soils exam 3, part 1

What is a soil colloid?

very small particles with high surface area and net surface charge

What are the main types of colloids?

organic and mineral

What gives organic colloids their net charge?

dissociation of H+ from carboxyl, phenolic OH and alcohol OH groups

What gives phyllosilicate clays their negative charge?

isomorphous substitution in tetrahedral or octahedral sheets

What are the building blocks of phyllosilicates?

silica tetrahedra and aluminum octahedra

What is isomorphous substitution?

replacement of one ion by another of similar size but different charge, creating permanent charge

1:1 clay example and characteristics

kaolinite, low CEC, non expanding

2:1 expanding clay example and characteristics

smectite, very high CEC, high swelling

2:1 non-expanding clay example

illite (fine-grained mica)

Non-crystalline silicate examples

allophane and imogolite

What minerals dominate highly weathered soils?

Fe and Al oxides (goethite, gibbsite)

CEC definition

the total quantity of negative charge available to attract cations (cmolc/kg)

What is ECEC?

effective CEC using unbuffered salt (NH4Cl)

Effect of pH on CEC of organic matter

CEC increases as pH increases

Cation order of attraction (strong to weak)

Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+

What is cation saturation?

the percent of CEC occupied by a specific cation

What is charge density?

Influence of valence + hydrated radius on attraction strength

Flocculation vs dispersion

come together, separate

Which clay has the highest CEC

smectite (smash)

Which clay has the lowest CEC?

kaolinite (KO)

Which clays have a short range order ~30cmolc/kg

allophane and imogolite

Which clay swells the most?

smectite

Which clays do not swell?

kaolinite and Al/Fe oxides

What is a 1:1 clay?

one tetrahedral and one octahedral layer

What is a 2:1 clay?

two tetrahedral layers around 1 octahedral layer

dioctahedral =?

Al

trioctahedral =?

Mg

Ammonium acetate (NH₄OAc) at pH 7 measures…

buffered CEC

How do crystalline silicate clays form?

They form from the weathering of primary minerals (feldspar and micas) which then break down into layered structures made of Si tetrahedra and Al octahedra

Why do crystalline silicate clays have a net negative charge?

Because of isomorphic substitution (lower charge ions replace higher charge ions creating permanent negative charge)

Soil physical and chemical properties =

colloid type and amount

Soil buffering capacity =

CEC

What does “soil physical & chemical properties = fn(colloid type, colloid amount)” mean?

Soil properties depend on the type of colloids present (1:1 and 2:1 clays, oxides, OM) and the amount of these colloids. Different colloids have different surface charges, CEC, and swelling behaviors, so they control nutrient retention, aggregation, water holding, and shrink–swell. The more colloids a soil has, the stronger these properties are expressed.

Explain soil buffering capacity = fn(CEC)

Soil buffering capacity depends on CEC, because CEC determines how many acidic cations (H⁺ and Al³⁺) colloids can hold. These acidity pools release or absorb H⁺ when acids or bases are added, preventing large pH changes. Soils high in clay and organic matter have high CEC and buffer strongly, while sandy soils with low CEC change pH quickly. Therefore, buffering capacity increases as CEC increases.

What are the acidic cations?

H+ and Al3+

pH

concentration of H+ in soil solution

buffer

system that resists change to pH when acids or bases are added

Why is soil buffered, and how does the buffer system work?

Soil is buffered because colloids hold large pools of H⁺ and Al³⁺ that can release or absorb H⁺; when acids or bases are added, these pools adjust to restore equilibrium, preventing rapid changes in pH.

Active acidity

H+

Salt replaceable acidity

exchangeable H+

Residual acidity

polymers bound by organic matter