exam schedule exam i: friday, may 22 nd exam ii: friday june 5 th exam iii: friday june 19 th review...
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Exam Schedule
Exam I: Friday, May 22nd
Exam II: Friday June 5th
Exam III: Friday June 19th
Review sessions will be on the Thursday preceding the exam.
Soil Texture
Three separates: Sand, Silt, ClayImportance: indicator of pore size, surface area, water movement, reactivityThere are 12 textural classes based on relative abundance of sand, silt, clayFlorida soils tend to range from sandy to sandy clay texturesTexture-by-feel assesses grittiness, smoothness, plasticity to estimate textureLaboratory analysis relies on sedimentation of soil particles in waterStokes’ Law determines the settling rates of particles from waterLarge particles (sand) settle quickly, small particles slowly.A hydrometer measures the density of a soil suspensionBased on the density, the mass of particles remaining suspended is determined
Particle Size Large/coarse Medium Fine/Small
SandLoamy SandSandy Loam
Sandy clay LoamSilty clay Loam
Silt LoamLoam
Clay LoamSandy ClaySilty Clay
Clay
Pore Size Large Medium Small
Reactivity Weak Moderate Strong
Texture
Texture and Civilization?
Tigris-Euphrates
Nile
Earliest Civilizations
Civilization
Year-round supply of waterEnduring SunlightTrustworthy harvestsBuilding materials
Population increasesSubstantial homesRelative Safety, peace
Neolithic Founder CropsWheatBarleyFlaxChick PeaLentil
cows, goats, sheep, and pigs
NileJordanTigrisEuphrates
Periodic Flooding
Flooding
Flooding and Soil Texture
Sand 2.0-.05 mmSilt 0.05 – 0.002 mmClay < 0.002 mm
Clay
Sand 1 mm V = 112 cm/secSilt 0.05 mm V = 0.281 cm/secClay 0.002 mm V = 0.0004 cm/sec
Stokes’ Law: V = kD2
K = 11,241 cm-1 sec-1
Flooding slows flow
River channel
Sedimentation
Sand Clays/Silts
Flood peaks in mid-September
Blue NileThe Nile
KenyaUgandaTanzania
Mesopotamia
Euphrates
Alluvial Plain
Flood: March through June
Agriculture and IrrigationIrrigationCanalsDikesWeirsReservoirschannels
History and Soil Texture(knowledge from clay and stone)
Stone and Clay
Egypt
Sumer
Stone
Clay
Architecture and Sculpture
Egypt
The Language of Power
sacred, ceremonial, literary, and scientific language
Sumerian
Soil Structure, Density, Porosity
Soil StructureSoil Structure
Arrangement or grouping of individual soil particles into secondary units.
Soil StructureSoil Structure
PedsAggregates
Units of soil Structure
Chemical Processes –
electrostatic attraction between clayparticles and between organic particles.
Biological Processes-macro-organisms (burrowing, tunneling,wastes)
-roots, fungal hyphae (compression, fibers, exudates)
-microorganisms (organic residues)
Formation
Soil StructureSoil Structure
Importance
Effects on porosity, water retention, water movement
Small pores
Large pores
Intra-aggregate pores
Inter-aggregate pores
(within)
(between)
Water moves easily and is poorly retained in inter-aggregate poresWater moves slowly and is strongly retained in intra-aggregate pores
Soil Structure is Desirable
Poor soil structure caninhibit infiltration of water, water movement,growth of roots.
Structure is the arrangement or grouping of individual soil particles into larger secondary units.
Clays and organic matter possess natural electricalCharge which can electrostatically bind particles together.
Macro-organisms and microorganisms can aid inbinding of individual particles into larger aggregates..
Aggregation allows for both movement and retentionof water via macropores and micropores, respectively..
Poor soil structure can inhibit infiltration of water, water movement, growth of roots.
Summary
Soil Density
DensityDensity
Density = Mass Volume
gcm3( )
2.65 g/cm3
Soil Bulk DensitySoil Bulk Density
Density of soil including the particles and Density of soil including the particles and the pore spacesthe pore spaces
xy
z
Volume = xyz
B.D. ranges between 1.1 and 1.6 g/cm3
(for mineral soil with 1 – 5% organic matter)
BD = mass OD soil volume solids + pores
Bulk vs. Particle DensityBulk vs. Particle DensityBulk density measures the mass of the soil solids in relation to the volume of the soil solids and the soil pores.
xy
zParticle density =
Mass of particle
Volume of particle
No pores
Bulk density = mass solids
volume xyz
BD = 1.6 g/cm3 PD = 2.65 g/cm3
Sampling for Density
Known Volume
Known Volume
Dry and Weigh: mass/volume = Bulk Density
Factors Affecting Bulk DensityFactors Affecting Bulk Density
• organic matter• aggregation• arrangement of particles• compaction• depth in profile
Porosity (pores are weightless)
Organic MatterOrganic Matter
Typical 1-5% organic matterMineral Soil:
bulk density = 1.1 –1.6 g/cm3
Organic Soils: > 20% organic matter
bulk density = 0.1 – 0.6 g/cm3
Factors Affecting Bulk DensityFactors Affecting Bulk Density
AggregationAggregation
One sand grain One aggregate ofClay sized particles(Zero porosity)(50% porosity)
Aggregation generally increases overall porosity, decreases density
Packing ArrangementPacking ArrangementDiscrete particle size classesContinuum of particle sizes
Particle Size
Arrangement(compaction)
Sorting
Depth in ProfileDepth in Profile
Lower organic matterFewer rootsCompaction from above
=> Higher bulk density
Aggregation can mitigateSome of these effects.
Summary Summary
Bulk densities of typical mineral soilsrange between 1.0 and 1.6 g/cm3.
Organic matter increases porosity, decreases BDOrganic soils can have BD as low as 0.1 g/cm3.
Compaction decreases porosity, increases BD
Aggregation increases porosity, decreases BD
Depth in profile decreases porosity, increases BD
Porosity
Bulk Density and Total PorosityBulk Density and Total Porosity
0 2.65Bulk Density (g/cm3)
100%Porosity
)( BDPD
1 - % Porosity = [ ] X 100
2.65 g/cm3
Bulk density high porosity lowBulk density low porosity high
A
E
B
BD = 1.1 g/cm3
BD = 1.15 g/cm3
BD = 1.6 g/cm3
P = 59 %
P = 57%
P = 40 %
Bulk Density and Porosity
A
EE
B
Pore Size DistributionPore Size Distribution
Macropores > 0.8 mm in diameterlarge, freely draining
sands, inter-aggregate pores
Micropores < 0.8 mm in diametersmall, storage of water
clays, intra-aggregate pores
The effect of total porosity and of pore size distribution is largelyrelated to the movement and retention of water as well asthe movement of soil gases
Knowledge of texture, structure, bulk density, and porosity allow deduction of the patterns
of movement of water and gases in soils
Next: Water in Soils
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