Download Soils Background

Soils Basics
Soil
Natural body that occurs on the land surface,
occupies space, and is characterized by one or
both of the following:
• Horizons or layers, or . . .
• The ability to support rooted plants in a natural
environment
– Upper limit is air or shallow (>2.5 m) water
– Lower limit is either bedrock or the limit of biological
activity
– Lower limit for classification set at an arbitrary 2 m
Nonsoil
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Badlands
Beaches
Rubble lands
Rock outcrops
Glaciers
Deepwater habitats
Factors That Influence
Soil Development
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Climate
Parent material
Topographical relief
Organisms
Time
Humans
Landscape Position
• Critically influences water flow and soil formation
• Most wetlands, even groundwater seeps, are on
some sort of concave surface
Hill Slope Elements and Curvature
After Pennock et al., 1987
Slope
Block
Overland and Throughflow:
Convergent landscapes
Contour
Upslope
Potential hydric soil
zone
Runoff
Infiltration
Percolation
Divergent
Upslope
Convergent
Throughflow
“Epiaquic”
“Endoaquic”
Modified from Pennock et al., 1987
Soil Catena
Summit
Backslope
Footslope
Toeslope
Key Soil Properties
Properties that are important to hydric soil
development and recognition:
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Color
Organic matter
Texture
Horizonation
Drainage
Permeability
Aspects of Soil Color
• Hue
• Value
• Chroma
10R 5/8
Coloring Agents in Soil
• Organic matter
– OM will mask all other coloring agents.
• Iron (Fe)
– brown colors are the result of Fe oxide stains coating
individual particles
• Manganese (Mn)
– resulting in a very dark black or purplish black color
• Lack of coatings
– Color of the mineral soil grains (stripped)
Soil Color
Coating of Fe2O3
Mineral grain
(gray)
Remove Fe
Bright
Gray Soil
Soil
“Bright-colored” soil is bright because the gray-colored
mineral grains are coated with a thin layer of “paint” formed
by Fe oxides. Stripping the paint off the particles leaves the
mineral grains exposed.
Hue
Red
0
2.5R
5R 7.5R
10R
Yellow-Red
0
2.5YR 5YR 7.5YR 10YR
Yellow
0
2.5Y
5Y 7.5Y
10Y
Value
• 10/0 - Pure White
The Lightness
or Darkness of
Color
• 5/0 - “Gray”
• 0/0 - Pure Black
Chroma
“Neutral”
Color
/0
“Pure”
Color
/2
/4
/6
Increasing strength of color
Increasing grayness
/8
What is important?
• Chroma is most important, with value close
behind
• Hue is also important
– Gley page hue is present
– Red parent material soils
– Gray parent materials
Soil Color
Munsell Soil Color Book,
7.5YR page
Munsell® or Earth Colors ®
• The Munsell notation system is a
system for recording color.
• The Earth Colors and Munsell
books use the same colors.
– Differences
are from personal
interpretation of color, fading of
pages over time, smudging of chips
with use, and quality control from
the factories.
Reading Soil Colors
• Optimum conditions
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Natural light
Clear, sunny day
Midday
Light at right angles
Soil moist
Color Patterns in Soils
• Matrix (predominant) color
• Color of redoximorphic features (mottles)
• Contrast, abundance, location, and size of
redox features
Redox Feature
Matrix
Contrast
• Contrast refers to the degree of visual
distinction between associated colors
– Faint -- evident only on close examination
– Distinct -- readily seen at arms length
– Prominent -- contrast strongly
Several indicators require distinct or prominent contrast!
Defining Distinct Redox
• Different Munsell page (hue),
OR
• Value differs by more than 2 (i.e. 10YR 3/6
vs 10YR 6/6),
OR
• Chroma differs by more than 1 (i.e. 10YR
4/4 vs 10YR 4/6)
Abundance and Size of Redox
• Few -- less than 2%
• Common -- 2 to 20%
• Many -- more than 20%
• Fine -- < 5 mm
• Medium -- 5 to 15 mm
• Coarse -- > 15 mm
Several indicators require at
least 2% abundance
From Munsell Color Chart & Field Book
Two Categories of Soil Material
Differences in organic matter
content:
• Organic horizons
• Mineral horizons
Two Categories of Soil Material
Organic horizons
• Consist of decomposed
organic material
• An organic soil has at least 16
inches of decomposed organic
material in the upper 32
inches, or any thickness over
bedrock
Two Categories of Soil Material
Mineral horizons
• Primarily sand, silt,
and clay, with varying
amounts of organic
matter
Soil Texture - Relative Sizes of Soil
Particles
Sand (0.05-2.00 mm)
Silt (0.002-0.05 mm)
Clay (<0.002 mm)
Soil
Texture
Triangle
Loamy Sands?
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70-87% sand, 0-30% silt, and 0-15% clay,
loose and single-grained,
most individual grains can be seen and felt,
are slightly cohesive when moist, and
fragile casts can more readily be formed
with them vs. sands
Specifically, What Kind
of Loamy Sand?
• Loamy fine sand. 50 percent or more* fine
sand; or less than 50 percent* very fine sand
and a total of less than 25 percent* very
coarse, coarse, and medium sand.
OR
• Loamy very fine sand. 50 percent or
more* very fine sand.
• *by weight, not volume
How to Know…
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Accurate texturing procedure
Soil survey information may help
Fine sands are slightly gritty
Very fine sands will feel smooth
Sieve analyses
• - If >50% (by weight of total soil) lvfs, then loamy
• - If <50% (by weight of total soil) lfs, then sandy
Texturing Standards
Crops and Soils Club Phone: 715-425-3395
Department of Plant & Earth Science
University of WI – River Falls
410 South Third St., River Falls, WI 54022-5001
• A. Texture kit contains samples of 10 soils, listing
percentage of sand, silt and clay and includes a
soil textural triangle.
– $25.
• B. Sand-size card – Samples of USDA vfs, fs, s,
cos, vcos mounted on a 4 1/2 x 7 1/4 6-hole punch
card. $8.00
Major Horizon Designations
Surface Organic Layer
O horizon
Surface Mineral Layer
A Horizon
Subsoil
B Horizon
Underlying
Material
C Horizon
Bedrock
R Horizon
Horizon
Terminology
Oi
Oe
Oa
Ap
A1
A2
A
E
AB
Surface
Layer
Subsurface
Layers
Surface
Soil
BA
Transition
Layer
Bt
Subsoil
AC
Layer of
Accumulated
Lime
Bk
BC
C1
Underlying
Material
C2
Bedrock
R
Cr
2C
A horizon
Bw horizon
C horizon
A horizon
E horizon
Bh horizon
Bs horizon
Permeability
• A measure of the ability of water or air to
move through the soil profile
• Permeability is “measured” in inches per
hour
– Permeability is actually an estimated property
based primarily on texture and structure
– Measured values are saturated hydraulic
conductivity
Permeability
Capillary Fringe
• Based upon permeability
• The zone above the free
water table that is
effectively saturated
– Water held at tension
– Theoretical values much
higher than “real life”
– Difficult to measure
Capillary Fringe
Largest Pores Not Filled
Saturated
All Pores Filled with Water