Transcript No Slide Title

Soils
In this lecture period, we wish to learn:
• What is "soil" and why are soils
important and fragile?
• What is weathering and what are the
factors responsible for weathering?
• How are nutrients cycled through soils?
• How are human activities influencing
the Earth’s soils, and hence
productivity?
• Definition: soil is a dynamic natural
body capable of supporting a vegetative
cover
Why Are Soils Important?
• All our elements came from the dying throes of
large stars
• Our proteins are built from amino acids
(comprised of a NH3 group, a carboxyl group,
and a variable CH chain) coming from plant life
• Plants need nutrients to help form proteins
• Plants synthesize amino acids from sunlight,
water, and nutrients derived from interaction
with soils
Composed primarily of weathered materials, along with
water, oxygen and organic materials, soil covers most of
the land surface with a fragile mantle.
Nutrients
Although living tissue is composed of carbon, hydrogen, and
oxygen in the approximate proportion of CH2O, as many as 23
other elements are necessary for biochemical reactions and for the
growth of structural biomass.
Examples of important nutrients:
• Nitrogen: the proteins found in plants and animals contain
about 16% by weight of nitrogen.
• Phosphorus: part of the important protein ribulose biphosphate
carboxylase and is part of ATP - adenosine triphosphate, the
universal molecule for energy transformations
• Calcium: Is a major structural component of plants and animals
• Others: Magnesium, Potassium, Iron, Sulfur, etc.
C, N, and S come from the atmosphere;
Ca, Mg, K, Fe, and P from rock weathering processes
Soil Horizons
Typical soil profile developed on a granite bedrock in a
temperate region.
“O” horizon: at the surface, composed of organic
matter or plant litter
"A" Horizon: marks the beginning of the mineral soil.
Usually darkest in color and contains the most organic
content of the mineral soil profile. Minerals in the A
Horizon are mostly clays and insoluble minerals.
"B" horizon: has relatively little organic material, but
contains the leached soluble minerals, including silicate
clays, humus, Fe and Al oxides. Material is leached
from the O and A horizons, enriching the B horizon.
"C" horizon: parent material - slightly broken-up
bedrock, typically 1-10 meters below the surface
Many other types of soils exist, depending on climate
Types of Soils
• Soils vary in color, content, pore space, depth,
and acidity (pH)
• Texture refers to relative amounts of different
sizes and types of particles
• Five major soil orders are mollisols, alfisols,
spodosols, oxisols, and aridisols
• Most of world’s crops are grown on grassland
mollisols, and on alfisols exposed when
deciduous forests are cleared
Soil Texture
• Soils vary in content
of clay (very fine
particles), silt (fine
particles), sand and
gravel (coarser
particles)
• Soil texture (chart)
affects soil porosity
• Soil porosity and permeability affect how water,
gases and nutrients are available to plants
• loams, with roughly equal amounts of sand and
silt, with less clay, are best for most crops
http://res.agr.ca/CANSIS/
Soils of the U.S.
World Soils
Source: FAO
http://hum.amu.edu.pl/~zbzw/glob/glob1.htm
Global Vegetation
http://hum.amu.edu.pl/~zbzw/glob/glob1.htm
Soil’s Goods and Services
“below that thin layer comprising the delicate
organism known as the soil is a planet as lifeless
as the moon” (Jacks and Whyte)
• Vegetation requires soil to grow
• all nutrients come from soil
• soil, along with the vegetation and bacterial life it
supports, purifies water and ameliorates runoff
• soil is necessary for life
Plant Roots
"A single plant of winter rye, 50 cm high, was found to have a
root system consisting of 143 main roots, 35,600 secondary roots,
2.3 million tertiary roots, and 11.5 million quaternary roots. The
root system was found to have a total length of 600 km and a total
surface of about 250 square meters“ - Emiliani, 1992.
• Plants obtain inorganic minerals from the soil (e.g.
NO3) and incorporate their elements into biochemical
molecules.
• Animals may eat plants and each other, and
synthesize new proteins, but the building blocks are
the amino acids originally synthesized in plants. There
are no vitamin pills in the natural biosphere!
