1. The Principles of cation and anion exchange capacity

2. Student activity
In your group discuss how a magnet works, and how are nutrients retained in the soil. 2 mins

3. Learner outcomes
Explain the principles of cation and anion exchange capacity in the soil and their relationship to texture and organic matter.

4. Ions
An ion is an atom that has a positive or negative electric charge. An ion with a positive charge is called a cation and a one with a negative charge is called an anion.

6. In soil science, cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, available for exchange with the soil solution.

7. Clay and humus have electrostatic surface charges that attract the solution ions, and hold them. This holding capacity varies for the different clay types and clay-blends present in soil, and is very dependent of the proportion of clay+humus that is present in a particular soil. One way to increase CEC is to favour the formation of humus.
In general, the higher the CEC, the higher the soil fertility.

8. Special properties of clay and humus
Both clay particles and humus particles are what are known as colloids and such have certain properties:
They are extremely small (0.002mm)
They have a large surface area relative to their size
They have negative electric charge

9. Cont...
Because they have a negative charge they will attract and hold positive ions (cations).e.g.
Hydrogen
Calcium
Magnesium
Potassium
The significance of that is that these cations will resist being leached from the soil and therefore remain available to the plant.
Nitrogen which is held as a negative ion is not attracted to the particles and therefore is easily leached out.
The ability to exchange ions is an important factor in soil fertility and is called cation exchange capacity (CEC)