An Introduction to Soil

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An Introduction to Soil

• Bryan McElvany
• Research Coordinator
• Warnell School of Forest Resources
• Patrick Davies

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_at_!

• Your first dirty word for the day

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_at_T
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_at_IT
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_at_IRT
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DIRT

• This is the stuff under your fingernails

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Why is soil science intimidating?

• Whole new vocabulary
• Hard to visualize
• Somewhat complex topics like chemistry and
  physics

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What is soil?

• Several different ways to define soil
• 1) Geologic definition Loose surface of the
  earth as distinguished from solid bedrock
  (support of plant life not required).
• 2) Traditional definition Material which
  nourishes and supports growing plants (includes
  rocks, water, organic material, air).

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What is soil made of?

• Mineral Matter
• Organic Matter
• Air
• Water

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What is the function of soil?

• Plant medium
• Recycling system for nutrients
• Animal home
• Engineering medium
• System for water supply and purification

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The ability of soil to perform those functions is
dependant upon

• Soil Texture
• Soil Structure
• Soil Chemistry
• Soil Age
• Site Factors (topography, water, etc)
• Etc.

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Soil Formation Factors

• There are five main factors that influence soil
  formation processes.
• Type of Parent Material
• Climate
• Topography
• Biotic Components
• Time

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Parent Material

• Two major divisions in type of parent material
• Residual-Formed in place (Residuum)
• Transported-Parent material was moved by
• - wind (Eolian)
• - ice (Glacial)
• - gravity (Colluvial)
• - water
• rivers (Alluvial)
• oceans (Marine)
• lakes (Lacustrine)

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Soil Formation Processes

• Transformations modification of soil
  constituents. Mineral weathering, organic matter
  breakdown.
• Translocations movement up, down, or laterally
  of soil constituents.
• Additions movement of soil material in from
  outside sources. Organic material from leaves,
  dust from atmosphere.
• Losses movement of soil material out of the
  soil. Transportation to groundwater, erosion of
  surface materials.

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Soil Formation Processes
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Soil Formation in Action

• The soil formation processes, operating under the
  influence of the soil formation factors, is the
  driving force in soil genesis.
• Important questions to remember are
• What transformations and translocations took
  place in the parent material to form this soil?
• What materials were removed?
• What was added?
• How did the climate and topography affect those
  processes over time?

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Soil Horizons

• As soils develop they form layers called
  Horizons.
• These horizons have distinct characteristics
  produced by soil forming processes.
• Master horizons include the O, A, E, B, and C
  horizons.
• Not every soil contains every horizon based on
  the level of development of the soil.
• Depth of each horizon varies between different
  soils.

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O Horizon

• Layer of accumulated organic matter such as
  leaves, grass, twigs
• Material can be in various states of
  decomposition
• Generally dark in color

O
A
E
B
C
R
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A Horizon

• Zone of incorporation of organic matter into the
  mineral soil
• Generally its the most productive horizon
• High biological activity
• Generally dark in color
• Topsoil

O
A
E
B
C
R
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E Horizon

• Zone of Eluviation or leaching
• Eluviation is the movement of soil material out
  of a layer of soil
• Soluble minerals and organic material move out of
  this horizon
• Generally a lighter washed appearance in color

O
A
E
B
C
R
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B Horizon

• Zone of illuviation or deposition
• Accumulation of materials washed from horizons
  above
• This material is commonly clay, humus,
  sesquioxides, or a mixture of the three
• Variable in color from reds and yellows to browns
  and grays

O
A
E
B
C
R
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C Horizon

• Parent material
• The unconsolidated material that has been
  affected little by the soil forming processes

O
A
E
B
C
R
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Soil Texture

• Soil Texture is the relative proportion of the
  primary particles in the soil.
• Sand 2mm 0.05mm
• Silt 0.05mm 0.002mm
• Clay lt 0.002mm

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Soil Texture

• This proportion of sand, silt, and clay can be
  grouped into textural classes.
• Soil texture has a great deal to do with many
  other soil characteristics.

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Soil Texture

• Coarse textured soil soil that has a higher
  proportion of larger particles. Sand, Loamy
  sand, Sandy loam.
• Fine textured soil soil that has a higher
  proportion of smaller particles. Clay, Silty
  clay, sandy clay.

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Soil Structure

• Soil structure is the arrangement of soil
  particles into aggregates.
• Structureless soils do occur as either single
  grained or massive.

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Pore Space

• The voids between soil particles are called
  pores. These can either be filled with water or
  air.
• Soil texture plays a major role in total pore
  space as well as size of pores.
• Coarse textured soils (sandy) have larger pores,
  but less total pore space.
• Fine textured soils (clayey) have smaller pores,
  but more total pore space.

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Soil Permeability

• Permeability is a measure of how fast air and
  water can move through a soil.
• Soil texture and soil structure both pay a role
  in soil permeability.

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Soil Texture and Permeability
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Soil Structure -PermeabilityGranular Blocky
Platy
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Soil Water

• Saturated soil is when that soils pores are full
  of water.
• Gravitational water is that water that moves out
  of the soil due to gravity. This water is
  generally in the larger Macro-pores.
• Capillary water is that water that is held in the
  soil due to adhesion and cohesion against the
  pull of gravity. This water is generally held in
  the smaller Micro-pores and as a film around soil
  particles.

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Soil Water

• After a major rain event, once the gravitational
  water has left the soil, the soil is at Field
  Moisture Capacity.
• The wilting point is reached when soil water
  levels decline to the point that all remaining
  water is held too tightly by soil particles to be
  removed by the plant.

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Soil Water
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Available Water Capacity

• Available water capacity is a measure of the
  water available to plants.
• Commonly defined as the difference between the
  amount of water at field moisture capacity and
  the amount at the wilting point.
• This is the water a plant has a chance of
  utilizing.

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Available Water Capacity
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The End

• Thanks