Soil Colloids

Question 1

Most chemically active portion of the soil

Answer 1

Soil Colloids

Question 2

Soil particles [____] or [_____] in diameter

Answer 2

<1μ, 0.001 mm

Question 3

Types of Soil Colloids

• Inorganic
  (______)
  (______)
  (______)
• Organic
  (______)

Answer 3

Silicate clays
Oxides of Fe and Al
Allophanes

Humus

Question 4

A silicate clay crystal

Answer 4

Micelle

Question 5

Types of Silicate clays

Answer 5

1:1
2:1 non expanding type
2:1 expanding type
2:1:1 or 2:2

Question 6

One tetrahedral sheet attached to one side of an octahedral sheet

• Represented by the mineral kaolinite
• Other 1:1 include halloysite, nacrite, dickite

Answer 6

1:1 Silicate clay

Question 7

If a trivalent is the central cation, two-thirds of the available cation sites are filled

Answer 7

Dioctahedral

Question 8

• Symmetrical arrangement of two tetrahedral sheets and a central octahedral sheet
• Represented by montmorillonite or smectite group of clay; also by vermiculite
• Isomorphous substitution is common (Al3+ to Si4+, Mg2+ to Al3+)
• Expand or contract as water molecules are added or removed

Answer 8

2:1 Expanding Silicate Clay

Question 9

• Same composition/structure as the
  other 2:1 type but doesn’t expand
  due to the presence of K+ ions
  “bridging” the silicate clays
• Represented by muscovite (mica)

Answer 9

2:1 Non expanding silicate clay

Question 10

• Structure: two silica tetrahedral sheet and two Mg-dominated octahedral sheet
• Represented by chlorites

Answer 10

2:1:1 or 2:2 Silicate clay

Question 11

Commonly dominant in the highly weathered soils of tropics and subtropics

Answer 11

Hydrous oxide of Fe and Al

Question 12

A colloidal matter that is either amorphous or the crystalline structure is not sufficiently ordered to be detected by x-rays

Answer 12

Allophanes and other amorphous minerals

Question 13

Composed of C, H and O, that is non crystalline

Answer 13

Humus

Question 14

Prevalent in soils developed from volcanic ash (Andisols)

Answer 14

Allophanes and other amorphous minerals

Question 15

Sources of negative charges

Answer 15

Dissociation of exposed OH groups at the broken edges of the mineral
Dissociation of functional groups in organic colloids

Question 16

The interchange between a cation in solution and another cation on the surface of any surface-active material such as clay or organic matter

Answer 16

Cation Exchange

Question 17

• The sum of the exchangeable cations that a soil can adsorb
• Expressed in terms of moles of positive charge adsorbed per unit mass (cmol+ kg-1 soil)
• Used to be milliequivalent per 100g soil (m.e. 100g-1 soil)

Answer 17

Cation exchange capacity

Question 18

The quantity of a substance that exactly reacts with, or is equal to the combining value of, an arbitrarily fixed quantity of
another substance in a particular reaction

Answer 18

Equivalent weight

Question 19

Proportion of CEC that is made up of exchangeable bases

Answer 19

Base saturation

Question 20

Characteristics of cation exchange

Answer 20

Reversible
Loss=gain
Instantaneous or rapid