Soil Formation

What is soil?

Soil is the medium in which plants are grown?

What does soil provide plants with?

• Water

• Oxygen

• Mineral nutrients (Nitrogen, Phosphorous, Potassium)

• Anchorage or physical support of root system

Factors of Soil Formation

• Parent material.

• Biota (living organisms)

• Time

• Climate

• Topography

Parent Material

• Determines soil texture.

• Basic composition of soil.

• Includes rocks and other deposits.

Climate (Weathering)

• Rainfall.

• Temperature changes.

Biota

• Animals.

• Microbes.

• Insects.

• Plants.

Topography

• Elevation.

• Slope.

• Grade.

• Exposure.

• Depresions.

Time

Added layers to soil horizons.

Types of Parent Material

• Glacial Till

• Alluvium

• Lacustrine

• Organic

• Eolian

• Colluvium

• Marine

Glacial Till Parent Material Description

Product of glacial weathering. As glaciers move (flow) they carry away existing surface material and gorge deep into the bedrock (scouring, plowing); as they recede, they leave material behind (glacial till).

Glacial Till Material

Mixture of unconsolidated materials that range in size from boulders to clay particles.

Result of Glacial Till Parent Material

Bedrock

Types of Bedrock

• Igneous

• Sedimentary

• Metamorphic

Igneous Bedrock

Formed from molten rock (magma and lava). This material has not changed since its formation, and froms up 95% of the Earth’s crust.

Igneous Bedrock Examples

Granite, gabbro, and bassalt.

Sedimentary Bedrock

Formed from rock shards, minerals, and other material derived from pre-existing rock. Over time, these sediments become consolidated into rock.

Sedimentary Bedrock Examples

Sandstone, shale, conglomerate, and limestone.

Metamorphic Bedrock

Formed when Igneous or Sedimentary rock undergoes transformation leading to physical or chemical change (changes in temperature and pressure).

Metamorphic Bedrock Examples

• Marble from limestone.

• Stable from shale.

• Quartzite from sandstone.

Alluvium Parent Material Description

Material that has been transported by running water (rivers and streams). The suspended material is deposited as the running water looses velocity: heavy sand first, silt second, and light clay last.

What are the Alluvium deposits know as?

These deposists are know as sediments, and would be found on a flood plain or Delta after a flood.

Lacustrine Parent Material Description

Sediments that were deposited in a glacial lake bottom. These sediments are carried to the lakes by rivers and streams.

What happens when lakes freeze (Lacustrine Parent Material)?

When lakes freeze, or there is lower river flow, the lake water becomes less turbulent allowing the suspended sediments (sand, silt, and clay) to settle at the bottom of the lake and stratify.

How do Lacustrine sediments get exposed?

Lacustrine deposist get exposed as land levels rise or lakes are filled with sediment, and then new rivers cut through the old lake bed creating a channel through the deposits (like the Thompson River in Kamloops).

Organic Parent Material Description

These materials (from plants and other organisms) accumulate in lake or swamp water. These maerials are slowed down by the limited Oxygen supply. That is, the remains of plants and animals.

Organic Parent Material Examples

• Animals

• Bacteria

• Fungi

• Protist

• Slime

• Moulds

• Algae

Eolian Deposit Parent Material Description

Wind trasported material. It includes:

• Sand dunes - formed by sand in arid locations.

• Loess - silt blown in from floodplains.

• Tephra - volcanic material of any size.

Colluvium Parent Material Description

Weathered material transported by gravity. From mud flows or at the foot of a mountain slope.

Colluvium Material Example

Due to the recent fires there is less oot system and, therefore, material may ‘slide’ down and sediment at the foot of a mountain.

Marine Parent Material Description

Fine silts and clays deposited on ocean floors. Uplifting ocean floor above sea level results in marine parent material.

Effect of Climate on Soil Formation

Weathering of material.

Types of Weathering

• Freezing and Thawing.

• Heating and Cooling.

• Wetting and Drying.

• Grinding Action.

• Rainfall.

Freezing and Thawing in soil Formation

Water expands and exerts up to 150 tonnes per sq. feet of pressure as it freezes. This amount of pressure will split any rock no matter how big or small.

