Soils 101
January 2011
Patricia Steinhilber, Ph.D.
Ag Nutrient Management Program
University of Maryland College Park
Department of Environmental Science and Technology
What is Soil?
• the outer layer of the Earth’s crust
(geologists)
• the ecstatic skin of the Earth (William Logan)
• the living skin of the Earth (Ian Pepper)
• crucible of terrestrial life (Daniel Hillel, soil
physicist)
• the pedosphere
− the interface between the lithosphere,
hydrosphere and atmosphere (ecologists)
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What are Soils?
• reactive, dynamic, three-phase ecosystems
composed of solids, liquids and gases
Minerals
Air
Water
Organic Matter
25%
25%
48%
2%
topsoil several days after
rainfall or irrigation
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Question 1
• What % of the soil volume would
be water-filled after several days
of heavy rain?
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Question 1
• What % of the soil volume would
be water-filled after several days
of heavy rain?
• 50%
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First Things First:
Soil Solids
• mineral or inorganic solids
−often classified based on particle size
• organic solids a.k.a. “organic
matter”
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Major Mineral Classes in a
Soil
Mineral
Class
Size of Mineral Particles
Feel of
Particles
sands
0.05 – 2 millimeters (mm)
gritty
silts
0.002 – 0.05 millimeters (mm)
smooth
clays
less than 0.002 millimeters (mm) sticky
when wet
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(like flour,
cornstarch
or talcum
powder)
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of Environmental
Environmental Science
Department
Scienceand
andTechnology
Technology
60% silt
20% clay
20% sand
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of Environmental
Environmental Science
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andTechnology
Technology
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Effect of Soil Texture on Soil
Properties
Soil Property
Textural Groups
coarse-textured
medium-textured
fine-textured
sands, loamy sands,
sandy loams,
loams, silts loams,
silt, sandy clay
loams
clays, sandy clays,
silty clays, silty clay
loams, clay loams
water-holding
capacity
low
moderate
high
susceptibility
to erosion
low
high
moderate
leaching
potential
high
moderate
low
nutrient
retention
capacity
low
moderate
high
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The Other Soil Solid
Material: Organic Matter
Humus
75%
10%
Biomass
15%
Residues &
By-Products
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Biomass: What It Is
• the living component of the soil
• consists of a range of creatures
− as small as microscopic viruses &
bacteria
− as large as roots, worms and other
creatures that are visible to the
unaided eye
− and everything between
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Source: USDA
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Question 2
How many bacteria are there in a
handful of soil?
A) 10,000 – 50,000
B) 1,000,000 – 10,000,000
C) 300,000,000 – 50 billion
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Question 2
How many bacteria are there in a
handful of soil?
C) 300,000,000 – 50 billion
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Biomass: What It Does
• participates in nutrient cycling
−break down plant and animal
materials (residues), using what they
need and leaving behind what they
don’t
• mineralization
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Biomass: What It Does
(cont.)
• creation of biopores
−larger organisms move through soil
creating channels or pores
−channels promote water infiltration
and create a healthy balance between
large and medium pores
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Residues and By-products:
What They Are
• dead stuff - crop residues, dead roots
and bodies of soil creatures
• by-products - materials that plant
roots and soil creatures release or
exude into the soil
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Question 3
• What antibiotic used to treat
bacterial infections in humans
originates from the by-product of
a soil organism?
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Question 3
• What antibiotic used to treat
bacterial infections in humans
originates from the by-product of
a soil organism?
• Streptomycin
−Isolated from Streptomyces by Dr.
Selman Waksman of Rutgers
University. Led to a Nobel Prize in
Medicine in 1952
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Residues and By-products:
What They Do
• fuel and nutrients for soil organisms
− energy and nutrient source for most of the
soil creatures
• formation and maintenance of soil
aggregates (structure or architecture)
− sticky and gummy by-products of residue
decomposition hold soil particles together in
clumps or aggregates
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Last But Certainly Not Least:
Humus
• relatively stable end product of residue
decomposition
• composes the majority of organic matter
• resists further decomposition (1% per
year)
• it is not a good nutrient or energy
source for soil creatures
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Humus: What It Does
• very small in particle size & high
surface area
• charged sites at many locations on
the surface
• effective at holding water and
nutrients
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The Other Half of Soil:
Soil Pores (Void Space)
• soil water
− adequate (but not too much) quantity
− adequate supply of nutrients
− minimize runoff and leaching
• soil air
− source of oxygen for roots and most soil
organisms
− constantly enriched with carbon dioxide
from roots and soil organisms
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The Interplay of Air and Water:
Soil Aeration
• The exchange of O2 and CO2
between the soil pores and the
ambient atmosphere
Hillel
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Comparison of Gasses in the
Atmosphere and in Soil Air of
Well-Structured Topsoil
(several days after rainfall)
Gas
In Soil Air
nitrogen
In the
Atmosphere
79%
oxygen
20.9%
20.6%
carbon dioxide
0.035%
0.300%
79%
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Balance Between Water
and Air
• macropores (large pores)
− drain quickly after rain or irrigation
− allow rapid infiltration of rainfall and
replenishment of oxygen in the root zone
• mesopores (medium-sized pores)
− “storage pores”
− hold water in form most plants can use
• micropores (very small pores)
− water is held too tightly to be use to most
plants
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Now, a Video Interlude • Dr. Ian Pepper, soil microbiologist,
Arizona State University
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Let’s talk about the hydrologic
cycle and soil water…
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Biological Classification of
Soil Water
• excess or gravitational water
− water that drains from soil 1-3 days after a
rainfall or irrigation (in macropores)
• available
− water that is in a form crop plants can use
(in mesopores)
• unavailable
− water that is held to tightly by the soil to be
usable by most crop plants (in micropores)
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Water and Textural Groups
(% water)
Coarse- MediumFineTextured Textured Textured
40
50
60
Total
Excess
25
15
15
Available
12
27 *
20
3
8
25
Unavailable
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Environmental Science
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Soil Solution
• keep well supplied with nutrients
• minimize adverse conditions for
plant growth
−acidity
−salinity
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Soil tests can help
identify
limiting nutrient
factors.
