Deck 8: The Colloidal Fraction: Seat of Soil Chemical and Physical Activity

Question 1

You are working with a soil colloid known to have a reasonably high potassium level, a cation exchange capacity of about 20 cmol_c/kg of clay and little tendency to swell when wetted. Which of the following is it most apt to be the dominant clay mineral in this soil?

A) vermiculite
B) smectite
C) kaolinite
D) humus
E) fine-grained mica

Answer 1

E

Question 2

The swelling/shrinking tendency of some silicate clay minerals is due primarily to _____.

A) the presence of cations attracted by the negative charges on the internal surfaces
B) the movement of water molecules in and out of the interlayers of the crystal
C) varying thickness of the film of water covering the external surface of the particles
D) expansion in the width of the interlayers due to movement of larger ions such as K⁺
E) the high Mg²⁺ contents in the octahedral layers

Answer 2

B

Question 3

Some soil colloids exhibit positive charges under highly acid conditions. To what are these charges likely due?

A) protonation of some hydroxy groups by excess H⁺ ions
B) the adsorption of Al³⁺ ions on the colloid
C) the release of some OH^- ions leaving an unsatisfied positive charge on the colloid
D) substitution of lower valent atoms for higher valent atoms in the octahedral sheet
E) increase in the level of H⁺ ions on the exchange complex

Answer 3

A

Question 4

The electronegative charge on 2:1 type silicate clays is due primarily to _____.

A) ionization of surface OH groups
B) substitution of one metallic atom for another in the crystal structure of the clay
C) extremely small size of the individual colloid particles
D) the effect of pH on the presence of H⁺ ions in the exchange complex
E) the preponderance of tetrahedral sheets compared to octahedral sheets

Question 5

Anion exchange by formation of outer sphere complexes is of least importance in the plant absorption of _____.

A) NO_3- B. Cl^-
C) SO_42-
D) MoO_4-2
E) H₂PO_4-.

Question 6

Allophane differs from other silicate clays in all ways but one of the following (select the one).

A) nature of parent materials from which it forms
B) degree of crystallization of the particles
C) the presence of negative charges on the colloidal particles
D) widespread occurrence around the world
E) the period of time it has been subject to weathering

Question 7

Humus is an important soil colloid. It differs from 2:1 type minerals in all but one of the following characteristics (select the one).

A) structural framework of the particles
B) CEC dependence on soil pH
C) influence of isomorphous substitution
D) capable of adsorbing cations
E) influence on soil aggregate stability

Question 8

You find that the CEC of a sandy loam soil at pH 5.0 is 8 cmol_c/kg and at pH 8.2 it is 14 cmol_c/kg. What is the most likely reason for this difference?

A) increase in the pH-dependent charges on 1:1 type silicate clays
B) removal of Al³⁺ ions from the exchange complex at higher pH.
C) ionization of carboxyl and phenolic groups to release H⁺ ions
D) removal of Al atoms from tetrahedral layers at low pH.
E) replacement of trivalent Al³⁺ ions with divalent Ca²⁺ ions at high pH.

Question 9

A well drained soil under a rain forest in Brazil contains significant amounts of a silicate clay. Which of the following is it most apt to be?

A) chlorite
B) fine-grained mica
C) smectite
D) kaolinite
E) vermiculite

Question 10

Soils rich in which type of clay would provide the most troublesome site on which to build a home?

A) Fe, Al oxides
B) kaolinite
C) fine-grained micas
D) chlorite
E) smectite

Question 11

Which cation would you expect to be mostly tightly held in a soil at pH 4.5?

A) K⁺ B. Ca²⁺
C) Al³⁺
D) Mg²⁺
E) Na⁺

Question 12

The negative charges associated with smectite clay crystals are due mostly to _____.

A) isomorphous substitution of Mg for Al in the octahedral sheet
B) substitution of Al for Mg in the tetrahedral sheet
C) high pH associated with the formation of the mineral
D) substitution of Si for Al in the tetrahedral sheet
E) OH groups on the external surface of the crystal

Question 13

Which of the silicate clays would likely have the highest level of potassium in the crystal structure?

