SCSC 301 CH 8 HOWE

Soil Colloids: Size

Extremely small

Soil Colloids: Surface area

Their tiny size gives them a massive surface area per gram, which allows for a high capacity to hold water and nutrients.

Soil Colloids: Charge

Typically negative

Soil Colloids: Adsorb ions & water

- Charges attract ions of opposite charge

- Cations: H+ , Al3+ , Ca 2+ , Mg 2+

- Anions: NO3- , Cl- , SO42-

- Ions are loosely held

- Water attracted to colloid surface & ions

Layer Silicates (Phyllosilicates)

Built of sheets of tetrahedral Si-O and octahedral Al-O or Mg-O layers

Responsible for most of the CEC (cation exchange capacity) in mineral soils.

– Kaolinite

– Smectite

– Vermiculite

– Mica

– Chlorite

Fe/Al Oxides

Made entirely of iron or aluminum oxides (Fe-O or Al-O). NO Si

Low CEC and often show anion exchange capacity (AEC).

Found in highly weathered tropical soils.

- Goethite

- Hematite

-Gibbsite

Allophane & Imogolite

Amorphous (non-crystalline), short-range order minerals.

Composed of Si-O + Al-O arranged in tubes or spheres.

Common in volcanic soils.

Humus

Derived from decomposed organic matter.

Composed of C, H, O, N, S, and P.

Extremely high CEC due to functional groups like -COOH and -OH.

pH-dependent charge that increases with higher pH.

layers

- Silicate clays are made of stacked sheets of:

Tetrahedral sheets (Si⁴⁺ surrounded by O²⁻)

Octahedral sheets (Al³⁺ or Mg²⁺ surrounded by O²⁻ or OH⁻)

- These sheets bond together to form layered structures, which differ between clay types

Interlayer

The space between layers is called the interlayer.

It may contain:

- Water molecules

- Exchangeable cations (e.g., K⁺, Ca²⁺)

- Hydrated cations

- In some minerals (like mica or chlorite), the interlayer has fixed elements (e.g., K⁺ or Mg-hydroxide), making them non-expanding

1:1 Type Mineral:

Kaolinite

- One tetrahedral sheet + one octahedral sheet

- No interlayer swelling – layers are held tightly by hydrogen bonds.

- Low surface area and low CEC (~3–15 cmolc/kg)

- Stable structure; dominant in highly weathered soils (e.g., SE US, tropics)

2:1 Type Minerals

• Smectite

• Vermiculite

• Mica

• Chlorite

Smectite

- High shrink-swell capacity

- Very high CEC (~80–150 cmolc/kg)

- Interlayer contains hydrated cations and water, which can enter/exit, causing expansion

Vermiculite

- Moderate swelling

- Very high CEC (~100–180 cmolc/kg)

- Often contains Mg²⁺ or Ca²⁺ in interlayer

- Holds more water and nutrients than mica, less than smectite

Mica (Illite)

- Non-expanding due to fixed K⁺ in interlayer (fits tightly into cavities)

- Moderate CEC (~20–40 cmolc/kg)

- Weathering of mica leads to formation of vermiculite or smectite

Chlorite

- Similar to mica, but with an extra hydroxide (Mg-OH) layer in interlayer

- Non-expanding

- Moderate to low CEC (~10–40 cmolc/kg)

Weathering of Silicate Clays: weather from less stable to more stable forms:

Mica → Vermiculite → Smectite → Kaolinite → Fe/Al oxides

As weathering progresses:

- CEC decreases

- Clays become less reactive but more stable

- This is common in tropical or humid climates

Permanent Charge

- Caused by isomorphous substitution in the clay sheets (mainly in 2:1 clays).

- Does not change with soil pH.

Main source of CEC in:

Smectite

Vermiculite

Mica

Chlorite

pH-Dependent Charge

- Found in all colloids: clay edges, Fe/Al oxides, humus

- As pH increases (more OH⁻), more functional groups deprotonate, increasing negative charge

- At low pH, positive charge or neutral surfaces may occur

Cation Exchange Capacity (CEC)

The soil’s ability to hold and exchange positively charged ions (cations).

- Higher CEC = more nutrients retained.

Influenced by:

1.) Type of clay (2:1 clays have more CEC than 1:1)

2.) Organic matter (very high CEC)

3.) Soil pH (higher pH increases pH-dependent charge → more CEC

% Base Saturation

Percentage of CEC sites occupied by basic cations (Ca²⁺, Mg²⁺, K⁺, Na⁺).

High %BS = more fertile, less acidic soil

.

Low %BS = more acidic, likely more Al³⁺ or H⁺ on exchange sites.

Anion Exchange Capacity (AEC)

The soil’s ability to hold and exchange anions (e.g., phosphate, nitrate).

Typically found in:

Fe/Al oxides

Very acidic soils

AEC is usually much lower than CEC, but important for phosphorus management