Understanding Soil Texture and Structure

Understanding Soil Texture and Structure

Objectives:• Describe the concept of soil texture and its

importance;• Determine the texture of a soil sample;• Describe soil structure, its formation, and

importance; and • Identify various soil structures.

Soil TextureSoil Texture:

Fineness or coarseness of a soil

What are the three soil particles?◦Sand ◦Silt ◦Clay

Soil TextureWhy is soil texture important?

◦It affects Water-holding Capacity Permeability Soil Workability Ability of plants to grow

Determine Soil TextureWhat would you do to determine

the texture of a soil sample?◦You can feel it

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?

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

Sand: 20%Silt: 30%Clay: 50%

Sand: 40%Silt: 45%Clay: 15%

Sand: 17%Silt: 44%Clay: 39%

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

Activity – The Ribbon Test

Watch the video

Activity – The Ribbon Test

You need:◦ Soil◦ Water

Now roll your sleeves up

Put a small amount of your soil in your hand

Moisten itSqueezeDoes 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?

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

Two ways to form 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

Soil structure:◦Affects tilth◦Affects permeability◦Determines whether it resists

beating action of raindrops or not Minimizes formation of crusts

Why is Soil Structure Important?

Identify Soil StructuresEight of them

Granular◦Aggregates are small, non-porous,

and strongly held togetherCrumb

◦Aggregates are small, porous and weakly held together

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

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

Soil Structure affect on water