Heating and Cooling in Soil Formation

Rock will expand and contract as it heats and cools. This causes fracturing, which may result in exfoliation of exposed rock caps (loosening or scaling).

Wetting and Drying in Soil Fromation

Wetting and drying causes soil to swell and contract, which then results in abrasion among the soil particles creating finer particles than previously existed.

Grinding Action in Soil Formation

Friction caused by water, wind, gravity and glaciars. The friction of rock and soil particles moving against each other causes desintegration and erosion.

Rainfall in Soil Formation

• As rain falls, calcium and magnesium are leached (down through the soil) from the soil) resulting in changes of pH and, therefore, soil amendments.

• Rainfall may carry fine soil particles (clay) into lower layers below the topsoil where they concentrate and create a hardpan. Thus, inhibing root penetration and water drainage.

Soil Horizons

Layers of soil or soil material.

Types of Soil Horizons

• A - Topsoil

• B - subsoil

• C - Parent Material

• D - Bedrock

Depths of Soil Horizons

• A - 15-30 cm

• B - 30-100 cm

• C - 30 cm to many meters

• D - Thousand of meters

Topsoil Characteristics

• Surface layer containing living organisms.

• The horizon where root development occurs.

Subsoil Characteristics

• Low in Oxygen.

• Contains few if any living organisms.

• Little organic matter and biological activity.

Temperature in Soil Formation

Climate affects plant growth and, hence, the organic matter concentrations in the soil. In areas with more rainfall and higher temperatures there is more vegetation and, therefore, more organic matter concentration in the soil (richer soils).

Biota Description

Plants, animals, and humans cause physical weathering which contributes to the soil formation process.

Biota Soil Formation Examples

• Plant roots can split rocks and secrete enzymes which break down rocks.

• Animals and insects burrow.

• Humans cultivate, plow, and remove vegetative cover (like trees) which adds to physical disintegration.

Topography Factors in Soil Formation

• Slope

• Depression

• Elevation

• Exposure to Sun

Slope in Soil Formation

Area of thin soils as less water moves into the profile due to rapid runoff; also subject to rapid surface erosion during periods of heavy rainfall. This results in less organic material and plant growth.

Depressions in Soil Formation

Areas of more moisture accumulation that cannot drain away (as it does in slopes). In arid areas, it results in salt concentration problems as evaporation is the only way to reduce water level.

Elevation in Soil Formation

Higher elevation areas have a cooler climate but often more available moisture and, therefore, more plant growth.

Exposure to Sun in Soil Formation

South facing slopes have higher temperatures resulting in less moisture and less vegetation (like the South Thompson Valley).

Time in Soil formation

Soil develops distinct layers, horizons, as time pases.

How long would it take for a soil profile to develop?

Under ideal conditions a soil profile may develop within 200 years. Under less favourable conditions, it may take several thousands of years or not develop at all.

Soil Horizons Characteristics

• Horizontal layers of soil.

• Layers differ from each other in physical and chemical properties.

• The major soil horizons are known as top soil, sub soil, and parent material.

Topsoil Characteristics

• Higher amounts of organic material due mainly to plant growth.

• Usually a dark colour.

• Zone of leaching; clays and carbonates leach from the topsoil layer (creating a hardpan in layer B - Subsoil).

Subsoil Characteristics

• Less organic material than topsoil layer.

• Lighter in colour than topsoil.

• Zone of accumulation of clays (grayish) and might create a hardpan.

Major Soil Components and their Composition

• Mineral particles (sand, silt, clay) - 45%

• Organic Matter - 5%

• Soil water - 20-30%

• Soil air - 20-30%

Sand Characteristics

• .050 - 2.00 mm in size.

• Mainly quartz.

• Hard and resistant to weathering.

Silt Characteristics

• .002 - .05 mm

• Mainly quartz.

Clay Characteristics

• Smaller than .002 mm

• Plate like structure.

• Very fine particles formed from mica feldspars, and various oxides.

• More chemically reactive than sand and silt.

Soild Soil Components

Mineral particles (sand, silt, clay) and organic matter.

Soil Components Occupying Pore Space

Air and water.