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Soil is the top layer of the Earth’s surface.
Soils are the transition between the biotic and
abiotic worlds.
Soil is a mixture of:
1.
2.
3.
4.
Minerals
Water
Gases
HUMUS-Dead “things” that have broken down and
become organic material—decayed plant and animal
remains
(thanks to decomposing fungi
and bacteria)
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Water
(~25%)
Air
(~25%)
Humus
(~5%)
Mineral
(~45%)
Humus is
organic matter!
Soils are important:
◦ As a habitat for growing crops
 Food
 Fabrics/Dyes
 Rubber/Building Materials
◦ As foundations of buildings
◦ As beds for roads and highways
◦ As a filter for pollution that comes from rain and
water runoff
◦ Medium for photosynthesis
There are THREE main contributing
factors
1. The materials (rocks) from which they formthese are called PARENT MATERIAL
2. The ENVIRONMENT in which they developed
(climate, vegetation, soil life, topography)
3. Length of time soils have been developing—the
AGE of the soils
Good rule of thumb: 500-1000 yrs per inch of
topsoil
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Mineral Matter
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Older soils are:
◦ Provides nutrient
minerals for plants
◦ Provides pore space
for water & air
◦ More weathered
◦ Lower in certain
essential nutrient
minerals
Humus: black/dark brown organic material
remaining after decomposition
– Leaf litter, animal dung, & decaying organisms
– Increases the soil’s water-holding capacity by acting
like a sponge
WEATHERING: Any process where rock breaks
down (changes chemically and physically)
There are two types of weathering:
Physical or Mechanical Weathering: Large rock mass
is broken into smaller fragments of the same
type-NO CHEMICAL CHANGE
Chemical Weathering: mass of rock is broken down
by chemical reactions
Followed by Erosion (Wind, Water, Ice)
What are “agents” of physical weathering?
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◦ Tree roots/plant roots
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Frost wedging:
1. Water collects in cracks in rock
2. Water expands when it freezes
3. Pieces of rock break off
1.Water collects
in cracks in
rock
2.Water expands
when it freezes
3.Water melts;
Pieces of rock
break off
Rock
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Most chemical weathering involves gases in
the atmosphere (oxygen and carbon dioxide)
and/or water
Example: Think about metal rusting
◦ You go from metal to rust—a
chemical reaction has taken place
All these happy little critters break down
dead plant and animal material and
aerate the soil—they make topsoil!
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Earthworms & Ants:
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Mycorrhizae fungi
◦ Cycle minerals
◦ Aerate the soil
◦ Assist plants in
reproduction by burying
seeds
◦ found on roots of plants.
◦ Help plant get nutrients
from soil in exchange for
food
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Retention of nutrients (prevent leaching)
Cycling nutrients to the right forms for plants
(fungi & bacteria)
Building soil structure – good for water
movement and root development
Production of plant growth promoting
hormones
Decomposition of toxic compounds
Dr. Greg Pillar, Queens College
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Nitrogen fixation*
Nitrification*
Assimilation
Ammonification*
Denitrification*
*uses bacteria
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No gaseous phase
Limiting factor
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Texture
pH
Permeability
Porosity
Nutrient Levels—Nitrogen/Phosphorus
Soil texture: The way a soil "feels"
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Depends on the fraction each size of particle in
the soil
Sand, silt, and clay are names that describe the
size of individual particles in the soil.
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SAND are the largest particles and they feel
"gritty.“
SILT are medium sized, and they feel soft, silky or
"floury"
CLAY are the smallest sized particles, and they feel
"sticky" and they are hard to squeeze.
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Texture is determined by its % sand, silt, and
clay.
Loam:
ideal for
agricultural
soil
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Texture determines the
type of soil, and the
soil’s properties.
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Predicts soil properties-fertility, etc.
Sandy soils
 low organic matter
 Poor retention water and nutrients (it flows through)
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As silt and clay fractions increase, soils
 More organic matter
 Better buffered
 Better retention of water and nutrients
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But you can have TOO much clay
 Hard to cultivate-too sticky when wet; too hard when dry
 Shrinks and swells
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The best soils are a mix of all three and are called
LOAM
SOIL
DEF.
PROPERTY
WHY
CARE?
OTHER INFO
Texture
Determines
many soil
properties;
how
workable;
holding
capacity
Sand = largest, silt =
middle, clay =
smallest;
SAND—bad holding
capacity; low OM;
workable; CLAYgreat holding; but
shrink/swell; LOAM
= 1/3 mix—BEST!!
Way a
soil
feels;
Based on
sand,
silt, and
clay
amounts
Porosity
◦ amount of soil pore spaces
(aeration)
Permeability
– ability of water to percolate through
____ porosity = ____ permeability
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As water infiltrates the soil, it can carry
dissolved materials with it.
Leaching: Removal of dissolved materials
from the soil by water percolating
downward
Illuviation: Deposition of leached material in
lower layers
◦ Some substances completely leach out of the soil
because they’re so soluble that they migrate right
down to the groundwater.
