Soil Science 100

Mid Quarter Exam 1 Six digit ID number _ _ _ _ _ _

25 October 1999

Value 100 points

Please answer all questions completely.

1. (9) Give one property of each of the following kinds of horizons that distinguishes the horizon from the others.

O horizons, A horizons and B horizons

The O horizon has significantly more organic matter than either the A or B. It is an organic horizon compared to the A or B that are mineral horizons. It will be darker than either the A or B most of the time. The A horizon can be distinguished from the B by color (darker) and structure (granular or fine blocky). The B horizon is a horizon of accumulation and so will have more clay, more oxides (be more red), and have larger structural units than the A and much less organic matter than the O.

2. (9) Clay has three meanings in soil science. What are the three meanings?

It is a soil texture class

It is a mineral size

It is a kind of mineral

3. (8) Examine the morphologic data for two A horizons in the following table and then answer the questions that follow the table.

Soil Property	A horizon 1	A horizon 2
Color	Black, 10YR 2/1	Reddish brown, 7.5YR 4/4
Structure	Strong, medium grannular	Strong fine granular
Cation exchange capacity	275 cmolc/kg	300 cmolc/kg
1.5 MPa water content	22%	28%
15 KPa water content	37%	40%

a. Which soil has more clay? 2

b. Which soil has more plant available water? 1

c. Which soil has more organic matter? 1

d. Which soil has more iron oxides? 2

4. (4) What soil property or properties did you use to determine your answers to questions 4 a through d.

a. The CEC and 1.5 MPa water content.

b. The field capacity (15 KPa) and permanent wilting point (1.5 MPa).

c. The color.

d. The color.

5. (12) For each of the answers in question 3 (a through d) select one of the properties you listed as an answer to question 4 and explain why it led you to your answer in question 3.

a. Clay, organic matter, and iron oxides contribute to the CEC of a soil. Horizon 2 has a higher CEC. Based on the color it has less organic matter so it should have more clay to have the higher CEC. The higher iron oxides might contribute more than the clay, but we don't know that. In addition, the higher 1.5 MPa water content is surely a function of the clay content.

b. The difference between field capacity (15 KPa) and permanent wilting point (1.5MPa) water contents is higher for horizon 1.

c. The darker color indicates more organic matter in horizon 1.

d. The redder color indicates more iron oxides in soil 2.

6. (9) Briefly explain what is a 1:1, 2:1 and 2:1:1 clay mineral.

A 1:1 clay mineral consists of 1 silica tetrahedral sheet and 1 aluminum octahedral sheet. A 2:1 clay mineral consists 2 silica tetrahedral sheets and 1 aluminum octahedral sheet.

The 2:1:1 clay mineral consists 2 silica tetrahedral sheets and 1 aluminum octahedral sheet with a magnesium hydroxide sheet in the interlayer space between 2:1 sheets.

7. (8) What are two sources of negative charge in clay minerals.

Isomorphous substitution is the most important source and variable charge at broken edges or hydroxyl surfaces is another.

8. (15) A plant root at 80 cm depth is actively respiring. (5 for each part a, b, c)

a. Describe the relative concentrations of CO₂ and O₂ at the root surface compared to pores far from the root.

At the root surface, the CO2 concentration will be higher and the O2 concentration will be lower than in the bulk atmosphere.

b. What processes are taking place that move gas from the root into pores away from the root and gas to the root from pores away from the root.

Diffusion will move CO2 away from the root and O2 towards the root. Mass flow or convection will move O2 and CO2 towards the root as the root consumes oxygen and water.

c. List three soil properties that affect the rates of transfer.

The total volume of pores, the size distribution of the pores, the connectivity of the pores and the tortuosity of the pores. Some students will say that structure is a soil property that affects rates and it does indirectly by influencing pore properties (give partial credit).

9. (9) Assume you have a soil sample with the following properties. Calculate the bulk density, porosity and gravimetric water content of the sample.

Property	Value
Airdry weight	298.2 g
Ovendry weight	284. g
Sample volume	200. cm³
Particle density	2.63 g/cm³
Saturated weight	376. g

Bulk density = ovendry mass/sample volume = 1.42 g/cm3 or Mg/m3

Porosity = 1-(bulk density/particle density) = 0.46

Gravimetric water content =

water content of airdry soil/ovendry weight = .05

10. (10) Draw a moisture release curve for a loamy sand soil texture and a clay soil texture. Label the two axes and the two curves. Explain why you drew the curves as you did based on what you know about soil water holding capacity.

The clay has more total pore space so it holds more water than the loamy sand at saturation. It releases water more slowly than the loamy sand because the pore size distribution is skewed towards finer pores.

11. (7) In the laboratory exercise in which you measured texture, what were you measuring when you took readings at 40 seconds and 2 hours? Name three potential sources of error in your measurements.

You were measuring the density of the suspension. At 40 seconds this included the water, sodium hexametaphosphate dispersing solution and silt plus clay. At 2 hours, it included the water, dispersing solution and clay.

Plenty of sources of error. Not reading the hydrometer correctly, making wrong calculations, not stirring well, not using a blank, using the wrong initial weight of soil in the calculations, not measuring at the correct time etc.