Midterm 1

Question 1

Which answer correctly describes the master soil horizon “A”?

Answer 1

a horizon that is typically found near the surface, is darker in color and coarser in texture than the horizons underneath.

Question 2

True or false: increasing the organic matter content of a soil is likely to INCREASE the soil’s water holding capacity.

Answer 2

true

Question 3

The bulk of gaseous interchange in soils takes place by

Answer 3

diffusion

Question 4

True or false: 2:1 clays have a net electroPOSITIVE charge in ACIDIC soils.

Answer 4

false

Question 5

In which of the following situations would you expect downward movement of soil water through the profile to be most rapid?

Answer 5

uniformly textured sandy loam profile

Question 6

True or false: B horizon is pale.

Answer 6

false

Question 7

True or false: the matric potential is due to the attraction of water molecules to soil solid surfaces.

Answer 7

true

Question 8

True or false: the cation exchange capacity of a soil is determined primarily by the amount and kind of colloids in the soil and by the pH.

Answer 8

true

Question 9

The swelling/shrinking tendency of some silicate clay minerals is due primarily to

Answer 9

the movement of water molecules in and out of the interlayers of the crystal

Question 10

True or false: alluvial/fluvial parent materials are those that have been deposited by streams.

Answer 10

true

Question 11

Which of the following actions would significantly increase ventilation?

Answer 11

draining the soil of excess water

Question 12

Mechanical weathering processes result in

Answer 12

the disintegration of rocks due to differential expansion of minerals

Question 13

A smooth feel and matte surfaces indicate a high percentage of which soil separate?

Answer 13

silt

Question 14

Forest fires result in sufficiently high soil temperatures to cause the chemical alteration of certain organic compounds in soils. The main effect of these compounds is to

Answer 14

reduce water infiltration and percolation through the profile

Question 15

The reaction 2 KAlSi3O8 + 2 H+ + 9 H2O –> 2K+ + H4Al2Si2O9 + 2H+ + 4 H4SiO4 is a form of chemical weathering called

Answer 15

hydrolysis

Question 16

Which of the listed soil colloids has a reasonably high potassium level, a CEC of about 20 cmolc/kg and a low tendency to swell when wetted?

Answer 16

fine-grained mica

Question 17

Which statement about the soil water potential is correct?

Question 18

You find that the CEC of a sandy loam soil at pH 5.0 is 8 cmolc/kg and at pH 8.2 it is 14 cmolc/kg. What is the most likely reason for this difference?

Answer 18

ionization of carboxyl and phenolic groups to release H+ ions

Question 19

Granite is an example of a(n)

Answer 19

igneous rock

Question 20

Which of the following is NOT a characteristic of soil colloids?

Answer 20

low water holding capacity

Question 21

Explain the term “permanent charge” of a 2:1 clay mineral. What type of charge is it? Why do 2:1 clay minerals carry it?

Answer 21

A permanent charge is a charge that does not respond or change with changes to the pH. The 2:1 clay minerals carry it because the crystals form with the wrong center.

Question 22

Sketch the basic structure of a 2:1 clay mineral. (the physical structure of the layer and the chemical composition of the sheets)

Question 23

The 4 types of soil colloids are sesquioxides (Al/Fe oxides), _____, _______, and amorphous clays.

Answer 23

organic matter
crystalline silicate clays

Question 24

State the chemical formula for carbonic acid.

Answer 24

H2CO3

Question 25

Briefly explain how carbonic acid forms in soils. Which chemical are involved in the formation of carbon acid? What are the sources of the chemicals that combine to form carbonic acid?

Answer 25

Carbonic acid forms from a combination of CO2 and water.
All soils that have life have carbonic acid because it is produced by the respiration of plants and microbes.

Question 26

An extraction procedure reveals that a soil contains the following amounts of various exchangeable ions. Note that all ions are reported in units of cmolc/kg.

What is the cation exchange capacity (CEC) of this soil in cmolc/kg?

Ion cmolc/kg
Al3+ 1
Ca2+ 5
K+ 3
H+ 1
Mg2+ 3
NH4+ 5
PO4–2 1

Answer 26

18
(don’t count PO4 because is a negative ion)

Question 27

A valley with shallow soils overlaying impermeable bedrock is drained by a single river. It receives 700 mm of precipitation per year, approximately half as snow and half as rain.

From measurements of the river’s depth, width, and velocity, it is determined that approximately 450 mm of water is discharged each year.

How much water is lost from the valley to the atmosphere by evapotranspiration?

Answer 27

250

Question 28

The strength of absorption of ions, when they are present in equivalent quantities, by most colloids is

Answer 28

H+ > Mg2+ > Na+

Question 29

A silty soil with the following properties was analyzed in the lab. What is its bulk density?
[Important: State both the numerical answer and its units.]

field mass-dry mass-org. C-depth-diameter-volume
298 g 262 g 0.77% 6cm 6cm 170cm^3

Answer 29

1.54 g/cm^3

Question 30

The five state factors that explain pedogenesis are climate, parent material, ___,_______, and ______.

Answer 30

organisms
relief
time

Question 31

what is the greenhouse gas has the redox potential of well aerated (>400 mV)

Answer 31

carbon dioxide

Question 32

what is the greenhouse gas has the redox potential of poorly aerated (100-400 mV)

Answer 32

nitrous oxide

Question 33

what is the greenhouse gas has the redox potential of anoxic (-200 mV)

Answer 33

methane

Question 34

True or false: increasing the organic matter content of a soil is likely to INCREASE the soil’s water holding capacity.

Answer 34

true

Question 35

the expression P-ET-D estimate the ____ a watershed

Answer 35

soil water content of

Question 36

True or false: water will move from one site in a soil with a water potential of -100kPa to an adjacent site with a water potential of -1200 kPa

Answer 36

true

Question 37

which statement about the soil water potential is correct

Answer 37

not the one about -10kPa to 0 kPa and NOT the one about percolation

Question 38

True or false: organic matter has a much higher cation exchange capacity at pH 5.5 than at pH 8

Answer 38

false

Question 39

the electronegative charge of 2:1 clays orginiates from

Answer 39

isomorphic substitution of a central atom for another with lesser charge in the crystal structure of the clay

Question 40

If you were seeking a soil colloid with a high capacity to adsorb cationic pollutants (e.g. Cd2+), but also with a consistent capacity to do so even as the pH varied from 4.0 to 7.5, which of the following would you choose?

Answer 40

NOT GIBBSITE

Question 41

List the six plant macro-nutrients

Answer 41

nitrogen
phosphorus
magnesium
calcium
sulfur
potassium

Question 42

Walking barefoot across a tile floor feels colder than walking on a wooden floor because the tiles have a ___ than wood

Answer 42

larger thermal conductivity

Question 43

explain the term field capacity

Answer 43

the amount of moisture in a soil after it has been freely drained of excess water by gravity

Question 44

draw a diagram showing how the temperature of a moderately-weather soil without plant cover as a function of soil depth in summer vs winter.
briefly describe how soils impact the temperature at the land surface? what is the main effect of soils on regional temperatures?