Cation Exchange Capacity and Soil Organic Matter

What are colloids?

very tiny particles which stay suspended in the soil water which have an electric charge and very large surface area

3 Types of Colloids

1. Clay colloids- platy and layered with a permanent charge

2. Metal oxide- round coated particles, pH dependent charge

3. Organic Colloids- humus, non crystalline, ph dependent charge

Why colloids matter?

When water flows through soil, colloids hold nutrients in place and not wash away

Cation Exchange Capacity

• soils ability to hold and exchange cations on colloids surface

• the higher the CEC the greater the nutrient retention

• cations are held on negatively charged colloids, these cations are exchangeable with hydrogen ions released by plants

How plants use CEC

1. plant roots release hydrogen ions

2. hydrogen ions displace the cations from the colloid surface

3. the freed cation enters the soil water

4. Plant absorb these nutrients

What influences CEC

1. texture- clay has a greater surface area, more sites for cations

2. mineralogy- kaolinite=low CEC (1:1 structure), fewer charged sites, smectite=high CEC (2:1 structure), greater surface area

3. Organic matter- extremely high CEC

4. pH- increase in pH increases CEC

Soil organic matter and carbon

• everything that has lived or living in the soil

• SOM primarily from the decomposition of plant and aniaml residues

• 58% exists as carbon

Importance of soil organic matter

• improves soil structure

• increase water holding capacity

• boosts nutrient holding capacity

• helps CEC

Measuring soil organic carbon

*Loss of ignition- burning off the organic material and measuring the weight loss

• limitations- not specific to carbon, overestimates SOM not accurate