UIUC NRES 201 Exam 2 Review,
NRES 201 Topic 1 uiuc Curriculum
Revision Compilation.
How does the hydrometer method determine soil texture? - ANSWER
Sedimentation by the hydrometer method determines the percentages of sand, silt,
and clay. Sand settles first, then silt -Clay is estimated by the difference.
How do bulk and particle densities differ? - ANSWER Particle density does not
include volume between particles and is unaffected by soil structure, pore space,
compaction, and tillage.
How does bulk density affect root growth? - ANSWER High bulk density impedes
root growth because of poor aeration, slow movement of nutrients and water, and
the buildup of toxic gases.
What are the effects of soil compaction on particle density, bulk density, and percent
pore space? - ANSWER Soil tillage increases bulk density, lowers porosity, and
decreases aggregate stability. All of the changes occur as a result of organic matter
depletion.
How is the percent pore space calculated from bulk and particle densities. -
ANSWER 100- [Db/Dp*100]
How do macro and micropores compare for soil air and water movement? -
ANSWER Macro: allows for rapid air and water movement, accessible to fine roots
and microscopic animals.
Micro: low conductance for air and water movement, some are too small for root
hairs or microorganisms.
Why is tillage more detrimental to aggregate stability for Mollisols than for Oxisols. -
ANSWER Tropical Oxisols have a high content of hydrous oxides - Because of
these oxides, aggregate stability is: Much greater than for temperate soils, Less
dependent on soil organic matter, and Less affected by tillage
What is Soil Organic Matter (SOM) and what are its functions? - ANSWER 3
components:
1. Living organisms (biomass)
2. Organismal remains (residue)
3. Decay (humus)
Functions:
Physical: improves structure, aggregation, and macroporosity. Enhances water
infiltration and aeration, increases water holding capacity. Reduces erosion and
crusting, promotes root growth, decreases evaporation
Biological functions: Essential for heterotrophs
Ecological functions: SOM is major surface Carbon reservoir
How does residue decomposition vary with C/N ratios? - ANSWER Decomposition
is decreased by less disposable residue to a higher C/N ratio.
Faster with low C/N
, UIUC NRES 201 Exam 2 Review,
NRES 201 Topic 1 uiuc Curriculum
Revision Compilation.
Slower with high C/N
How do soil orders vary in organic matter content? - ANSWER Histosols: highest
SOM
What effect has synthetic N fertilization had on SOM levels in the Morrow Plots? -
ANSWER N fertilization increased residue C inputs but not soil organic carbon
storage
Synthetic N fertilization does not replace crop rotation
How do polarity and H bonding affect the properties of water? - ANSWER Polarity:
covalent bonding that leaves two electropositive H and one electronegative O
H bonding: arises when a H atom of one water molecule is attracted to the O end of
another
What causes capillarity? - ANSWER Water rises in a tube when: some of the
molecules climb up the tube wall by adhesion and they pull the others up by
cohesion
water-water = cohesion
water-surfaces = adhesion
What affects the height and rate of capillary rise in soils? - ANSWER Height of rise:
clays>loams>sands
Rate of rise: sands>loams>clays
What are the 4 potentials that determine the energetics of soil water? Which is most
important? - ANSWER 1. Matric: represents the amount of work that must be
expended to extract water that has bonded to soil solids **Most important***
2. Osmotic: represents the amount of work needed to remove water from a solution
with a given concentration of solutes
3. Gravitational: represents the amount of work required to move a given mass of
water against gravity
4. Hydrostatic: represents positive pressure of water in a saturated zone below the
water table
How is soil water classified with respect to saturation, field capacity, the wilting
coefficient, the hygroscopic coefficient, and oven-dryness? - ANSWER 1.
Saturation: all pores filled with water, maximum retentive capacity.
2. Field capacity: soil water content after gravitational water has been drained from
micropores
3. Wilting coefficient: soil water content that supplies no water to plants
4. Hygroscopic coefficient: soil water content when evaporation ceases
5. Oven-dryness: only chemically bonded water is present.
How is gravimetric water content calculated? - ANSWER Wet weight of soil - oven
dry weight/ oven dry weight
NRES 201 Topic 1 uiuc Curriculum
Revision Compilation.