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Soil minerals are often present as ions.
Minerals ions are either positively or
negatively charged.
Clay particles have mostly negative
charges on their outer surfaces.
◦ Positively charged mineral ions are attracted to
the soil particles and are held for plant use.
 potassium K+ and magnesium Mg2+
◦ Negatively charged mineral ions are repelled
by the soil particles and are washed away from
roots.
 nitrate NO3-
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Why would 100% sand be undesirable for plants?
Why would 100% clay be undesirable for plants?
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Answer SAND / SILT / CLAY for the following:
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◦ A soil with a lot of _____ would have good drainage, but
poor nutrient-holding characteristics.
◦ A soil with a lot of _____ would have poor drainage, but
good nutrient-holding characteristics.
◦ A soil with a lot of _____ would have low porosity.
◦ A soil with a lot of _____ would have high permeability.
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Most soil pH is 4-8. Plants are happiest at 6-7. 
Plants are affected by soil pH.
(1) The solubility of certain nutrient minerals varies with
differences in pH
 Low pH = Al & Mn are more soluble, and can be absorbed in toxic
amounts.
 High pH = some salts are less soluble, and are less available to the
plants.
(2) Soil pH affects the leaching of nutrient minerals.
 Acidic soil = positively charged ions are less soluble and are
unavailable for plants.
PESTICIDE TIE-IN: If soil is too acidic, applied pesticides,
herbicides, and fungicides will not be absorbed (held in the soil)
and they will end up in runoff
Plants the like
strongly acid
soil (pH 4.0 to
4.0)
• Sundew
• Pitcher Plants
• Venus flytrap
• Azalea
• Rhododendron
• Camellia
Plants that prefer
slightly acid soil
(pH 5.0 to 6.5)
Plants that prefer
neutral soil
(pH 6.0-8.0)
• American holly
• Orchids
• Many evergreen
trees and shrubs
• Strawberries
• Potatoes
• Carrots
• Fescue grasses
• Alfalfa
• Bluegrass
• Most vegetables
(lettuce, tomato)
• Grains (corn,
wheat)
http://www.wvu.edu/~Agexten/hortcult/turf/liming.htm
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CaCO3 or MgCO3
Not a fertilizer!
Important functions:
1. Corrects soil acidity
2. Adds important plant nutrients— Ca & Mg
3. Reduces solubility & toxicity of certain elements
in the soil
Al, Mn, Fe
This toxicity could reduce plant growth under
acid conditions.
4. Promotes availability of major plant nutrients.
Zn, Cu, P
5. Increases bacterial activity & mutualistic
relationships.
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Stabilizes the soil pH (more basic)
Replaces organic matter and restores
biological activity
Improves the soil structure and increases
permeability and infiltration
Improves moisture holding capacity, reducing
erosion and run-off
Supplies beneficial microorganisms to soils
Soil stores important nutrients
Phosphorus (P)
Nitrogen (N)
Potassium (K)
Calcium (Ca)
Magnesium (Mg)
Dr. Greg Pillar, Queens College
http://www.lesco.com/
•Nutrients: essential chemical elements needed for
the growth of healthy plants
•Plants get nutrients from soil after the nutrients have
dissolved in the soil solution (the water around soil
particles)
•Each nutrient has specific roles in producing healthy
plants
FUNCTIONS
 Used to make chlorophyll
 Responsible for rapid growth and health green
leaves
 Not enough
 Plant leaves
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nitrogen?
turn yellow
Too much nitrogen?
 Long weak stems/no flowering
MOVEMENT THROUGH and TO THE SOIL:
 Nitrogen Cycle 
FUNCTION
 Responsible for root development and growth
 Helps plant produce flowers and fruit
 Important in photosynthesis and respiration, energy
transfer and storage
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Not enough phosphorus?
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Too much phosphorus?
 Purplish or deep green leaves
 Poor root system
 No symptoms
 Plants NEVER have too much phosphorus—there’s not
a lot out there—what does exist bonds to soil or leaches
away quickly
MOVEMENT THROUGH and TO THE SOIL:
 Phosphorus cycle
Maryland
Soil
Profile
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Soil Profiles
◦ the sequence of layers (horizons) from the surface downward
to rock or other underlying material
Soil Layers
“O”- organic horizons, litter derived from dead plants
and animals
A- eluvial, mineral horizons which lie at or near the
surface and are characterized as zones of
maximum leaching (E = exit)
B- illuvial, washed in, layer of accumulation (I = Into)
C- unconsolidated material under A&B layers
Bedrock
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Florida Soil
Profile
Some Questions
• What effect does temperature have on soil?
– It controls how fast organic material breaks down
• How might pH affect soil?
– It changes what nutrients are available and toxins
(e.g. metals) in the soil
• What is a soil profile?
– The cross-section of soil that shows different
layers