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Roger Weller, geology instructor
Soils
Marti Stoner
Physical Geology
Spring 2006
Soils
Soils
are one of our most important natural resources. Soil is a thin layer of material on the
Earth's surface. Soil is a porous natural material made up of many materials
including rocks, weathered minerals, and dead decaying plants and animals. The ratio of these materials vary from place
to place depending on how it was formed. Soils are very old -- as old as
200,000 years. Some soil profiles are
formed in as little as 2000 years. There are 5 soil forming factors; parent
material, climate, organisms, location, and time. There are four main mineral types found in
soil; sand, silt, clay, and loam.
Soil Formation
The formation of soils is a process
that takes a very long time creating a thin layer of soil. Physical and chemical
weathering cause the rocks on the surface of the Earth to break down into
smaller pieces.
Soil
and topsoil are produced at about an average of 1 ton/year, and lost at a rate
of 10-40 times faster than it is produced. Worldwide there is a total of 1200
million hectares of agricultural land of which we are losing at the rate 6
million hectares per year.
http://www.seafriends.org.nz/enviro/soilgeosoil.htm
The
soil is formed from a "parent material". This material could be several different
types; bedrock, organic material, old soil surface, or a deposit from water,
wind, glaciers, volcanoes, or material moving down a slope.
The climate is another factor that
helps in forming soil. Environmental
forces such as sunshine, heat, rain, ice, snow, wind, weather and breakdown the
material, and influence how fast or slow the process of soil formation takes. When weathering takes place, four components
are released from the rock; minerals in solution (the basis of plant
nutrition), oxides of iron and alumina, various forms of silica, and stable
wastes as very fine silt (mostly fine quartz) and coarser quartz (sand). These
have no nutritious value for plants. If
the climate is favorable, the oxides of iron and alumina combine with silica to
form clay, a new mineral.
Decaying plants and animals add
organic matter. In addition to adding organic matter to the topsoil, plants add
hydrogen helping the water percolate through, moving some parts of the minerals
in the soil vertically. Microorganisms (bacteria, fungi, algae, protozoa and
more) help breakdown plant matter producing carbon dioxide. Carbon dioxide mixed with moisture in the
soil will dissolve mineral materials.
Some bacteria also extract nitrogen from the air, to assist in plant
growth. Insects, earthworms roundworms,
millipedes, centipedes help decompose plant material by feeding upon it and
loosening the soil by burrowing through it.
Mice, squirrels, hares, caribou, moose, bison, camels, horses etc all
contribute to the enrichment of the soil also.
The location of where the soil is
can determines how the climate affects the breakdown of the parent
material. The drier soils are the ones
that are facing the sun on slopes. At the bottom of a hill the soil is moister
than the soils on the slopes.
Soil Profile
Every soil profile is made up of
layers called soil horizons. The main horizons are A-horizon, B horizon, and
C-horizon. Horizon A is the upper layer of soil at the surface of the earth's
crust, more commonly known as topsoil. On top of the A-horizon is the O horizon
made mostly of organic matter from the
vegetation, which helps prevent erosion, and adds nutrients to the soil once it
is broken down.
http://www.seafriends.org.nz/enviro/soil/geosoil.htm#soil
The A-horizon is made of mineral matter mixed with some humified (decomposed)
organic matter. You will find most plant roots and soil organisms in the A-horizon. It
has very little nutrients because the nutrients have been used by plants or
leached downward into the B Horizon.
B-horizon, also called the zone of
accumulation because it is the area where nutrients are leached from above, and
new material from below accumulate.
In the B-horizon concentrations of clay, iron or organic matter can be
found. Some lime may be present also.
C-horizon is found below the
B-horizon and is mainly made up of big
rocks (parent material) that have been through some weathering process as it is
breaking down to form new soil.
Soil Types
Soils are porous bodies that retain water and are made
of a silicate group of minerals called silicate clays, mainly consisting of the
chemical elements oxygen, silicon, aluminum, and iron. Potassium, calcium, magnesium, sodium,
carbonates, oxides, phosphates and sulfates are other minerals that are in the
soils.
There are four main mineral types
found in soil: sand, silt, clay, and loam. The large pieces in the soil are
sand. It feels rough when you rub it
because it has sharp edges, and it doesn't have many nutrients in it. In
between the sand and clay particle size there is silt. Silt feels smooth and
powdery and it has a smooth non-sticky feel when it is wet. The smallest particle in soil is known as
clay. Clay is smooth when dry and unlike silt, it is sticky when wet. Soils
high in clay content are called heavy soils. Clay is not very porous but it can
have a lot nutrients. Particle size has
a lot to do with a soil's drainage and nutrient holding.
http://www.seafriends.org.nz/enviro/soilgeosoil.htm
The colors of soils are caused by what elements the
soil is made of. Darker soils have a
high moisture and organic content.
Surface soils are usually darker than subsoils. Red, yellow and gray colors of subsoils tell
us about the oxidation and moisture states or iron oxide content of the
soil. Good drainage can be found in
soils that have red and yellow hues.
Soils that have very little aeration are usually grey hued.
Types of soil vary around the world depending on
different conditions of temperature and humidity. Tundra soil is mostly made up of organic
matter, it is boggy and unable to thaw completely, and has a short growing
season.
Grey podsols soils are in areas made mostly of clay
from the B-horizon with a sandy A-horizon because the soils are acidified by
the resin from the trees, causing the nutrients and clays to be leached
downward. The growing season is during cold summers.
Deeper soils (grey/brown) are formed where evaporation
is basically the same as the rainfall. Deciduous forests hibernate in the
winter and have a long growing season during the summer months.
Fertile black chernozem soils, rich in humus, are
formed in places here the water table is deeper. These dryer conditions promote deep-rooting
grasses that produce enough acid to keep a layer of clay, which in turn does
not allow the soil nutrients to be leached.
Chestnut soils are formed in areas where the water
table becomes deeper, and where rainfall is less, than the evaporation rate,
causing soils to form that have very little clay, humus and organic material,
and high concentrations of iron oxides, making it hard to grow plants.
You can find rigorous erosion and slow weathering in
desert soils. It is difficult to grow
most plants in desert soils because they lack many nutrients. Desert soil is easily disturbed by wind and
occasional rain. These are the areas where you can find gypsum in the horizons.
Life on Earth would not be possible
without healthy vital soils because they are necessary for plant food and the
formation of the oxygen we breathe. The way humans make use of soils has a huge
impact on soil formation. The quality of the soil ecosystem is important to
growing plants and the formation of more soil.
URL Addresses
http://library.thinkquest.org/J003195F/soil.htm
http://soil.gsfc.nasa.gov/basics.htm
http://soil.gsfc.nasa.gov/soilfert/npk.htm
http://www.enviroliteracy.org/subcategory.php/36.html
http://www.gi.alaska.edu/ScienceForum/ASF5/531.html
http://www.seafriends.org.nz/enviro/soil/geosoil.htm