To a geologist, or an earth or soil scientist,soil
is material that supports plant growth. It is usually within the top meter
of material on the ground.
Soil is an open system leaching from top down.
To an engineering geologist, however, soil refers to any
unconsolidated material overlying the bedrock.
Type of soil is primarily a function of climate, organic activity,
relief, and time. (ClORPT)
Cl = Climate (warmer, wetter climates produce soil faster)
O = Organic Activity (Roots and animal burrowings, as well as acids
from organic sources, have a major role).
R = Relief of Land (Soil tends to be washed away from the tops of
mountains and gathers in the lowlands.)
P = Parent Material (porous materials and silty sands will decay
into soil faster than granites.)
T = Time (The more time processes are at work, the more soil forms.)
It is not strongly dependent on parent material..
Organic matter consists of plant litter on the surface, and humus
dispersed throughout the soil profile.
Organic matter is an important component of soil.
Trees, shrubs, herbs and grass produce plant litter.
The plant litter becomes humus, which is finely divided organic
matter which the plants can use.
Fifty years of farming may result in the loss of 20 to 40% of
organic matter in temperate areas.
Formation of Soil
Soil formation occurs in two main processes:
Mechanical Breakdown - Ice or salts get into cracks, and break
down material. Also, expansion and contraction from heating and
cooling breaks down material.
Chemical Breakdown - Mostly in the form of the reaction of water
on rocks near the surface, although also occurs from acidic
breakdown. This process is mainly aerobic (involving oxygen) vs
anaerobic (not involving oxygen).
Several acids from plants and atmosphere such as the Carbonic,
sulfuric, nitric, and various organic acids.
From the atmosphere, for example:
CO2 + H2O = H2CO3
Limestone dissolves relatively easily
Silicates dissolve more slowly leave dominantly clays and Al and Fe
Oxydation and Hydration - Similar to the formation of Rust.
Hydrolysis - Formation of clays.
These are a combination of mechanical and chemical effects.
Mechanical - Roots, animal burrowings, etc.
Chemical - Plants produce acids and other chemicals that react
with the rocks and soil.
One kilogram of soil may contain as much as 500 billion bacteria, 10
billion actinomycetes, and one billion other fungi.
Bacteria and fungi are consumers of organics, which is dominantly an
aerobic process. At temperatures above 75°F, there is little humus
present in the soil.
Some bacteria fix nitrogen on leguminous plants.
Soil is loaded with roots.
Burrowing animals mix and aerate soil.
Color is independent of bedrock.
Black color in soil is due to humus.
Red, yellow or brown color is due to oxidized iron and a low humus
Grayish and bluish colored soil in humid climate is due to reduced
iron in a boggy soil.
Gray to white soil in dry climate is due to low humus and high salt
Example:Long Island has black or brown top soil. Maryland and areas
further south tend to have a more reddish soil. This is due to the
fact that it is warmer there, so there is less humus to darken
Carbon in the Soil
Soil has three times as much carbon as all land vegetation.
Each year soils release 5% of their carbon to the atmosphere as CO2.
This is a quantity 10 times that produced by the burning of fossil
Arctic tundra, peat and wetland soils are world reservoirs of soil
Methane Hydrate (CH4 + H2O) (A greenhouse gas) - There is more
methane hydrate in the oceans and arctic soils than all the fossil
fuels in the world conbined. It is very pervasive throughout the
tundra soil. If the tundra is heated up (by global warming, for
example), the carbon in the soil may become oxidized to CO2 and there
may be the release of methane to the atmosphere from the methane
hydrate in the permafrost. Both CO2 and methane are greenhouse gases.
This is an example of positive feedback, for the greenhouse effect.
Time to develop soil
For 1 meter of soil to develop, it takes:
20,000 to 100,000 years, if breaking down granite in an equatorial
Roughly 10,000 years, if breaking down glacial deposits in the
midwest praries ( the same amount of time since the last ice age).
Water -- a major source. Can leave behind several strong erosion
Rills -- Relatively small scale. Farmers can easily plow over
Gullies -- Larger. Can not be plowed over, and increase furthur
Siltation -- Water erosion also results in the sedimentation of
streams. Soil that is eroded off the land is dumped into streams
and ponds, clogging them up. When this occurs in areas of high
fertilizer use, this can also cause hypoxia in streams from
eutrophication initiated by fertilizer washoff.
Wind -- can be a major factor, especially in fairly dry regions.
Example: The Dust bowl, in the 1930's.
Mitigation -- ways to stop soil erosion.
Green manure -- for example, alfalfa or clover. These crops cover
the fields in winter and help to enrich soil while growing as well
as when they are plowed under in spring..
Windbreaks Trees planted along the edges of fields.
Contour Plowing -- plow parallel to the contours. Helps prevent
erosion by slowing runoff down sloped land areas.
Strip-cropping -- plant parallel to contours alternating plant
types. One a type that has exposed soil, such as corn, the other
type that completely covers the ground such as alfalfa.
Terracing -- shaping steep terrain to provide flat areas suitable
Soil erosion can also result from over-grazing.
Cutting down forests to provide grazing land or farmland often
results in massive soil erosion, because the roots decay and can no
longer hold the soil.
Off road vehicles also do damage and increase soil erosion by
tearing up the ground and killing vegetation
During the early stages of development (such as new construction for
houses) there is extreme erosion, since for an extended period of time,
there is nothing protecting the soil and ground from being washed away.
Inorganic Soil Pollution
Often found in the vicinity of mines.
Also get some from farming, in the form of pesticides.
Industrial processes create a lot of inorganic pollutants.
Lead, Arsenic, Mercury, and Cadmium (all main industrial pollutants) tend
to accumulate in the human body for long periods of time, as they are
difficult to flush out of our system. Organic Soil Pollution
The main source of organic pollution is from herbicides, pesticides,
and fungicides used in agriculture and in gardens.
Oil spills are another large organic soil pollution source.
Associated with irrigation. The water used for irrigation has
dissolved salts that are left behind as the water evaporates
Affects 25% of farmlands.
Dilution with water -- uses up and contaminates a lot of water.
Washing with water -- also uses up and contaminates a lot of water.
Vapor extraction -- expensive process - deals with pumping the
vaporous fumes out of the ground, and into the atmosphere.
Contaminant-eating bacteria - ideal in that it is fairly inexpensive
and extremely effective. However, is a very slow process. This process
can be somewhat aided by building a "High Technology Restaurant for
Bacteria", which add oil and/or other nutrients into polluted soils
and groundwaters in order to aid the feeding of contaminant-eating
Contaminant-concentrating plants -- certain plants concentrate
specific elements (for example, Loco weed concentrates selenium from
the soil.). Over time the soil becomes depleted in the element. Each
year the plants can be harvested and disposed of or sometimes the
element can be extracted for use.
Polluted soil is often dug up and shipped to landfills or hazardous waste
sites instead of being cleaned