Exam 2 Flashcards
Soil texture
-relative proportion of primary particles (sand silt clay)
Larger particles are called ______ and have an effective diameter of
- sand
- 2 - .05mm
- have irregular size and shape
Smaller particles are called ___ and have an effective diameter of
- clay
- less than .002 mm
- colloidal are less than .001 mm
Medium size particles are called ____ and have an effective diameter of
- silt
- .05 - .002mm
Equation for specific surface area
=surface area divided by unit mass or volume
Characteristics of sandy soils
- low water holding capacity, organic matter, fertility, and compaction potential
- rapid water drainage, well aerated
- susceptible to wind erosion and resist to water erosion
Characteristics of silty soils
- medium water holding capacity, organic matter, fertility, drainage, and aeration
- ver susceptible to wind and water erosion
Characteristics of clay soils
-slow drainage and poor aeration
-high water holding capacity, organic matter, fertility
-if dispersed, susceptible to wind and water erosion
if aggregated, not susceptible to wind and water erosion
Colluvial soil texture _____
Lacustrine soil texture ____
- sand
- clay
Soil structure
- aggregation of primary particles into secondary units
- peds, aggregates
GRADES OF SOIL STRUCTURE
Weak soil structure:
Moderate soil structure:
Strong soil structure:
WEAK: poorly formed, indistinct peds, barely observable in place
MOD: well formed peds, evident in undisturbed soil
STRONG: well formed peds distinct in undisturbed soil
Genesis of soil structure: aggregate formation
-physical processes
-wetting and drying
freezing and thawing
-physical effects of roots and activity of soil organisms
-long vs short term tillage
Biological processes that enhance aggregate stability
- soil organic matter (SOM)
- microbial decomposition products
- root exudates
- fungal hyphae exudates
Soil structure genesis II, Aggregate STABILITY
- Inorganic material (interacts with organic matter)
- silicate clay
- Fe and Al oxides
- Cations
Site quality
-relative measure of the vegetative production capacity of a site for a give purpose
Benefits of soil structure
- INCREASED
- porosity
- aeration
- infiltration
- percolation
Partical density
- mass per unit volume of soil solids
- compressed
- assumed particle density is 2.65 Mg/m^3
Bulk density
-mass per unit volume of dry, undisturbed soils
-soil particles plus pore space
=mass (dry) divided by volume
Total pore space =
- soil moisture tension at 0 bars or 0 kPa
- =100 - ((BD/PD)*100)
- PD = 2.65
what is the assumed particle density?
2.65Mg/m^3
Factors influencing total pore space and bulk density
- STRUCTURE: BD is lower for well developed granular structure
- ORGANIC MATTER: DB decreases with increasing OM bc there are more small particles, OM contributes to structure
- DEPTH: DB increases with depth in profile bc there is less freezing/thawing and physical activity
Factors influencing TPS and BD
-management practice
-traffic on moist soils = soil compaction
-DB increases with this
-tillage
-DB increases long-term and decreases short term
-long-term is a potential harm
-texture
-DB decreases with finer texture soils so TPS
increases
Hectare furrow slice (HFS)
- 2.2 million kg/HFS
- 2,000,000 lb/ac (AFS)
- kg/ha divided by 1.1 = lb/ac
Equation for soil weight
-BD x Volume
Ways of measuring bulk density
1) core method: V=(pi)h(r^2), BD=m/v
2) clod method: V=displaced water, BD=m/v
3) Excavation: V - with beads, water, urethane foam, BD=m/v
4) Gamma Radiation
5) Freezing: V=water displaced BD=m/v
6) 3D scanning technique
Macropores
- large diameter pores
- water flows freely if drainage is unimpeded
- .08 to 5+mm in size
Micropores
- small diameter pores
- hold water against the force of gravity
- .03 to .08 diameter
Harvesting impacts of soil structure
- most severe when soils are wet
- recovery may require freezing
- poorly sorted sands are most susceptible
Soil physical properties influence soil water:
-availability, storage, and movement
-function of pore size distribution and attraction of soil
solids for water
Hydrogen bonding equals high
- boiling pt
- viscosity
- specific heat
consequences of polarity
- hydrogen bonding
- cohesion (water molecule attraction for each other
- adhesion (water molecule attraction for soil surfaces)
Free energy
- measure of capacity of water to do work
- water moves from high to low free energy
- affected by the three sources in the soil
The three little forces affecting free energy
- Matric (attraction of soil solids for water, reduces free energy)
- Osmotic (attraction of ions for water, reduces free energy)
- Gravitational (elevation increases free energy)
Total soil water potential
- sum of component potentials
- amnt of work required to transport unit quantity of pure water reversibility and isothermally
Field capactiy
- moisture content of soil 2-3 days after a soaking rain
- when soil moisture potential is -.1 to -.33 bars
- or -10 kPa
Wiliting point
- moisture content of soil at which plants wilt
- when soil moisture potential is -15 bars or -1500 kPa
1 bar equals ____ kPa
100
Available water equation
=FC-WP
Hydroscopic coefficient
- moisture content of air dry soil
- when soil moisture potential is -31 bars or -3100 kPa
Gravimetric moisture content
=(wet-dry)*100/dry
-percentage
Volumetric moisture content
gmc x BD
-percentage
Factors influencing available water
- texture
- organic matter
- thickness
soil compaction impacts on available water
- air filled porosity
- soil structure
- soil depth/layering
Hysteresis
- moisture content of a soil at a given soil water potential
- higher when the soil is drying that when it is wet
What does Ksat represent
- saturated hydroulic connectivity
- pore size/shape/connectivity
- decreases with time and depth
