SSC 100

Fall 2002                                                                                  ID_KEY_

Exam 1                                                             Lab section___________

 

Value 90 points

 

Answer all questions in the space provided.

 

1.        a. (6) What is the difference between texture and structure?

 

Texture is the particle size distribution and structure is the organization of the particles.

 

b. (5) Why do texture and structure matter?

 

Texture (particle size distribution) controls many soil processes and provides surface area for many processes.  Structure partly controls pore size distribution and total porosity.

 

 

2.      (9) Name the three phases that combine to make a soil.

 

solid, liquid and gas.

 

 

3.      (5) What is the major structural difference between crystalline and amorphous minerals?

 

Crystalline minerals have long-range order.  Amorphous minerals do not.

 

Please put your name on this page of the exam______________

 

4.      (20) Complete the following table quantitatively (e.g. using numbers) or qualitatively (e.g. high, low)

 

 

Property

Mineral

 

Kandite

Smectite

Vermiculite

Chlorite

Number of tetrahedral layers

1

2

2

2

Number of octahedral layers

1

1

1

1

Shrink-swell capacity

Low

High or Very high

High or very high

Low/none

Surface area

Low

High

High

Low to Moderate

Cation exchange capacity

Low

high

Very high

Low to Moderate

 

5.      a.  (6) Name the two sources of charge in clay minerals.

 

Isomorphous substitution and pH dependent charge.

 

      b. (4) How are they different?

 

Isomorphous substitution is “permanent” charge because it is derived from the atomic constituents in the crystal.  pH dependent or variable charge is not permanent because it varies with soil pH.

 

6.      (9)  List three properties that characterize humus.

 

It is colloidal soil organic matter that decomposes slowly and colors soils brown or black.  It has high surface area and high pH dependent charge.  The main building blocks are C, H and O.

 

 

7.      (5)Put the following colors in order from most red to least red. 10YR, 2.5 YR, 7.5 YR, 5 Y, 10R

 

10R, 2.5YR, 7.5YR, 10YR, 5Y

 

 

8.      (10) Using the data from the table for a soil sample, answer the following questions.

 

a.       What is the gravimetric water content?

b.       What is the bulk density?

c.       What is the porosity?

d.       What is the volumetric water content at saturation?

e.       What is the mass water content at 50% of saturation?

 

 

 

Sample Property

Value

Oven dry weight (g)

300

Air dry weight (g)

310

Volume of an air dry sample (cm3)

220

Particle density (g/cm3)

2.62

Water density (g/cm3)

1.0

 

a.  GWC = (ADW-ODW)/ODW = 310-300/300 = 0.033

 

 

b.        Mass/Volume = 300/220  = 1.36 g/cm3

 

c.        Porosity = 1-(bulkdensity/particle density)  = 1-1.36/2.62) = 0.481

 

d.        volumetric water content = volume of pores at saturation.

 

220 cm3 *0.481 = 105.8 = 106 cm3 pores and 106 cm3 water.

 

e.  106 cm3 is the volume of water, assuming water density is 1 g/cm3, there are 106 g of water in the sample when saturated.  50% of the water is 53 g.

 

9.      a. (4) Name the two processes by which gas moves through soil.

 

Mass flow or convection

Diffusion

 

 

b.        (4) For each process name what soil physical properties control the flux of gas through soil?

 

Mass flow is controlled by the total pore size distribution and the connectivity.

 

Diffusion is controlled by the volume of air-filled porosity (or soil water content), and total volume of pores and connectivity.

 

10.  (3) Name three characteristics of ion exchange reactions.

 

They are characterized by being:

quantitative property of the solid phase,

rapid,

stoichiometric,

selective in that not all ions are treated the same,

concentration dependent and

reversible.