Midterm 1 Flashcards
Which answer correctly describes the master soil horizon “A”?
a horizon that is typically found near the surface, is darker in color and coarser in texture than the horizons underneath.
True or false: increasing the organic matter content of a soil is likely to INCREASE the soil’s water holding capacity.
true
The bulk of gaseous interchange in soils takes place by
diffusion
True or false: 2:1 clays have a net electroPOSITIVE charge in ACIDIC soils.
false
In which of the following situations would you expect downward movement of soil water through the profile to be most rapid?
uniformly textured sandy loam profile
True or false: B horizon is pale.
false
True or false: the matric potential is due to the attraction of water molecules to soil solid surfaces.
true
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.
true
The swelling/shrinking tendency of some silicate clay minerals is due primarily to
the movement of water molecules in and out of the interlayers of the crystal
True or false: alluvial/fluvial parent materials are those that have been deposited by streams.
true
Which of the following actions would significantly increase ventilation?
draining the soil of excess water
Mechanical weathering processes result in
the disintegration of rocks due to differential expansion of minerals
A smooth feel and matte surfaces indicate a high percentage of which soil separate?
silt
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
reduce water infiltration and percolation through the profile
The reaction 2 KAlSi3O8 + 2 H+ + 9 H2O –> 2K+ + H4Al2Si2O9 + 2H+ + 4 H4SiO4 is a form of chemical weathering called
hydrolysis
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?
fine-grained mica
Which statement about the soil water potential is correct?
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?
ionization of carboxyl and phenolic groups to release H+ ions
Granite is an example of a(n)
igneous rock
Which of the following is NOT a characteristic of soil colloids?
low water holding capacity
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?
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.
Sketch the basic structure of a 2:1 clay mineral. (the physical structure of the layer and the chemical composition of the sheets)
The 4 types of soil colloids are sesquioxides (Al/Fe oxides), _____, _______, and amorphous clays.
organic matter
crystalline silicate clays
State the chemical formula for carbonic acid.
H2CO3
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?
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.
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
18
(don’t count PO4 because is a negative ion)
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?
250
The strength of absorption of ions, when they are present in equivalent quantities, by most colloids is
H+ > Mg2+ > Na+
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
1.54 g/cm^3
The five state factors that explain pedogenesis are climate, parent material, ___,_______, and ______.
organisms
relief
time
what is the greenhouse gas has the redox potential of well aerated (>400 mV)
carbon dioxide
what is the greenhouse gas has the redox potential of poorly aerated (100-400 mV)
nitrous oxide
what is the greenhouse gas has the redox potential of anoxic (-200 mV)
methane
True or false: increasing the organic matter content of a soil is likely to INCREASE the soil’s water holding capacity.
true
the expression P-ET-D estimate the ____ a watershed
soil water content of
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
true
which statement about the soil water potential is correct
not the one about -10kPa to 0 kPa and NOT the one about percolation
True or false: organic matter has a much higher cation exchange capacity at pH 5.5 than at pH 8
false
the electronegative charge of 2:1 clays orginiates from
isomorphic substitution of a central atom for another with lesser charge in the crystal structure of the clay
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?
NOT GIBBSITE
List the six plant macro-nutrients
nitrogen
phosphorus
magnesium
calcium
sulfur
potassium
Walking barefoot across a tile floor feels colder than walking on a wooden floor because the tiles have a ___ than wood
larger thermal conductivity
explain the term field capacity
the amount of moisture in a soil after it has been freely drained of excess water by gravity
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?
