Soils

Soils

• Products of weathering

Soil Characteristics

• Regolith– Loose material covering solid rock• Dust, sediment, broken rock

– Soil is the only part that supports the growth of plants

Soil Characteristics

• Composition• Texture• Structure

Soil Composition

• Mineral matter– Broken rocks

• Organic matter (humus)– Remains of dead organisms– Source of nutrients– Increases ability to retain water– Percentage varies

• Bogs are almost all humus, deserts have very little

Soil Composition

• Water– Provides moisture for chemical reactions– Provides nutrients in a form that plants can use

• Air– Source of CO2

Soil Texture

• The proportions of particle sizes• Influences ability to support plant life• Based on % of clay, sand and silt

Soils

SAND:• Drains quickly; dries out

Soils

LOAMY SAND:• falls apart when handled.

Soils

LOAM:• feels spongy• Drains well without drying out• Contains more nutrients

Soils

SANDY LOAM:

Soils

SANDY CLAY:

Soils

SILT LOAM:

Soils

CLAY:• Drains slowly• Hard for plant roots to grow

Soils

SILTY CLAY:

Soils

SILTY CLAY LOAM:

Soils

CLAY LOAM:

Good Quality Soils

• Typically consist of 50 % organic and mineral matter

• Other 50 % consists of pore spaces – Air and water circulation

Soil Structure

• Soil particles form clumps that give soils a particular structure– Determines:

• How easily it will erode• How easily it can be cultivated• Water infiltration

Soil Formation

Parent Material

• Source of the mineral matter• Bedrock– Residual soil

• Unconsolidated material– Transported soil

Parent Material

• Affects:– Rate of weathering and soil formation• Unconsolidated material forms soils more quickly than

residual

– Plant fertility• Determines types of plants that can grow

Soil Formation

• Climate:– Temperature, humidity, precipitation– Most important factor

• Time:– Older soils are thicker

Soil Formation

• Organisms– When living things die, decomposers break them

down• They become part of the soil• Add nutrients to soils• Plant acids speed up weathering

– Burrowing• leaves holes for air and water circulation

Soil Formation

• Topography/Slope– Steeper slopes have more erosion, less water

Soil Profiles

• Soil forms top down

• At different depths, soil varies in:– Composition, texture, structure and color

Soil Profiles

• Mature soils tend to have 3 distinct horizons

Soil Horizons

• O– Humus, typically considered part of A

• A – Topsoil, mostly organic matter

• B– Subsoil, lower limit of roots and burrowing, – Hardpan – clay that forms a compact layer

• C – Partially weathered parent material

Pedalfer

• Forms in temperate areas with more than 63 cm of rain each year– Much of the eastern US– B horizon

• Large amts. of iron oxides (reddish brown)

Pedocal

• Forms in drier western US– Less clay (b/c weathering is slower)– Contain calcium carbonate – light brown

Laterite

• Hot, humid, tropical areas• Lots of water– Distinctive orange-red

• Almost no organic material (quickly used)