Fertilizers can enrich
the soil solution when
soil tests indicate an
insufficiency.
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Question 4
• What are the plants nutrients?
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What Elements do
Plants Require?
Non-Mineral
Nutrients
carbon
oxygen
hydrogen
Mineral Nutrients
macronutrients
primary
secondary
nitrogen
phosphorus
potassium
calcium
magnesium
sulfur
micronutrients
iron
chlorine
zinc
boron
copper
cobalt
nickel
manganese
molybdenum
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Forms in Which Nutrients Exist
• cation – positively charged ion
• anion – negatively charged ion
• neutral - uncharged
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So Which Nutrients Exist
in What Form?
•
•
•
•
•
•
•
•
•
•
ammonium - NH4+
potassium - K+
calcium - Ca+2
magnesium - Mg+2
iron - Fe+2, Fe+3
zinc - Zn+2
manganese - Mn+2, Mn+4
copper - Cu+2
cobalt - Co+2
nickel - Ni+2
•
•
•
•
•
nitrate - NO3phosphate - H2PO4-, HPO4-2
sulfate - SO4-2
chlorine - Clborate - H3BO3, H2BO3-,
B4O7-2
• molybdate - MoO4-2
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soil acidity:
the adverse
condition on
humid
regions
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Adverse Conditions Due to
Soil Acidity (Low pH)
Adverse Condition
Soil Component Affected
inadequate supply of
some nutrients
calcium and magnesium may be present
in inadequate amounts in acidic soils
excessive or toxic
supply of some
nutrients or nonessential elements
aluminum, iron, zinc, manganese and
copper can be present in excessive and
even toxic amounts in acidic soils
some nutrients bound phosphorus and molybdenum are so
in unavailable forms
tightly bound to soil clays that they are
not available to plants in acidic soils
inhospitable
environment
bacteria, such as those needed for
nitrogen fixation, can not thrive in acidic
soils
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Soils are Biochemical
Reactors
• the various components (soil air,
soil water, minerals and organic
matter) interact
• a wide array of chemical and
biochemical processes occur
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Soil Model
biomass
organic
matter
residues &
by-products
soil
solution
soil
air
sand
silt
clay
humus
pore space
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minerals
Clays and Humus
• center of chemical reactivity in soil
• engine driving chemical transformations in soil
geoscienceworld.org
fullwiki.org
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Solid-Liquid Interface
• adsorption of water and nutrients
occurs at surfaces
• small particles have more surface area
than the same weight of large particles
• surfaces are often charged; most charge
is negative
• negatively charged surfaces attract
positively charged ions (cations)
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Cation exchange is a big deal!
• cation exchange capacity (CEC)
− a soil’s ability to hold cations on charged
surfaces
− a measure of the negative charge on the
particles
− correlated to clay and humus content of
soils
• exchange phase
− those cations held on surfaces of charged
particles
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Importance of CEC
• nutrient cations exist on exchange
phase
−are protected from leaching
−can replenish the soil solution when
plant uptake or leaching removes
nutrients
−the “storehouse” of cationic nutrients
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Relationship of Textural Class
to Clay Content and CEC
Textural Group
Range of Clay
Content (%)
Cation Exchange Capacity
(CEC)*
0 - 15
1-5
15 - 20
5 - 10
0 - 25
5 - 15
clay loams, sandy
clay loams, silty
clay loams
20 - 40
15 - 30
clays, sandy clays,
silty clays
>49
>30
sand and loamy
sands
sandy loams
loams and silt
loams
*cmol/kg or centimoles per kilogram, or an older mode of expression,
milliequivalent per 100 grams.
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Soil:
The Most Complex
Biomaterial on Earth
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Soil – We can’t grow without it.
Questions?
Smithsonian – Dig it! The Secrets of Soil
Hillel, 2008
front cover
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• psteinhi@umd.edu
• www.anmp.umd.edu
What would you like to cover in Soils 102?
http://www.ted.com/talks/bonnie_bassler_on_
how_bacteria_communicate.html
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