Please answer all questions in the space available.
1. (9) Consider a hill in Northern California as shown in the diagram below. For each of the two sites on the hill, draw a diagram of the soil temperature distribution from the surface to two meters on June 21 and December 21. Carefully label the curves and all axes. The mean annual air temperature is 60 °F.
Note that summer temperatures are higher at the surface then subsoil and opposite in winter. Note that all tend towards the mean annual temperature. Note that the south facing slope is warmer than the north facing slope.
2. (10) Explain why you drew the curves as you did.
South facing slopes get more solar radiation than north facing. This makes them warmer and drier, thus influencing processes and products. The south facing slope in the picture is steeply sloping meaning it gets more direct radiation than a flat surface, further increasing the difference between the two sites. Heat flow downward is attenuated with depth to near the mean. In the winter, the surface soil temperatures are lower than the subsoil because the rate of flow of heat is slow and it takes time for the heat pulse to move downward or upward.
3. (20) Determine the cation exchange capacity of a soil sample using the data from the following procedure. You remove the cations from a 20 g (oven dry weight) soil sample using a barium chloride solution. The final volume of the extracting solution is 50 mL. You then take a 10 mL aliquot of the extracting solution and dilute it to a volume of 100 mL. You analyze the 100 mL of solution and find that the concentrations (mg/L) of ions are Ca2+ = 48 mg/L, Al3+ = 32.4 mg/L, and H+ = 6 mg/L. What is the CEC of the soil? The atomic weights of the ions are Ca=40g, Al=27g, H=1g.
1.2 mg/g soil *1g/1000mg * 2cmolc Ca/0.40 g Ca * 1000g/kg soil = 6 cmolc Ca/kg soil
Using the same process, there are 9 cmolc Al /kg and 15 cmolc H/ kg soil.
Add these together and the result is 30 cmolc/kg soil
You lost points if units were wrong and/or if math was wrong. Not everyone got to the same place the same way as I did. I checked everyone's calculations as best I could and gave credit if the math and units were ok.
4. (16) Name the four components of total hydraulic head AND explain how each contributes to water flow in soil.
Pressure--water flows to decrease pressure
Gravity--water flows downward
Osmotic or solute--water flows to decrease salinity; e.g. salts decrease potential energy
Matric--water flows from wet to dry. This describes the change in potential energy due to interaction between the solid phase and the solution phase.
5. (8) For the diagram below, draw (using arrows to show head dimensions) each component of hydraulic head (label each appropriately) for the input and outflow side of the column.
6. (5) Calculate the saturated hydraulic conductivity for a 30 cm long, 5 cm diameter soil column in which the flow from the column is 3.0 cm3/min and the total change in hydraulic head over the length of the column is 15 cm. Area of the column is r2.
Q = A * K *DH/DX
3 cm3/min = 3.14 *(2.5 cm)2 * K * (15/30)
K = 0.30 cm/min
7. (15) Examine the diagram below and then answer the questions following the diagram. All pots have the same amount of the same soil.
In pot A, B and C the soil is sterilized. In pot A nothing has been added. In pot B, nitrogen and phosphorus have been added and in pot C nitrogen has been added. Pot D has neither been sterilized nor fertilized.
a. Why did the plant in pot D grow so well compared to the plant in pot A?
Pot D must have a rich microbial population that helps to provide N and P to the plant. Pot A does not because it has been sterilized. The microorganisms may be N fixers or OM decomposers (that release N). The plant may have a mycorrhizal association that helps it to get P.
b. Why did the plant in pot B grow as well as the plant in pot D?
Because all the N and P the plant needed was supplied.
c. Why did the plant in pot C grow better than the plant in pot A but not as well as either the plant in pot B or D?
The N helped to alleviate the N deficiency caused by the sterilization but the plant remains P deficient because it lacks the symbiosis needed to get sufficient P.
8. (5) How can the addition of organic matter decrease soil fertility in the short term?
If the carbon:nitrogen ratio is too high, the microorganisms will use the C and all of the N, thus making the N unavailable for plants. In addition, the microorganisms will use otherwise plant-available N as they continue to utilize the C in the OM, thus lowering the soil N-supplying capacity in the short term.
9. (12) Among the bacteria are photoautotrophs, chemoautotrophs and heterotrophs. What is one characteristic energy source for each of these groups?
Photoautotrophs utilize sunlight for energy.
Chemoautotrophs use the oxidation of inorganic compounds for energy.
Heterotrophs (like us) utilize preformed carbon sources (other than CO2) for energy.
