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Understanding Soil Texture and Structure
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Soil TextureSoil Texture:
Fineness or coarseness of a soil
What are the three soil particles?◦Sand ◦Silt ◦Clay
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Soil TextureWhy is soil texture important?
◦It affects Water-holding Capacity Permeability Soil Workability Ability of plants to grow
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Determine Soil TextureWhat would you do to determine
the texture of a soil sample?
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Determine the TextureCan be tested in a lab
◦The lab results will be in the form of percentages of the three soil particles.
So let’s say we had:◦40% sand◦10% clay◦50 % silt
How would this sample feel?
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Determine the Texture
What do you do with those percentages?◦40% sand◦10% clay◦50 % silt
Use the Textural Triangle to determine the textural class
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Sand: 20%Silt: 30%Clay: 50%
Sand: 40%Silt: 45%Clay: 15%
Sand: 17%Silt: 44%Clay: 39%
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Determine the TextureWhat if you can’t send it to a lab?What would you do?
The Ribbon Test◦You can find 5 texture classes in the
field with this method Fine-textured Moderately fine-textured Medium textured Moderately coarse-textured Coarse textured
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Activity – The Ribbon Test
Watch the video
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You need:◦ Soil◦ Water
Now roll your sleeves up Put a small amount of your soil in your hand Moisten it Squeeze Does it stay together? Roll it into a ball Will it stay in a ball? Form a ribbon Will it? How long before it breaks? What soil texture do you have?
Activity – The Ribbon Test
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Soil Structure, Function, ImportanceWhat is Soil Structure?
◦Arrangement of soil particles into aggregates. (clusters)
◦Many different sizes and shapesPeds
◦Naturally occurring aggregatesClods
◦Aggregates formed by tilling
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Two steps to forming soil structure◦Clumps of soil particles stick together
Plant roots surround soil and break up clumps Freezing and thawing of soil Soil gets wet and then dries out Soil is tilled Fungal activity
◦Weak aggregates are cemented together making them distinct and strong. Clay, iron oxides, and organic matter act as
cement Also gums produced by microorganisms
Soil Structure, Function, Importance
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Soil structure:◦Improves tilth◦Improves permeability◦Resists beating action of raindrops
Minimizes formation of crusts
Why is Soil Structure Important?
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Identify Soil StructuresEight of them
Granular◦Aggregates are small, non-porous,
and strongly held togetherCrumb
◦Aggregates are small, porous and weakly held together
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Platy◦Aggregates are flat, plates overlap
causing slow permeabilityPrismatic/Columnar
◦Aggregates are prism-like with the vertical axis greater than the horizontal;
◦Prismatic – flat caps◦Columnar – rounded caps
Identify Soil Structures
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Blocky◦Aggregates are block-like, with six or
more sides; all three dimensions are about the same
Structureless◦No apparent structure – 2 forms◦Single grain
Soil particles are individual and do not form aggregates
◦Massive Soil particles cling together in large uniform
masses
Identify Soil Structures
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Soil Structure affect on water