Soil Science 100 ID___KEY

Second Exam
Lab Section____________
Friday 17 November 2000
Value 100 points

Please answer all questions in the space available.

1. (12) Distinguish between heat, heat capacity and temperature. 4 points each

Heat is an increase in motion of molecules. [Not needed for full credit--It is produced when radiant energy is absorbed and converted into sensible heat.]

Heat capacity is the amount of heat that an object must absorb or lose to produce a one-degree change in its temperature.

Temperature is a measure of heating intensity.

2. (12) What are the four components of hydraulic head?

Pressure, gravity, matric and osmotic (or solute)

3. (14) a. Draw a fully labeled figure that shows how soil infiltration rate changes over time. b. Briefly explain why the soil infiltration rate curve looks the way you have drawn it.



4. (10) You found that all of the soils in the laboratory had more KCl extractable nitrate (NO3-) then ammonium (NH4+). Given that the data are correct, and based on what you know about the nitrogen cycle, how do you explain this finding?

The rate of nitrification (conversion of ammonium to nitrate) must have been greater than the rate of mineralization (conversion of organic N to ammonium).

(This following is not necessary for full credit) The nitrate accumulated in the dry soil. In nature, it would have been used or leached away.

 

5. (15) a. What is a mycorrhizal association? b. Name one way such associations benefit each of the participants?

A symbiotic association between fungi and plant roots.

The fungus gets readily assimilated carbohydrates from the plant. The plant may get immobile nutrients like P, water, and protection from predators or disease from the fungus.

 

6. (12) Complete the following table by calculating either the mass of the element needed for the number of cmolc or the cmolc represented by the mass of the atom. Note the atomic mass/mole of Mg = 24, Al= 27, K=39, and Ca= 40.

3 points each correct answer.

Ion

Mass (g)

cmolc

Mg2+

0.72

6

Al3+

0.81

9

K+

1.56

4

Ca2+

0.80

4

Mg has 0.14 g/cmolc * 6 cmolc = 0.72 g, Al has 3 cmolc in 0.27 g = 0.81/.027=3*3=9

K has 1 cmolc/.39g. 1.56/.39 = 4; Ca has 1 cmolc/0.20g. 0.80/0.20 = 4.

 

7. a. (15) List 3 reasons why cation exchange is significant to plants, water or people.

CEC allows soil to hold nutrients and toxins making nutrients available to plants, keeping toxins and nutrients out of water supplies.

Readily available source of nutrients

Protects the soil from leaching losses of cations

Potential for renovation of wastes

A pH and nutrient buffer

 

8. (10) The following figure is from chapter 5 of the text. Use it to answer the following question. Can the aquifer illustrated here supply water to an alfalfa field that requires a volume of 235 m3/day? Assume the pressure heads do not change during pumping. The aquifer has a cross-sectional area of 15,000 m2 and a hydraulic conductivity of 0.75 m/day. In addition to answering yes or no, for full credit, .show the calculations used to determine your answer.

This is a Darcy's law problem. Q = K*A* dH/dX

dH = (125 + 210) - 90 = 245m

dX = 9,999m

A = 15,000 m2

K = 0.75 m/day

Q = 0.75 * 15,000 * 245/9999 = 275.6 m3/day.

YES, the aquifer can supply the needed volume of water each day.