SSC 100

SSC 100 - 2001
Mid quarter exam
Answer sheet

These are my (MJS) answers to the questions. Partial credit was given for answers that were close to these.

1. (12) Name three properties that differentiate among layer silicates. Any three of these five is ok.

The number and sequence of tetrahedral and octahedral sheets
The layer charge per unit cell of structure
The type of interlayer bond and interlayer cations
The cations in the octahedral and tetrahedral sheets
The type of stacking along the c dimension

2. (5) Give examples (one each) of likely isomorphous substitution in the octahedral and tetrahedral sheets of clay minerals.

Al substitution for Si in the tetrahedral layer
Fe, Mg, Ca substitution for Al in the octahedral layer

3. (6) What are two important results of isomorphous substitution?

Strain in mineral structures.
Net negative charge.

4. (10) Explain why 1:1 clay minerals like the Kandites do not shrink and swell when the soil water content changes, but the 2:1 clay minerals like the Smectites do.

In the Kandites, hydroxyl atoms from the Al-octahedral layer face oxygen atoms from the next silicon tetrahedral layer. Hydrogen bonding across the interlayer space holds the sheets together. In the Smectite family, oxygen atoms face each other across the interlayer space, thus promoting repulsion and allowing hydrated cations to enter and exit the interlayer space.

5. (3) Using the attached textural triangle, determine the texture class name for each of the following particle size distributions.

Sample	sand	silt	clay	Texture Class Name
a.	32	45	23	loam
b.	48	10	42	sandy clay
c.	15	50	35	silty clay loam

6. (9) Using the following data, calculate the gravimetric water content, bulk density and porosity of the soil sample.

Soil Property	Value
Oven-dry weight	160.0 g
Sample volume	110.0 cm³
Air-dry weight	170.0 g
Particle density	2.63 g/cm³

Gravimetric water content = weight of water/oven dry weight;

GMC: 170 – 160 = 10/160 = 0.06

Bulk density = mass/volume = 160.0 g/110.0 = 1.45 g/cm³

Porosity = 1 – (bulk density/particle density) = 1- (1.45/2.63) = 0.45

7. (5) Why was sodium metaphosphate used in determining particle size distribution in the laboratory?

It is used as a dispersant so that the individual sand, silt and clay grains would settle independently, not as aggregates.

8. a. (5) What is pH dependent charge in soil? b. (2) Give one example of a major component of the soil solid phase that contributes most to pH dependent charge.

It is charge (negative or positive) that varies with the soil pH. Organic matter, edges of clay minerals, and oxides and oxihydroxide minerals all contain pH dependent charge. Organic matter and the oxides or oxyhydroxides are the major contributors. Two points for these, one point for clay edges.

9. (6) What are the two mechanisms by which the soil atmosphere moves through soil.

Diffusion and mass flow or convection. Three points each correct ans.

10. (12) Name four characteristics of pores that influence liquid or gas flow through soil.

Total volume of pores, pore size distribution, tortuosity, connectivity.

11. a (10) Explain why a concentration gradient develops for CO₂ and O₂ between a plant root and the soil surrounding the root.

b. (5) Why is the gradient steeper in wet soil compared to a dry soil?

Root respiration consumes O₂ thus depleting the amount near the root and contributes CO₂, thus increasing the concentration near the root. Lack of turbulence in the soil reduces the exchange of these gases with the atmosphere and soil atmosphere away from the root, thus creating the gradient. Diffusion of CO₂ away from the root and O₂ towards the root is slow, enhancing the gradient. Diffusion is slower in wet soil than dry soil where there is more air-filled pore space. Hence in a dry soil the gradient will be less steep than in a wet soil.