Soil

Soil Composition

Water and air content can fluctuate over time.

Soil FormationPhysical weathering of rocks by wind and water breaks into smaller rocks

Biological weathering as lichens break down rocks into mineral particles.Lichens are a mutualistic interaction between a

fungus and an algaeThe fungi secrete an enzyme that breaks down the

rock

Video Link: The Ecosphere: Succession and Soil Formation

Primary Succession

First plants to grow on bare rock are:lichen

Next plants are:mosses

followed by:ferns

etc., etc. until the climax community is established

Where are the animals?

The plants attract animals, When the animals die, their bodies decay and add organic material to the soil.

cc licensed flickr photo by DG Jones: http://flickr.com/photos/dgjones/2151134648/

Soil Horizons

Soil is made up of distinct horizontal layers; these layers are called horizons.

They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers ( subsoil, regolith and bedrock).

Soil TypesMineral content in soil can be sand (large particles), silt (medium size particles) or clay (small particles).

Most soils are made up of a combination of the three.

Soil Horizons� O Horizon - The top, organic layer of

soil, made up mostly of leaf litter and humus (decomposed organic matter).

� A Horizon - The layer called topsoil; it is found between the O horizon and the E horizon. Seeds germinate and plant roots grow in this dark-colored layer. It is made up of humus mixed with mineral particles.

� E Horizon - This eluviation (leaching) layer is light in color; this layer is between the A Horizon and the B Horizon. It is made up mostly of sand and silt. Most of its minerals and clay are lost as water drips through the soil.

Soil Horizons� B Horizon - Also called the subsoil - this

layer is between the E Horizon and the C Horizon. It contains clay and mineral deposits (like iron, aluminum oxides, and calcium carbonate) that it receives from layers above it when mineralized water drips from the soil above.

� C Horizon - Also called regolith: the layer between the B Horizon and the R Horizon. It consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer; very little organic material is found in this layer.

� R Horizon - The unweathered rock (bedrock) layer that is beneath all the other layers.