Transcript Slide 1

Characterizing Soil Horizons
Physical Properties of Soils
Idealized Surface Soil
Voids
Avenues
Storage
Distribution
Movement
Solids
Interactive Media
Minerals
Organic matter
Reactivity
Parent Material to Soil
Differentiation
Additions
Losses
Parent Material
Translocations
Transformations
Bedrock
Bedrock
Soil Horizons
Roughly parallel layers in the soil
with varying composition and properties
Criteria for Characterizing
Soil Horizons
Color
Texture
Density
Structure
Organic matter
Mineralogy
Chemistry
Soil Color
Hue
dominant spectral color;
related to the wavelength
of light. Related to the
proportions of red to yellow.
Value
related to total
amount of light
reflected.
Chroma
measure of the strength
of spectral color
Hue = 10 YR
Value = 6
Chroma = 3
Munsell Color
10 YR 6/3
Physical Criteria for Delineating Horizons
Color
Texture
Structure
Density
The Soil Mineral Component: Texture
the size of soil particles
Soil texture refers to the relative
amounts of three distinct size
separates comprising the soil
mineral component.
Sizes classes of particles
Sand
Silt
Clay
Soil Texture
Class
Sand
Silt
Clay
Diameter
Dominant Minerals
(2.0 – 0.05 mm)
Quartz
(0.05 – 0.002 mm) Quartz /Feldspars/mica
(<0.002 mm)
Secondary minerals
Quartz Distribution
clay
silt
sand
100%
Importance of Soil Texture
(Distribution of particle sizes)
Soil Porosity
Particle Surface Area
Water/Gas Movement
Reactivity
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 spaces
Small particles yield small pore spaces
Water moves rapidly and is poorly retained in
Coarse-textured sandy soils.
Water moves slowly and is strongly retained in
Fine-textured, clayey soils.
Rapid
Sands
Poor Retention
Clays/iron
Slow
Water Retained
Surface Area and Particle Size
2 mm
2 mm
1 mm
1 mm
1 mm
2 mm
Each face is 4 mm2
6 faces x 4 mm2 = 24 mm2
Each face is 1 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
Surface Area
Specific Surface Area = Surface Area
mass
water
units
cm2
g
nutrients
Interface with the environment
O.M.
gasses
microorganisms
Specific Surface Area
Separate
SSA (cm2/g)
Sand
30
Silt
1500
Clay 16 g of clay 3,000,000
100 g soil
Soil A
95 g sand
4 g silt
1 g clay
Soil B
90 g sand
5 g silt
5 g clay
95 g sand x 30 g/cm2 = 2850 cm2
90 g sand x 30 g/cm2 = 2700 cm2
4 g silt x 1500 g/cm2 = 6000 cm2
5 g silt x 1500 g/cm2 = 7500 cm2
1 g clay x 3 M g/cm2 = 3 M cm2
Total = 3,008,850 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
Florida Soils
clay
Sand
Loamy sand
Sandy loam
Sandy clay loam
Sandy clay
Clay
<10%
10 – 15%
15 – 20%
20 – 35%
35 – 55%
> 50%
70% sand, 20% silt, 10% clay
60% 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-Feel
Soil
Field Analysis
No Ball
Sand
No Ribbon
Loamy Sand
Increasing Ribbon Length
Low Clay
Medium Clay
High Clay
Grittiness, Smoothness
Texture by Feel
Sand = Gritty
Silt = Smooth
Clay = Sticky, Plastic
Laboratory Analysis of Soil Texture
Laboratory Analysis
Sedimentation – Sand, Silt, and Clay Fraction
drag
drag
gravity
Sedimentation
Sand
Sand
Silt
Silt
Clay
Clay
silt
sand
Quantifying Sedimentation Rates
Stokes’ Law
g (dp-dL) D2
Velocity V(cm/s) =
18ų
g = gravity
dp = density of the particle
dL = density of the liquid
ų = viscosity of the liquid
V = K D2
K = 11,241 cm-1 sec-1
1
cm · sec
Stokes’ 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
1
cm · sec
X
cm2
= 112.4 cm/sec
Stokes’ 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
1
cm · sec
X
cm2
= 0.00045 cm/sec
Settling Velocity (cm/sec)
500
450
400
350
300
250
200
150
100
50
0
clay
0
0.5
silt
sand
1
1.5
Particle Diameter (mm)
2
2.5
Sedimentation
The density of a soil suspension decreases
as particles settle out.
1 minute
Sand settles out
4 hours
Silt settles out
suspension
1 min.
silt
sand
4 hr.
Hydrometer Method
Stem
0 g/L
Scale
Bulb
weight
hydrometer
t=0
Pure distilled water (18o C) = 0 g/L
t = 1 min
Hydrometer Method
Add 40 g soil to 1 liter of water
Time = 0 sec
density = 40 g/L
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
Hydrometer Method
Time = 4 hrs
density = 4 g/L
What is being measured?
Clay content = 4 g
Clay % = 4 g clay = 0.10 = 10%
40 g soil
Silt % = 100% - (75% + 10%)
100% - 85%
= 15 %
Hydrometer Method
Sand = 75%
Silt = 15%
Clay = 10%
Sandy Loam
Reactivity, Water Movement
Next: Density, Structure, Porosity