Cation Exchange Capacity

Question 1

The amount of available nutrients that are held and ready to be used in the by plants is called…

Answer 1

Cation Exchange Capacity (CEC)

Question 2

Clay minerals and organic colloids possess what charge?

Answer 2

Negative

Question 3

Is the charge on clay molecules permanent?

Answer 3

Predominantly, yes. (It is not effected by pH)

Question 4

Clay colloids aren’t usually affected by pH, are Organic Colloids?

Answer 4

Yes. The source and type of charge varies with pH.

Question 5

Define Colloid.

Answer 5

A homogenous, non-crystalline substance consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance.

(Substances which repel others due to charge when in solution. This prevents ‘settling out’ )

Question 6

Clay and humus are soil colloids with a predominantly negative charge. As a result, they____ each other?

Answer 6

Repel

Question 7

Water has a polar charge and so ______ the colloidal suspension.

Answer 7

Reinforces

Question 8

What type of clays accounts for most of the charged clay particles in the temperate zone?

Answer 8

Broadly, Alumino silicates

e.g Kaolinite (AL₂Si₂O₅(OH₄))

Question 9

What is humus?

Answer 9

Humus is an ill-defined, high molecular weight carbon compound. It results from the decomposition of biological material by fungi and bacteria.

Question 10

Negatively charged colloids hold Cationic Nutrients in solution due to their charged attraction…

Answer 10

See photo…

Question 11

Attraction to colloids makes the nutrients in the cation exchange zone less prone to…

Answer 11

Leaching

Question 12

CEC is commonly expressed as…

Answer 12

cmoles+/kg

(Centimoles of positive charge per kg of soil)

Question 13

Correctly list the binding strength of nutrients to clay particles.

(Hard, advanced knowledge)

Answer 13

See photo

Question 14

Follows is an example of the CEC in different mediums.

Answer 14

The numbers shown for OM, Clay and Loam are the mass (g) of each element in 100g if all the CEC was occupied by ions.

This is an example of the varying proportions of minerals in suspension.

See Photo

Question 15

Anions such as Nitrate NO^-₃ and Chloride CL^- have a negative charge. What is their effect in relation to CEC?

Answer 15

Anions are more prone to leaching because they are not retained in the same fashion as cations. However, some clays in some conditions will form an attraction with Anions although this is rare and is poorly understood.

Question 16

pH (amongst other things) effects…

Answer 16

Nutrient storage capacity.