All about Soil

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Plant
Ecology
January 8
2017
Soil Profile , Soil Structure , Soil Texture , Soil Formation , Soil Organic Matter Chapter # 3
“Don’t Forget Why
You Started”

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Soil Profile Descriptions
• Soil Profile - A vertical section of the soil extending vertically through
all its horizons and into the parent material.
• Soil Horizon - A layer of soil, approximately parallel to the surface,
with properties that differ from the horizons above or below it – the
properties (characteristics) are produced by soil forming processes.
• Soil Layer - A layer in the soil deposited by a geologic force (wind,
water, glaciers, oceans, etc.) and not relating to soil forming process.

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Organic (O) Horizons
• O horizons or layers: Layers dominated by organic material.
• Identification Criteria
– >20% organic matter
– Dark color (
– Feels ‘Squishy”
– Identifiable dead leaves, grass, etc. accumulated at surface
A Horizons
• Referred to as topsoil
• Typically ranging from 6-30 centimeters thick
• Mineral horizon formed at the surface or below an O horizon.
• Characterized by an accumulation of well decomposed organic matter
intimately mixed with the mineral fraction.
• Identification Criteria
– Mineral soil material
– Mix of well decomposed organic matter and mineral material
– Surface mineral horizon
– Typically dark in color-darker than underlying horizons
E Horizons
• Mineral horizon in the upper part of the soil typically underlying an O or A
horizon.
• Light colored, leached horizons ranging from not being present to several
centimeters thick

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• Light color due to the natural color of the mineral grains.
• Formed by weak organic acids that strip coatings from mineral grains.
• Field Identification
– Zone of eluviation - removal of clays, Fe, Al, and humus
– Lighter in color than over or underlying horizon
– Near surface, below O or A horizons and above a B horizon
B Horizons
• Referred to as subsoil.
• The zone of accumulation (or illuviation) within the soil.
• Field Identification
– Subsurface horizon formed below an O, A, E horizon and above the C
horizon
– Formed as a result of soil forming processes
– Expressed often by color
– Illuvial concentration-zone of accumulation
C Horizon
• Referred to as parent material.
• These horizons and layers are little affected by soil forming processes
(unweathered geologic material).
• Field Identification
– Little affected by soil-forming processes
– Geologic layering
– Color of unweathered geologic material

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R Horizon
• Hard rock
• Field Criteria
– Can not dig it with a shovel or backhoe
Soil Texture
 Soil texture is the single most
important physical property of the
soil. Knowing the soil texture
alone will provide information
about:
 1) water flow potential,
 2) water holding capacity,
 3) fertility potential,
 4) suitability for many urban
uses like bearing capacity
Particle Diameter Size
 Soil particle diameters range over 6 orders of magnitude
 2 m boulders
 Coarse fragments > 2 mm
 Sand < 2 mm to 0.05 mm
 Silt < 0.05 mm to 0.002 mm

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 Clay < 0.002 m
Coarse Fragment
 > 2 mm
 Gravels,cobbles,
boulders
 Not considered part
of fine earth fraction
(soil texture refers
only to the fine earth
fraction or sand, silt & clay)
 Boulders left in valley of Big Horn
Mts.(Wy) by a glacier.
Sand
 < 2 mm to > 0.05 mm
 Visible without microscope
 Rounded or angular in shape
 Sand grains usually quartz if sand looks
white or many minerals if sand looks brown,
 Some sands in soil will be brown, yellow, or red because of Fe and/or Al
oxide coatings.
 Feels gritty
 Considered non-cohesive – does not stick together in a mass unless it is very
wet.
 Low specific surface area
 Sand has less nutrients for plants than smaller particles
 Voids between sand particles promote free drainage and entry of air

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 Holds little water and prone to
drought
Silt:
 < 0.05 mm to > 0.002 mm
 Not visible without microscope
 Quartz often dominant mineral in
silt since other minerals have
weathered away.
 Does not feel gritty
 Floury feel–smooth like silly
putty
 Wet silt does not exhibit
stickiness / plasticity /
malleability
 Smaller size allows rapid
weathering of non quartz minerals
 Smaller particles – retains more
water for plants and have slower
drainage than sand.
 Easily washed away by flowing
water – highly erosive.
 Holds more plant nutrients than sand.
Clay:
 Wet clay is very sticky and is plastic or
it can be molded readily into a shape or
rod.

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 Easily formed into long ribbons
 Shrink swell – none to considerable depending on the kind of clay.
 Pores spaces are very small and convoluted
Movement of water and air very slow
 Water holding capacity
Tremendous capacity to adsorb water- not all available for plants.
 Soil strength- shrink/swell affects buildings, roads and walls.
 Chemical adsorption is large

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Soil Microbes
What Are Microbes Doing For Plants?
• Providing nutrition
– Mycorrhiza – absorb water and nutrients
– Nitrogen fixation and siderophores– rhizobacteria
– Decomposition of dead material into plant usable
– Store nutrients in their cells
• Preventing pathogens
– Bacteriocins and antibiotics
– Competition
– Molding the architecture of the soil
– Aeration
– Aggregates that enhance water retention
0 2 4 6 8 10 12 14 16
Plant in sterile soil
" Plant + bacteria
"Plant + bacterial +bacterial feeding nematode
Plant + fungi
Plant + fungi + fungal feeding nematode
Plant + all above
Effect of microbes on weight of plant shoot in
mg.
Weight of Plant Shoot in mg

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MYCORRHIZAE ON ROOT
TIPS (AMANITA)
NODULES FORMED WHERE
RHIZOBIUM BACTERIA
INFECTED SOYBEAN
ROOTS.
What does the plant do to encourage the microbiota?
• Secretes small molecules such as sugars and amino acids, peptides
– Far more life in the rhizosphere than away from it.
– A large part of the plants energy stores are secreted into the soil.
• Protection: allows some bacteria to live in plant cells.

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• BACTERIA
COLONIZED ON ROOT
HAIR.
THE PLANT SECRETES SMALL
MOLECULES TO FEED THE
BACTERIA IT REQUIRES.
• FOOD SOURCES
OF SOIL
MICROBES