What physical properties result in higher Ksat
- sandy texture
- stable granular structure
Preferential flow
- non uniform water movement through soil (macropores)
- free water flows through megapores
Unsaturated flow
- driven by gradient in matric potential
- .001 rate of saturated flow
- higher for fine texture
Well aerated soil
-gas exchange sufficiently rapid to replenish oxygen and prevent buildup of carbon dioxide
Diffusion
- movement across a gradient in response to a gradient
- 10,000x greater in air filled compared to water filled pore
Field soil oxygen availability regulated by
- soil macroporosity texture and structure
- soil water content air filled porosity
- oxygen consumption by roots and soil organisms
4 methods of assessing soil aeration
- Content of O2 and other gases
- Air-filled soil porosity
- Oxygen diffusion rate
- Redox potential
Assesing soil aeration: content of o2 and other gases
- O2 > .1 L/L (10%) required most upland plants
- CO2 > .1 L/L (10%) toxic most upland plants
Assesing sol aeration: air filled soil porosity
-<20% of TPS or < 10% soil vol
Assessing soil aeration: oxygen diffusion rate
<0.3 ug/cm^2/min: top growth ceases
<0.2 ug/cm^2/min: root growth ceases
Assessing soil aeration: Redox potential (Eh)
-tendency of a system to reduce or oxidize
IRIS and MIRIS tubes
Ways to measure reduction
- IRIS: indicator of reduction in soils (in Montezuma)
- MIRIS: Mn indicator of reduction in soils
Redox potential (Eh) of a well drained soil
> .4volts
Factors affecting soil aeration
- macropore space
- biochemical reaction rates
- season
Impact of poor aeration
- reduced root growth
- reduced root nutrient and water uptake
- reduced microbial decomposition rate
- buildup of toxic materials
- WETLANDS
Aeration problems and management alternatives
- improve drainage (ditches, drainage tile, raised beds) to remove water
- manage for appropriate species
Hydric soils
- formed under conditions of saturation, flooding, or ponding long enough during to the growing season to develop anaerobic conditions in the upper part
- saturated, reduced soils
Soil temperature
- affects physical, chemical, and biological soil processes (winter burn)
- Q10: rate of chemical rx doubles with increase in 10 degrees C
Factors influencing soil temperature
- LATITUDE: decreasing NRG input N or S from equator
- ALTITUDE: decreases 6 deg C per 1000m increase in altitude
- ASPECT/SLOPE: in the northeast, SW slopes are warmer and drier than NE slopes
- DEPTH: temp variations cease below 10m depth
- COVER: ground (living and nonliving)
Soil thermal properties
- SPECIFIC HEAT: ratio of the amnt of heat nrg required to raise the temp of a substance 1 degree C compared to that required for the same vol of water
- HEAT OF VAPORIZATION: amnt of nrg required to vaporize water
Heat transfer is soil (Qh)
Qh=K(change in T)/x
-K = thermal conductivity
x=distance
Thermal conductivty (K) is influenced by:
from heat transfer equation
- soil moisture
- organic matter content
- soil texture
- bulk density
Soil temperature and root growth
- minimal range: 0 to 7 deg C
- optimal range: 10 to 25 deg C
- Max range: 25 to 35 deg C
Frost heaving
-formation of segregated ice lenses and volumetric expansion of water during freezing
3 requirements of frost heaving
1) prolonged period of subfreezing temp
2) frost susceptible soils
3) source of water
Accelerated erosion
-in response to human activities
Geologic erosion
-in the absence of human activities
Process of soil erosion
- detachment
- transport (wind, water)
- Deposition
Types of water erosion
- Sheet erosion
- Rill erosion (miniture gullies)
- Gully erosion
Common models to predict erosion
- WEPP SWAT models
- USLE RUSLE models
WEPP
-water erosion prediction project
-simulation model that predicts PROCESSES (hydrologic, plant growth, and litter decay), ON AND OFF SITE EFFECTS OF (raindrop impact, splash erosion, etc.)
-soil surface is classified into rill and inter rill
-rill: dG/dx = Di + Dr
-interrill component: Di = KiI^2GeCeSf
Dr = rill detatchment/deposition rate
Di = interrill sediment delivery rate to rills
SWAT
- soil and water assessment tool
- super model comprised of many individual models
- simulates hydrological processes in large, heterogenous watersheds
- hillslope erosion (sheet and rill) processes
- uses modified MUSLE subroutine among other models
USLE
- Universal Soil Loss Equation
- A = RKLSCP
- R=rainfall erosivity
- K=soil erodibility
- LS=topographic factor (slope length (L) x slope gradient (S))
- C=cover management factor
- P=support practice factor
Vadose zone
-zone of aeration above the permanant water table
Soil consistence
-resistance of a soil to compress under mechanical stress
Heat of vaporization
-the amount of energy required to turn a liquid into a gas
Wind erosion transport mechanics
1) SALTATION: short bounces along the ground. 50 to 90% of the movement in wind erosion
2) SOIL CREEP: lateral movement of soil 5 to 25% of total movement
3) SUSPENSION: “dustcloud” particles in the air 15-40% total movement
Factors affecting wind erosion
- wind velocity, and turbulence
- surface roughness
- soil properties (BD, particle size, moisture of soil)
- vegetation
CRP
- conservation reserve program
- paying farmers to remove land for production
- has significantly reduced the amnt of annual soil erosion
What is erosion?
-the detachment, transport, and deposition of particles
Calculation for macropore space
Calculation for micropore space
TPS-FC
TPS-macropore
how to determine which soil has a finer texture on graph
-the one with a greater soil moisture at a given soil moisture tension
Difference between mineral soil and organic soil
-mineral soil typically have a bulk density greater than 1, organic soils typically have BD much less than 1
