Soil Texture, Structure, and the Nature of Soil

Soils Rock!!!

Enduring Understanding

• Plants can grow in a wide variety of media which can be manipulated to produce healthy plants.

• Soil is the basic medium of most terrestrial plants on earth.

Essential Questions

• How do plants interact with the natural and artificial elements in their environment?

• How does the geology of the environment effect plant growth?

• How do humans impact soil?

• What is the difference between a plant’s food and its nutrients?

What’s In Soil• 50% Pore Space (25% air, 25% H2O)

• 45% Minerals (Sand, Silt, Clay)

• 5% Organic Matter (decaying material)

Soil Formation

Organic Matter• Dead plants and animals

• Compost

• Essential for plant growth

• Made of carbon-based material

• Replaces nutrients

SOIL AIN’T DIRT

• “Dirt” should only refer to displaced or dead soil!

Why Do Plants Need Soil?

• Nutrients

• Water

• Stability/support

• Plants can be grown in artificial soil called growing media (we use this in the greenhouse).

Why Do We Need Soil?

• FOOD!

• Plant products (ethanol, wood, etc.)

• Ecotourism

Good Soil No Soil

Valleys Desert

Volcanic Regions Beach

Mountains

Soil Texture• Sand-large particles• Silt-Medium Particles• Clay-Small Particles

– Sand < 2 mm to 0.05 mm

– Silt < 0.05 mm to 0.002 mm

– Clay < 0.002 m

Course Fragments (NOT SOIL!)

• >2mm in diameter

• Gravel

• Pebbles

• Rocks

• Boulders

Sand

• < 2 mm to > 0.05 mm• Porous

• Doesn’t hold water or nutrients

• Gives good drainage

Silt

• < 0.05 mm to > 0.002 mm• Flower like feeling• Medium size• Typically made from the mineral

quartz• Not sticky• Erodes easily• Holds nutrients better than sand

Clay

• < 0.002 mm• Small• Holds water and nutrients very well

(sometimes to well)• Very sticky holds soil together• It is important to have in soil contrary

to popular belief.

Soil Texture is…

• The percentage of sand, silt and clay

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– Silt– Silt loam– Silty clay loam– Loam– Sandy clay loam– Sand– Sandy loam– Sandy clay– Clay loam– Silty clay– Clay

Loam is the Best!!!

• Perfect blend of soil particles

• Good drainage

• Good moisture holding capacity

• Good black dirt

The Ribbon (Field) 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

• 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

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

Definitions

ClodsLoamPeds PermeabilitySoil structureSoil TextureSoil WorkabilityTexture TriangleWater Holding Capacity