Soil Colloids and Cation Exchange Policy

Colloidal complex of soils that consists of mainly clays and humus(organic matter)

soil colloids

Soil colloids play an important role in…

ion exchange and fertility

Clay’s small size translates to…

high surface area and adsorptive capacity

The four major types of Colloids

Crystalline silicate clays, non-crystalline silicate clays, Al and Fe Oxides, Humus-organic colloids

Crystalline vs non-crystalline silicate clays

crystalline clays are dominant and layered and non-crystalline is not layered

Al and Fe Oxides vs Humus-Organic Colloids

Al and Fe oxides are in highly weathered soils of warm regions

Humus-organic colloids are in mostly all soils(especially upper profiles) and is undefined structure

The building blocks of layered silicate clay structure

Si-tetrahedron, Al/Mg-octahedron, and sheets and layers

One cation by another of similar size but different charge

Isomorphous substitution

pH dependent charges are associated with…

hydroxyl(OH-) groups (as pH increases, deprotonation increases)

Organization of Select Layer Silicate Clays

Based on the number and arrangement of tetrahedral (Si) and octahedral (Al, Mg) sheets contained into crystal layers

Crystalline silicate clays are classified into what 3 groups

1:1 type, expanding 2:1, non expanding 2:1

Modified octahedral sheets found in highly weathered soils

Al and Fe Oxides

Organic Colloids are also known as

HUMUS (high negative charge)

In soil, the negative and positive charges are…

attracted to each other that result in many cations and anions present

In temperate-region soils, the …. is common and negatively charged

2:1 type silicate clays that absorb cations

In the tropics, soils are rich in…and positive charges are more abundant

1:1 clays, and Fe, Al oxides

Cation vs Anion Exchange Reactions

Cation- the colloidal site is negatively charged

Anion- colloidal site is positively charged

The total exchangeable cations a soil can absorb

Cation Exchange Capacity (CEC)

…. use cation exchange to assist in nutrient uptake from soil

plant roots