How does the hydrometer method determine soil texture? - ANSWER
Sedimentation by the hydrometer method determines the percentages of sand, silt,
and clay. Sand settles first, then silt -Clay is estimated by the difference.
How do bulk and particle densities differ? - ANSWER Particle density does not
include volume between particles and is unaffected by soil structure, pore space,
compaction, and tillage.
How does bulk density affect root growth? - ANSWER High bulk density impedes
root growth because of poor aeration, slow movement of nutrients and water, and
the buildup of toxic gases.
What are the effects of soil compaction on particle density, bulk density, and percent
pore space? - ANSWER Soil tillage increases bulk density, lowers porosity, and
decreases aggregate stability. All of the changes occur as a result of organic matter
depletion.
How is the percent pore space calculated from bulk and particle densities. -
ANSWER 100- [Db/Dp*100]
How do macro and micropores compare for soil air and water movement? -
ANSWER Macro: allows for rapid air and water movement, accessible to fine roots
and microscopic animals.
Micro: low conductance for air and water movement, some are too small for root
hairs or microorganisms.
Why is tillage more detrimental to aggregate stability for Mollisols than for Oxisols. -
ANSWER Tropical Oxisols have a high content of hydrous oxides - Because of
these oxides, aggregate stability is: Much greater than for temperate soils, Less
dependent on soil organic matter, and Less affected by tillage
What is Soil Organic Matter (SOM) and what are its functions? - ANSWER 3
components:
1. Living organisms (biomass)
2. Organismal remains (residue)
3. Decay (humus)
Functions:
Physical: improves structure, aggregation, and macroporosity. Enhances water
infiltration and aeration, increases water holding capacity. Reduces erosion and
crusting, promotes root growth, decreases evaporation
Biological functions: Essential for heterotrophs
Ecological functions: SOM is major surface Carbon reservoir
How does residue decomposition vary with C/N ratios? - ANSWER Decomposition
is decreased by less disposable residue to a higher C/N ratio.
Faster with low C/N
, UIUC NRES 201 Exam 2 Review,
NRES 201 Topic 1 uiuc Curriculum
Revision Compilation.
Slower with high C/N
How do soil orders vary in organic matter content? - ANSWER Histosols: highest
SOM
What effect has synthetic N fertilization had on SOM levels in the Morrow Plots? -
ANSWER N fertilization increased residue C inputs but not soil organic carbon
storage
Synthetic N fertilization does not replace crop rotation
How do polarity and H bonding affect the properties of water? - ANSWER Polarity:
covalent bonding that leaves two electropositive H and one electronegative O
H bonding: arises when a H atom of one water molecule is attracted to the O end of
another
What causes capillarity? - ANSWER Water rises in a tube when: some of the
molecules climb up the tube wall by adhesion and they pull the others up by
cohesion
water-water = cohesion
water-surfaces = adhesion
What affects the height and rate of capillary rise in soils? - ANSWER Height of rise:
clays>loams>sands
Rate of rise: sands>loams>clays
What are the 4 potentials that determine the energetics of soil water? Which is most
important? - ANSWER 1. Matric: represents the amount of work that must be
expended to extract water that has bonded to soil solids **Most important***
2. Osmotic: represents the amount of work needed to remove water from a solution
with a given concentration of solutes
3. Gravitational: represents the amount of work required to move a given mass of
water against gravity
4. Hydrostatic: represents positive pressure of water in a saturated zone below the
water table
How is soil water classified with respect to saturation, field capacity, the wilting
coefficient, the hygroscopic coefficient, and oven-dryness? - ANSWER 1.
Saturation: all pores filled with water, maximum retentive capacity.
2. Field capacity: soil water content after gravitational water has been drained from
micropores
3. Wilting coefficient: soil water content that supplies no water to plants
4. Hygroscopic coefficient: soil water content when evaporation ceases
5. Oven-dryness: only chemically bonded water is present.
How is gravimetric water content calculated? - ANSWER Wet weight of soil - oven
dry weight/ oven dry weight
