lecture 4

Question 1

Why are H-bonds in the 1:1 interspace of Kaolinite stronger than the K-bonds in the 2:1 interspace of Micas?

Answer 1

Hydroxides in octahedral layer face apical O atoms of tetrahedral layer forming H-bonds

The relative charge of the H ion is much higher than K due to smaller size = stronger bonding strength

Question 2

What are the characteristics of Fe and Al oxides and hydroxides / non silicates (sesquioxides)?

Answer 2

• Weathering products of clay minerals that have lost all Si4+ and most other cations except Fe3+ and Al3+
• Consist of modified octahedral sheets with Fe3+ and Al3+
• Have no silicon tetrahedral sheets
• Little or no isomorphous substitution - there are no more cations available
• Little cation exchange capacity
• Posses covalently bound OH-ions which may cause strong adsorption of certain anions like phosphate

Question 3

What are the characteristics of soil colour?

Answer 3

• Dark colours are usually indicative of high organic carbon contents
• Red colours are characteristic of soil rich in iron oxides
• Blue-grey colours indicate the presence of iron in its reduced form, the gibbsite is dominating- the hematite and goethite have been dissolved and transported away

Question 4

What chart allows us to identity types of soils?

Answer 4

The Munsell soil colour chart

Question 5

What is the importance of sesquioxides in soils?

Answer 5

• They dominate in soil colour where there is low organic matter content
• Form cementation in soil when large oxide or hydroxide content (e.g. laterite in Tropics)
• Provide anion exchange capacity!!

Question 6

What is cation exchange capacity? Where does it occur?

Answer 6

The ability to exchange cations between particle surface and soil solution. It occurs at the negative surfaces of clay minerals and humus.

Question 7

What are the sources of charge for the cation exchange capacity?

Answer 7

1. Isomorphic substitution within clay minerals
2. Broken bonds at the edge of clay minerals
3. Dissociation of bonds at the edge of the humus

Question 8

What is the process of cation exchange between soil particles and the soil solution? How does the reverse reaction occur?

Answer 8

Replacement of base cations with H+, e.g. from acid rain or organic matter

This process can be reversed with proper soil management. The displacement of H+ from the exchange complex by base cations can occur through fertilization or nutrient release from decomposing tissues. Fertilization increases the soil pH and recharges the base cations.

Question 9

What are the mechanisms of neutralization?

Answer 9

1. Reaction with carbonates
   - H2SO4 + CaCO3 –> CaSO4 + H2CO3
2. Reaction with exchangeable cations or weatherable minerals such as feldspars
3. Anion adsorption of SO4

Question 10

What are the characteristics of cation exchange capacity?

Answer 10

• Exchange reactions are not always reversible
• Released cation can either precipitate, volatilize, or strongly associate with an anion
• The relative strengths of adsorption order may be altered on certain clay minerals (influenced by how strongly its neighboring cations are adsorbed)

Question 11

Under what conditions do cations bind more tightly on the surface of colloids? Which cation has the strongest adsorption capacity?

Answer 11

The higher the charge and the smaller the hydrated radius of the cation, the more strongly it will adsorb to the colloid.

Al has the strongest adsorption capacity - this is why it is left behind even when the soil no longer has any buffering capacity

Question 12

What is the effect of soil organic carbon and pH and the cation exchange capacity?

Answer 12

The higher the SOC or pH, the higher the cation exchange capacity.

In acidic soils (low pH), there are more hydrogen ions (H⁺), which can occupy the negatively charged sites, reducing the overall CEC.

Question 13

What is the influence of soil pH on soil charge in clay minerals and how does that impact their preference for anion/cation exchange capacity?

Answer 13

Acid conditions = Positive charge –> prone to anion exchange capacity

Alkaline conditions = Negative charge –> prone to cation exchange capacity

Soils will always have the capacity to do anion and cation exchange capacity but it is dependent on the pH of the soil

Question 14

What are the base and acidic cations?

Answer 14

Base: Ca, Mg, Na, K

Acidic: H, Al, NH4

Question 15

Why are Al and Fe the last to mobilize during cation exchanges?

Answer 15

Due to their size and charge

Question 16

What is the relationship of cation exchange with pH? What are the long term effects of this process?

Answer 16

When H+ ions are introduced into the soil (through acid rain or organic matter), the soil solution becomes more acidic. This acidity can be buffered by releasing base cations into the soil solution through cation exchange (clay minerals receive H+).

HOWEVER, at a certain point the base cations available on clay minerals will run out and can no longer buffer against the acidity. In this situation, clay minerals will start releasing Al3+ into the soil solution due to their high positive charge.

This is why highly acidic soils are characterized by Al toxicity for plants.

Question 17

Why can soils rich in Fe and Al neutralize acid rain through anion adsorption?

Answer 17

Anion adsorption involves the binding of sulfate ions (SO₄²⁻) to Fe and Al oxides and hydroxides in the soil.

The resulting reactions release hydroxide ions (OH⁻), contributing to the neutralization of acidity in the soil solution.

Question 18

Would it be preferable to add calcium carbonate or calcium chloride to treat a soil impacted with acid rain?

Answer 18

Calcium carbonate. This reaction produces water and CO2, which does not increase the acidity in the soil.

Adding calcium chloride would keep the chloride in the soil solution and would not buffer the acidity

Question 19

What is the strength adsorption of cations from strongest to weakest?

Answer 19

Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+

Question 20

which mineral has the highest CEC? why?

Answer 20

Smectite because it has the largest surface area

Question 21

Rank the CEC in the different soils from least amount of CEC to highest.

Answer 21

Fe, Al oxides < Kaolinites (small H bonds and small inter layers) < smectites (high surface area) < soil humus (highest potential)

Question 22

Explain the anion exchange capacity of sesquioxides i.e. Al(OH)3 and Fe(OH)3

Answer 22

Anions such as Cl-, NO3-, SO42-, PO43_ and organic matter (COO-) can displace OH- groups from sesquioxides

Fe(OH)3 + PO43- –> Fe(PO4) + 3(OH-)-

MECHANISM OF NEUTRALIZATION becuase OH- groups are released