Soils Texture and Structure
and the Nature of Soil
Soils Rock!!!
Definitions
Clods
Loam
Peds
Permiability
Soil structure
Soil Texture
Soil Workability
Texture Triangle
Water Holding Capacity
What’s In Soil
• 50% Pore Space (25% air 25% H2O)
• 45% Minerals (Sand, Silt, Clay)
• 5 % Organic Matter
Soil Texture
• Sand-large particles
• Silt-Medium Particles
• Clay-Small Particles
Texture is important because it
affects
• Water holding capacity-how well the soil holds
available water
• Permeability-the ease in which air and water
may pass through the soil
• Soil workability-the ease with which soil may be
tilled and the timing of working soil after a rain
• Ability of plants to grow-some crops like carrots
and onions have difficulty growing in fine
textured soils
How do we determine texture?
• Lab method
– The soil is measured
in a lab by separating
the three major parts
(sand, silt, and clay)
• Field method
– This method involves
taking a sample in the
field and spraying it
with water and forming
a ribbon in your hand.
This method is more
technically known as
the ribbon method.
Lab Method
• When testing in the lab an accurate measurement can
be made resulting in 12 different textural classes of soils
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Silt
Silt loam
Silty clay loam
Loam
Sandy clay loam
Sand
Sandy loam
Sandy clay
Clay loam
Silty clay
Clay
The Ribbon Method
• When performing the ribbon method the measurement is
not as accurate but 5 classifications gives one a pretty
good idea what the soil is like
– Fine-textured--ribbon forms easily and is long and flexible
– Moderately fine-textured—a ribbon forms but it breaks into
pieces ¾ to 1 inch long
– Medium-textured—no ribbon forms. The sample breaks into
pieces less than ¾ of an inch long
– Moderately coursed-textured---no ribbon forms. The sample
feels gritty and lacks smoothness.
– Coarse-textured—no ribbon forms. It is composed almost
entirely of gritty material
The Lab Method
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•
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First put soil in a glass jar
Then fill the jar with water
Add soap
Shake
Let it sit over night and watch it separate.
– Sand falls to the bottom, silt settles next, then
comes clay and finally organic matter
After Using the Lab Method the Soil
Triangle will show what the sample is
Soil Structure
• The arrangement of soil particles into
aggregates of various sizes and shapes
– Natural aggregates are peds
– Clumps caused by tillage are clods
There are two steps that form
structure
• 1) A clump of soil particles sticks loosely
together through:
– Plant roots surrounding the soil and
separating clumps
– Freezing and thawing of soil
– Soil becomes wet and then dries
– The soil is tilled
– Fungal activity
• 2) Weak aggregates are cemented to
make distinct and strong. Clay, iron
oxides, and organic matter may act as
cements. When soil microorganisms
break down plant residues, they produce
gums that also glue peds together.
Soil structure is important for
several reasons
• It improves soil tilth
• It improves permeability
• It resists the beating action of raindrops,
minimizing the formation of crusts that
reduce crop stands.
There are eight types of structures
• Granular-aggregates are small, non-porous and tightly
held together
• Crumb-aggregates are small, porous, and strongly held
together
• Platy-aggregates are flat or plate-like slowing percolation
• Prismatic or Columnar-aggregates are prism like that are
very tall
• Blocky-aggregates are block-like with six or more sides
• Structureless- there is no structure
– Single grain-no aggregates
– Massive-soil particles cling together and form huge masses
Soil Profile