Transcript Components and Properties of Soils

Importance & Formation
of Soil
Ms. Keen
Importance of Soil
• Much of the United States’ success is
due to the productive capacity of the soil
– U. S. produces enough food and fiber for
its own use and exports these commodities
to other countries
Importance of Soil
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Habitat for soil organisms
Engineering medium
System for water supply and purification
Original source of most nutrients for
plant and animal life
– Provides nutrients for plant growth
– Animals get nutrients from plants or from
animals that eat plants
Soil Acts as a Recycling System
Soil Permanence
• Soils appear permanent and
unchanging
• Actually very dynamic, constantly
changing
• Changes must balance to maintain
productivity
Soil Formation
• Based on weathering of rocks and
minerals
• Physical and chemical decomposition
break down rocks and minerals into the
mineral matter component of soil
• On average, it takes about 100 years to
develop one inch of topsoil
Factors Affecting Soil
Formation
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Climate
Living organisms
Parent soil material
Topography
Time
Different combinations of these factors
lead to the formation of different soils
Climate
• Refers to the average course of weather
at a particular place as exhibited by
temperature, wind velocity, and
precipitation
• Temperature and precipitation greatly
affect the rate of weathering
• Increased temperature causes an
increased rate of chemical reaction and
increased growth of fungi, soil bacteria
and plants
Climate
• Adequate moisture causes an increased
rate of chemical reaction and increased
growth of fungi, soil bacteria and plants
• Excess moisture retards microbe growth
• Water carries minerals and organic
matter downward, forming horizons
• High rainfall areas generally have
leached, acidic soils
Average Annual Precipitation
United States
Climate
• Low humidity and high winds tend to dry
out soils, reducing chemical reaction
and microbe growth
• High winds can
cause cause
significant erosion
on some
unprotected soils
Photo courtesy of USDA NRCS
Living Organisms
• Bacteria and fungi
– break down organic materials
• Insects, “insect-like” creatures, and
animals
– break down and mix soil materials
• Plants
– roots break up mineral matter
• Humans
– land clearing and cultivation remove
vegetation and disturb soil structure
Parent Material
• Influences texture, structure,color, and
fertility
• Clay soils generally form from easily
weathered materials
• Sandy soils form from more resistant
materials
Topography
• On sloping land, loose material is
moved downward by runoff and gravity
– breaks up soil materials and deposits them
at lower levels
– exposes subsoil materials on upper slopes
• Influences percolation and drainage
– sloping land has more runoff and less
percolation
– low-lying soil may tend to be saturated,
leading to less biologic and chemical
activity
Time
• Soils form by chemical and physical
weathering of parent material over time
– chemical weathering results from chemical
reactions upon soil materials
– physical weathering results from
mechanical forces such as freezing and
thawing, erosion, and gravitational
movements
• Soils age and mature over time
Soil Maturity
• Initially, nutrients are released quickly, plant
growth increases, and organic matter
accumulates
• Eventually, soluble nutrients are lost through
leaching, plant growth decreases, and
organic matter is lost
• Fine clay particles move downward from
surface
• Mature soils gradually decline to lower levels
of production
Variety in Soils
Climate, living organisms, parent material,
topography, and time impact soil
formation and changes in soil
composition.
Variations in these factors have resulted
in the wide variety of soils that are present
today.
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Reproduction or redistribution of all, or
part, of this presentation without written
permission is prohibited.
Instructional Materials Service
Texas A&M University
2588 TAMUS
College Station, Texas 77843-2588
http://www-ims.tamu.edu
2006