Topic #1 -
What is a soil?
A dynamic (
continuously changing) natural body composed of mineral materials, organic materials,
gases, liquids and living organisms which can serve as a medium for plant growth.
Soil is one of our
“ultimate” resources, like water and air. Soil is a
renewable resource. A soil is both an ecosystem in
itself & a critical part of the larger terrestrial ecosystem.
How do soils form?
Soil Forming Processes
- soil constituents are modified or destroyed, and others are synthesized.
- physical and chemical weathering, formation of secondary minerals, organic matter (Oi)
- movement of inorganic and organic materials from one horizon (up or
down) to another
- moved primarily by water; however, soil organisms also mix materials within and between horizons
- clay, aluminum and iron oxides, humus, calcium carbonate (CaCO
) and soluble salts
- materials added to the soil profile from outside sources
- organic matter from vegetation, dust (eolian) and acids from the atmosphere, salts from groundwater
materials removed from the soil profile
- leaching of soluble constituents to groundwater or erosion of surface materials by wind or water
These processes lead to horizon differentiation (
formation of soil layers with distinctive physical, chemical,
biological and morphological properties).
Description of Soil Horizons
- a surface layer dominated by organic materials (> 20% organic carbon)
Oi - slightly decomposed organic matter; can still identify the original plant and animal remains
Oe - intermediately decomposed
Oa - highly decomposed (can not
identify the original source of the organic material)
mineral horizon (<20% organic C) which forms at the surface or beneath an O horizon
characterized by a darker color than the rest of the profile due to the accumulation of organic
matter; high biological activity
layer loses materials such as iron/aluminum oxides and clays)
- an intensively
leached horizon in which organic matter along with iron/aluminum oxides and
clay have been removed.
typically white or light gray in color due to the lack of coatings on the mineral surfaces
horizon formed beneath an A, E or O horizon and is a zone of accumulation.
may accumulate clay, iron/aluminum oxides, organic matter, carbonates, etc.
a layer of unconsolidated
material showing little weathering (alteration) and biological activity
beach sand, alluvium deposited by rivers, glacial till deposited by glaciers).
rock that can not
be dug with a shovel and shows little evidence of weathering