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Characterizing Soil HorizonsCharacterizing Soil Horizons
Physical Properties of Soils
SolidsVoids
AvenuesStorageDistributionMovement
Interactive MediaMineralsOrganic matterReactivity
Idealized Surface Soil
Parent Material to Soil
Parent Material
Bedrock
Additions
Losses
Translocations
Transformations
Bedrock
Differentiation
Soil Horizons
Roughly parallel layers in the soil with varying composition and properties
Criteria for Characterizing Criteria for Characterizing Soil HorizonsSoil Horizons
ColorColor
TextureTexture
DensityDensity
StructureStructure
Organic matterOrganic matter
MineralogyMineralogy
ChemistryChemistry
Value
Chroma
Hue
Soil ColorSoil Color
dominant spectral color; related to the wavelengthof light. Related to the proportions of red to yellow.
related to total amount of light reflected. measure of the strength
of spectral color
Hue = 10 YRValue = 6Chroma = 3
Munsell Color 10 YR 6/3
Physical Criteria for Delineating HorizonsPhysical Criteria for Delineating Horizons
ColorColor
TextureTexture
StructureStructure
DensityDensity
The Soil Mineral Component: Texture
the size of soil particles
Soil texture refers to the relative amounts of three distinct sizeseparates comprising the soil
mineral component.
Sand Silt Clay
Sizes classes of particles
Soil TextureSoil TextureDiameter
Sand (2.0 – 0.05 mm) Quartz
Silt (0.05 – 0.002 mm) Quartz /Feldspars/mica
Clay (<0.002 mm) Secondary minerals
Dominant Minerals
sandsiltclay
100%Quartz Distribution
Class
Importance of Soil Texture
Soil PorosityParticle Surface Area
(Distribution of particle sizes)
Water/Gas MovementReactivity
Soil Porosity
Porosity – the total volume of soil pores- the distribution of pore sizes
Sand Silt Clay
Texture, Pore Sizes, and Water
Large particles yield large pore spacesSmall particles yield small pore spaces
Water moves rapidly and is poorly retained in Coarse-textured sandy soils.
Water moves slowly and is strongly retained inFine-textured, clayey soils.
Sands
Clays/iron
Rapid
Slow
Water Retained
Poor Retention
Surface Area and Particle Size
Each face is 4 mm2 Each face is 1 mm2
6 faces x 4 mm2 = 24 mm2 6 faces x 1mm2 x 8 cubes = 48 mm2
If each of the resulting cubes was divided similarly, the surface area would increase 16 times more
2 mm 2 mm
2 mm
1 mm
1 mm
1 mm
Surface AreaSurface Area
Specific Surface Area = Surface Area mass
cm2
g
units
Interface with the environment
nutrients
gassesO.M.
water
microorganisms
Specific Surface Area
Separate SSA (cm2/g)Sand 30Silt 1500Clay 3,000,00016 g of clay
Soil A
100 g soil
Soil B
95 g sand 4 g silt 1 g clay
90 g sand 5 g silt 5 g clay
95 g sand x 30 g/cm2 = 2850 cm2
4 g silt x 1500 g/cm2 = 6000 cm2
1 g clay x 3 M g/cm2 = 3 M cm2
Total = 3,008,850 cm2
90 g sand x 30 g/cm2 = 2700 cm2
5 g silt x 1500 g/cm2 = 7500 cm2
5 g clay x 3 M g/cm2 = 15 M cm2
Total = 15,010,200 cm2
Soil Horizons
Texture
Clay Content
Surface Area
Potential Reactivity
Soil Textural Classes
Soil Textural Triangle
Sand <10%Loamy sand 10 – 15%Sandy loam 15 – 20%Sandy clay loam 20 – 35%Sandy clay 35 – 55%Clay > 50%
Florida Soils
clay
70% sand, 20% silt, 10% clay70% sand, 20% silt, 10% clay
60% sand, 10% silt, 30% clay60% sand, 10% silt, 30% clay
Measuring Soil Texture
Texture-by-Feel
Relative amounts of 3 soil separates:
Sand, Silt, and Clay
Gritty smooth plastic
Texture-by-FeelSoil
SandNo Ball
Loamy SandNo Ribbon
Grittiness, Smoothness
Low Clay Medium Clay High Clay
Increasing Ribbon Length
Field Analysis
Texture by Feel
Sand = Gritty
Silt = Smooth
Clay = Sticky, Plastic
Laboratory Analysis of Soil Texture
Laboratory AnalysisLaboratory Analysis
Sedimentation – Sand, Silt, and Clay Fraction
gravity
dragdrag
SandSilt
Clay
SandSilt
Clay
sandsilt
Sedimentation
Quantifying Sedimentation Rates
Stokes’ LawStokes’ Law
Velocity V(cm/s) = g (dp-dL) D2
18ų
g = gravitydp = density of the particledL = density of the liquid
ų = viscosity of the liquid
KV = D2
K = 11,241 cm-1 sec-1
1 cm · sec
Stokes’ LawStokes’ Law
V = K D2
K = 11,241 cm-1 sec-1
Sand: D = 1 mm 0.1 cm
V = 11,241 x (0.1)2
= 112.4 cm/sec
1 cm · sec X cm2
Stokes’ LawStokes’ Law
V = K D2
K = 11,241 cm-1 sec-1
clay: D = 0.002 mm 0.0002 cm
V = 11,241 x (0.0002)2
= 0.00045 cm/sec
1 cm · sec X cm2
050
100150200250
300350400
450500
0 0.5 1 1.5 2 2.5
Particle Diameter (mm)
Set
tlin
g V
elo
city
(cm
/sec
)
sandsilt
clay
SedimentationSedimentation
1 minute Sand settles out
4 hours Silt settles out
sand1 min.silt 4 hr.
suspension
The density of a soil suspension decreasesas particles settle out.
Stem
Scale
Bulb
weight
t = 0 t = 1 min
Hydrometer MethodHydrometer Method
hydrometer
0 g/L
Pure distilled water (18o C) = 0 g/L
Hydrometer MethodHydrometer Method
Time = 0 sec density = 40 g/L
Add 40 g soil to 1 liter of water
Time = 1 min. density = 10 g/L
Sand settled = 40 g– 10 g= 30 g
Sand (%) = 30 g sand = 0.75 = 75%
40 g soil
Time = 4 hrs density = 4 g/L
Hydrometer MethodHydrometer Method
What is being measured?
Clay content = 4 g
Silt % = 100% - (75% + 10%) 100% - 85%
= 15 %
Clay % = 4 g clay = 0.10 = 10% 40 g soil
Sand = 75%Silt = 15%Clay = 10%
Sandy Loam
Hydrometer MethodHydrometer Method
Reactivity, Water Movement
Next: Density, Structure, Porosity