _______molecules to form a crystal

layer

1:1 clays T‐O, T‐O, etc. Example  _______

 kaolinite

2:1 clays, T‐O‐T, T‐O‐T, etc. Example  ____________

montmorillonite

The layers are held together by ___________ or weaker inter‐particle bonds to form clay mineral particles.

H bonds

Interlayers are important features of ____________ clay

minerals

2:1 

__________ (a 1:1 mineral) has an interlayer that is

not accessible to water and ions

Kaolinite

In some layer silicates the interlayers expand and are accessible to water and cations. Name 2 things that this is important for?

• The internal surface area is important for cation exchange

• Interlayer expansion is responsible for clay shrink‐swell

properties

Electrical charge on soil clay minerals

One of the most important properties of clays.  

Comes from two sources:

– Isomorphous substitution is the largest source of charge in most soils.  

• It is always negative and occurs mostly in 2:1 clays.

– pH dependent charge (ionizable H+) is important on humus, oxides, and amorphous colloids.  

• The charge and its magnitude vary with pH (charge can be positive or negative).

This net neg. charge must be balanced by adsorbing ions

(cations) from soil solution.  This leads to:

cation exchange capacity.

Isomorphous substitution?

Cations of lower charge may substitute for higher

charge cations during mineral formation or

weathering.

Al³⁺

may substitute for _____

in the tetrahedral layer

Si⁴⁺

Divalent cations (e.g. Mg²⁺) can subsitute for ______

in the octahedral layer

Al³⁺

What creates a creates a net negative charge on the mineral?

Isomorphous Substitution

pH Dependent Charge

Originates from:

– Silicate clays

• Si‐OH or Al‐OH at particle edges

• The octahedral surface of 1:1 minerals

– Humus

• functional groups containing H (COOH, NH)

– Oxides

• Al‐OH or Fe‐OH groups at edges

pH Dependent Charge

can be :

positive or negative.

pH dependent charge becomes more positive at _________ pH,

more negative at ______ pH.

low, high

H⁺ may attach to or detach from ______ located on the clay

edges (depending on pH).  Creates a negative or positive charge

O atoms

Types of Soil Clay Minerals 

Iron and Aluminum Oxides?

The presence of metal oxides is indicative of intense weathering conditions (past or present).

– Important in ultisols and oxisols, some andisols

– No isomorphous substitution

• Charge

– pH dependent charge

net negative charge at high pH,

 net positive charge at low pH

• Examples are– Gibbsite Al₂ O₃

– Goethite FeOOH

– Hematite Fe₂O₃

Types of Soil Clay Minerals

Organic Colloids?

Humus

– Organic matter is often abbreviated as OM

– Humus refers to the fairly stable fraction of the soil

organic matter that remains after plant residues are

broken down by microorganisms

– Organic colloids are non‐crystalline and composed

mainly of C, H, O, N, S

– They are usually negatively charged in soils because of

ionization of functional groups

• pH dependent charge

Organic Colloids

Have a charge because of:

– pH dependent charge H⁺

may attach to or detach from O atoms located on the organic functional groups (depending on pH).  Creates a negative or positive charge. example:  Carboxylic acid 

Ion Exchange in Soils?

• An ion attached to the solid phase (on a

colloid) is exchanged with an ion in the solution phase (in pore water)

• Occurs on surfaces of

– clay minerals

– inorganic compounds

– organic matter

– roots

Major soil cations

• Acidic soils

– K⁺, NH₄⁺, H_⁺, Ca⁺², Mg⁺², Al⁺³

• Non‐acidic soils

 Na⁺,K^+, NH₄^+, Ca^+2,Mg⁺²

Cation Exchange

Because soil colloids are mostly negatively

charged, cation exchange is more important than anion exchange

– Exchangeable cations are a major source of plant

nutrients

– The ability of a soil to resist pH change (buffer

capacity) is related to the amount of cation exchange

– Cation exchange sites “hold” cations thereby slowing

leaching loss