Introductory Soil Science

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Introductory Soil Science

• Lecture Week 7
• Soil Profile Development
• and
• Soil Air Water

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Soil Profile Development
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Soil Profile Development

• The process of profile development continues
  throughout the life of the soil
• Involves
• humification
• weathering
• leaching
• Leaching has a marked effect on profile
  development

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Soil profile development

There are no distinct stages.
Bedrock
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Soil profile development

There are no distinct stages.
Starting point
Regolith
Bedrock
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Soil profile development

There are no distinct stages.
Starting point
Undifferentiated regolith - consists of
weathering products quartz clay soluble products
Bedrock
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Soil Profile Development

• Stage 1-weathering begins, development of
  regolith (C horizon)

C
Horizon Development
R
Time
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Stage 2Plants grow

Bedrock
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Stage 2Plants grow

Decomposing minerals provide nutrients which
allow plant growth clay minerals hold stores of
water and nutrients
Bedrock
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Stage 3Humus accumul-ates

Bedrock
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Soil Profile Development

• Stage 3-organic layer begins, develops above C
  horizon

O
C
Horizon Development
R
Time
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Stage 3Humus accumulates

Bedrock
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Stage 3Humus accumulates

humic acids
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
soluble nutrients
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
soluble nutrients secondary minerals-clay iron
and aluminium oxides
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
water moves nutrients down
soluble nutrients secondary minerals-clay iron
and aluminium oxides
Bedrock
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Soil Profile Development

• Stage 3 A B horizon development begins

O
A
B
C
Horizon Development
R
Time
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Stage 4Layer development

dark humus layer
nutrient and clay depleted
nutrient and clay enriched
Bedrock
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Stage 5 Nutrientrecycling

decay
root uptake
nutrients
Bedrock
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Soil Profile Development

• Stage 4 mature profile

O
A
B
Horizon Development
C
R
Time
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PARENT
MATERIAL
acted on by
SOIL
COLOUR
WEATHERING
influenced by
CLIMATE
PROCESSES
influences
SOIL
TEXTURE
forms
determine
REGOLITH
composed of
OLD
MINERALS-
NEW
QUARTZ
MINERALS -
CLAYS
acted on by
organisms
TIME
changes over
SOIL
produces
ZONAL SOIL
contain distinct
SOIL
make up
HORIZONS
PROFILE
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Soil Water Air

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Soil Air Water

• So far we have only considered the
  mineral/organic components of soil
• This ignores approximately 50 of soil volume
  which is unoccupied space pores or pore space
• Approx proportions for A horizon of loamy soil
  are
• Mineral matter 45
• Organic matter 5
• Air 25
• Water 25

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Physical Properties of Soils

• 4 Physical properties influence soil air and
  water. They are
• Texture
• Structure
• Bulk density
• Aggregate stability

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Texture
clay
Increasing porosity
sand
silt
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Structure

• Refers to the combination or arrangement of
  primary soil particles called peds

Platy
Columnar or prismatic
Blocky
Spheroidal
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Structure

• Size and ped type affect
• Pore size
• Infiltration rate
• Columnar peds allow faster downward infiltration
  than platy peds

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Bulk Density

• Bulk density is the weight in grams of 1 ml of
  soil after the moisture has been extracted by
  drying in an oven.
• Increase bulk density decrease porosity

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Bulk Density
Bulk Density
Pore Space
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Aggregate Stability

• Aggregate stability
• How well the soil aggregates resist breaking down
  under the influence of water
• If aggregates collapse when wet, pore space will
  be reduced as pores become blocked
• Reduces infiltration rate
• Increases surface run-off
• Tilth
• Refers to aggregates on the soil surface

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Tilth

• The ability of a soil to form a finely granular
  mixture suitable for sowing seed
• Self- mulching soils have an excellent granular
  surface texture which allows good water
  penetration
• Hard-setting soils are at the opposite extreme
• Compaction reduces infiltration

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Summary

• Physical properties of soil (texture, structure,
  bulk density and aggregate stability)
• affect soil porosity
• Are not independent of each other
• Are affected by human interference (cultivation,
  logging, passage of foot or vehicular traffic)
• Reversing adverse impacts is extremely difficult.

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Soil Water

• Water can exist in the soil in 4 ways
• Chemical water combined in the mineral structures
• Hygroscopic water held by electrical attraction
  to surface of soil colloids
• Capillary water held in small pores against
  gravity
• Gravitational water in larger pore spaces
  draining through the soil

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Chemically combined
Water unavailable
Hygroscopic
Wilting point
Capillary
Water available
Field capacity
Gravitational
Superfluous water
Forms of soil moisture
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Capillary rise
Height of rise depends on surface tension and the
weight of the water column Smaller diameter
columns support higher rise
Adhesion is greater than cohesion
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Capillary rise and water availability

• Soil pore space is equivalent to columns in the
  sense of capillarity
• Smaller pores will support higher rise
• 1mm glass column supports 3 cm rise
• 0.1mm glass column supports 30 cm rise
• Some soil pores are smaller than 0.001mm
• Capillary water is held against the force of
  gravity and is the only water available to plants
  after gravitational water has passed out of
  freely draining soil

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Wilting point and Field capacity

• Field capacity is the water held in the soil
  after saturation followed by 48 hours of free
  drainage
• Wilting point marks the lower end of the
  available water and is the point at which the
  water is held in the soil with such force that
  none passes into the plant roots and the plant
  wilts

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Soil texture and water availability
Available water field capacity wilting point
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Field Capacity
Water holding capacity
4
Available water
3
Wilting point
2
Unavailable water
1
Sandy loam
Sand
Loam
Clay loam
Clay
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Soil Air

• Good aeration is essential for plant growth and
  activity of soil micro-organisms
• Air capacity of air in soil at field capacity
• Loamy soils have the best balance

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Soil air
Air filled pores allow rapid diffusion
Oxygen
Carbon dioxide
Plants can tolerate Low oxygen but not high
carbon dioxide
Root zone