A) kaolinite
B) chlorite
C) smectite
D) fine-grained mica
E) vermiculite

Question 14

Which of the following silicate clays would be least apt to be formed from the mere alteration of 70 primary minerals?

A) smectite
B) vermiculite
C) chlorite
D) fine-grained micas
E) iron or aluminum oxides

Question 15

If you were seeking a soil colloid with a high capacity to adsorb cationic pollutants, but also with a consistent capacity to do so even as the pH varied form 4.0 to 7.5, which of the following would you choose?

A) montmorillonite
B) kaolinite
C) fine-grained mica
D) humus
E) chlorite

Question 16

An alkaline soil contains 20% clay and 3% humus. If the pure clay has a CEC of 40 cmol_c/kg and the humus 200 cmol_c/kg, calculate the CEC/kg of the soil.

A) 8
B) 30
C) 12
D) 16
E) 14

Question 17

As biotite weathers in soils, which of the following would not likely develop by mere alteration of this mineral?

A) fine-grained mica
B) kaolinite
C) smectite
D)chlorite
E) vermiculite

Question 18

Which of the following is not a characteristic of soil colloids?

A) very small size
B) high external surfaces
C) electronegative charges
D) ability to exchange ions with the soil solution
E) low water holding capacity

Question 19

If you were looking for a soil with high cation exchange capacity and were not concerned with the soil's physical properties, which of the following would most likely fit your needs?

A) Aridisol
B) Oxisol
C) Vertisol
D) Mollisol
E) Ultisol

Question 20

A 1 kg soil sample has 2 cmol_c of Al³⁺ on the exchange complex. Addition of which of the following would most effectively replace about half of this Al?

A) 1 cmol_c K⁺ B. 10 cmol_c OH^-
C) 4 cmol_c Ca²⁺
D) 4 cmol_c Na⁺ E. 2 cmol_c Na⁺

Question 21

The order of strength of adsorption of ions by most colloids when they are present in equivalent quantities is Al>Ca>K>Mg>Na.

Question 22

Isomorphous substitution involves the substitution of a tetrahedral sheet for an octahedral sheet.

Question 23

The leaf-like structure of silicate clays involves two kinds of horizontal sheets, one dominated by a plane of silicon atoms, the other by a plane of aluminum and/or magnesium.

Question 24

The high cation adsorption capacity of vermiculite is due primarily to the substitution of silicon atoms for aluminum in the tetrahedral sheet.

Question 25

A tetrahedral sheet is characterized by a plane of aluminum atoms surrounded by oxygen and hydroxyl groups.

Question 26

Humus has a much higher cation adsorption capacity at pH 5.5 than at pH 8.0.

Question 27

While kaolinite particles are larger than those of other major silicate clays, the surface area of kaolinite per kilogram is much smaller than that of the 2:1 type minerals.

Question 28

Vermiculite and associated silicate clays have a net electropositive charge in slightly acid soils.

Question 29

A 2:1 type silicate clay has one octahedral sheet sandwiched between two tetrahedral sheets.

Question 30

Soil colloids are too small to be seen with an ordinary light microscope.

Question 31

Kaolinite is most prevalent in soils developed from volcanic ejecta (Andisols).

Question 32

Iron and aluminum hydrous oxides are characteristic of soils that are highly weathered chemically.

Question 33

In most soils of the arid regions of Nevada, smectite is more prominent than in most soils in warm, humid Georgia.

Question 34

Even at pH values near 7.0 some colloids such as iron and aluminum oxides have negative surface charges less than 10 cmol_c/kg.

Question 35

Smectite and vermiculite are more prominent in extremely weathered soils than in soils where only modest weathering has occurred.

Question 36

You would expect the cation exchange capacities of Ultisols to be lower than those of Aridisols if the soil textures are about the same.

Question 37

Of the silicate clays, smectite has the least tendency to expand upon wetting and shrink upon drying.

Question 38

The cation exchange capacity of a soil is determined primarily by the amount and kind of colloids in the soil and by the pH.

Question 39

The primary source of charge for 2:1 type clay minerals is ionization of the surface hydroxyl groups on the tetrahedral sheets.

Question 40

While 2:1 type clays have significant internal surfaces, their external surfaces are generally much more extensive.