Hubbard Brook Experimental
Forest
Sources of Nutrients
The pool of nutrients held in the soil and vegetation is
many times larger than the annual receipt of nutrients from
the atmosphere and rock weathering
Life husbands nutrients effectively on land, storing much
of the total in the humus
Net Primary Production
Net Primary Production (NPP) = photosynthesis - respiration
NPP can be measured in units of energy or in tons of carbon.
Most often we think in terms of the amount of carbon
produced by life.
NPP (global) = 60 x 1015 g C /yr
Measurement of NPP in ecosystems is not easy, but methods
are well developed for forests, grasslands and the oceans. The
direct harvesting of plants for food, fuel and shelter accounts
for about 6% of the total.
Now, back to soils
Chimney Rock
Igneous Rock Weathering
a) Physical weathering (disintegration). As
rocks heat and cool they expand and contract,
cracking apart. Water, by erosion and freezethaw cycles, and abrasion by wind-blown
sand or glacial ice masses, grind down rocks.
b) Chemical weathering (decomposition)
is due to water, oxygen, and acids that
results from biochemical activity. Most
intense in wet, hot areas, this results in
plant nutrients in solution.
Together, physical and chemical weathering
transform the igneous rock into sand and
clay particles and dissolved salts.
Feldspar
a general term for a group of aluminosilicate minerals
containing sodium, calcium, or potassium and having a
framework structure.
the most common mineral in the Earth's crust
In humid tropics many granite boulders can be kicked into a
pile of grains - because the feldspar grains, which originally
form an interlocking crystal network, weather to a loosely
adhering clay called kaolinite.
K Al Si3 O8 ---> Al2Si2O5(OH)4
(feldspar)
(water)
(kaolinite)
Notice this equation does not balance exactly - the extra potassium (K) and
silicon (Si) not appearing on the right hand side are carried away in solution.
We say that the potassium and silicon is leached away and that the feldspar is
hydrated (water consumed).
This process is accelerated by carbonic acid!
Rock Weathering
Igneous rocks + acid volatiles = sedimentary rocks + salty oceans
• Due to interaction of the atmosphere with the Earth’s exposed crust
• Volcanic gases (e.g., C, S) dissolved in water produce acids that react
with surface minerals.
• Later, oxygen in the atmosphere reacted with reduced materials.
• With the advent of land plants, soil minerals have been exposed to
high concentrations of carbon dioxide maintained in soil pores as a
result of decomposition and the metabolic activities of roots.
• The reaction of this carbon dioxide with soil water produces Carbonic
acid (H2CO3), which determines the rate of rock weathering in most
ecosystems.
CO2 + H2O ---> H2CO3
(gas) (liquid)
(solution)
• Acid rain, produced by human effluents of nitrogen and sulphur
gases will increase the rate of rock weathering in downwind areas.
Rates of Weathering
a) A 3000-year
old Eqyptian
Obelisk
b) Same Obelisk
100 years after
moving to New York
Weathering rates are a strong function of local climate
How Fast Does a Rock Decay?
• Iron Nails rust quickly in soil (years).
Iron nails rust more slowly in drier environments (100's years)
• Aluminum cans decay very slowly
• Glass decays even more slowly.
• Plastic is considered non biodegradable.
• Soils aid rock decay, as do melt/freeze cycles and bacteria
Soils are both a factor in weathering and a consequence of it.
The production of soil is a positive feedback process.
Rates of Weathering of clean rock surfaces
(m/1000yr)
Rock Cold Climate Warm, Humid Climate
basalt
10
100
granite
1
10
marble
20
200
Bottom line: average rate of formation of topsoil is 10-20 millimeters
per thousand years
Acceleration of Weathering
Weathering proceeds via the
interaction of the moist atmosphere
(or soil) with bare rock.
The greater the surface area of rock,
the faster the weathering.
The total surface area (for the same
original volume of rock) increases as
the number of particles increases.
As a boulder breaks into smaller ones
along fractures and joints, much more
of the bulk's surface becomes
available for chemical decay.
This is a positive feedback
mechanism
Products of Weathering Reactions
Igneous rocks + acid volatiles = sedimentary rocks + salty oceans
The products of rock weathering are carried to the
ocean where they accumulate as dissolved salts or
in deposits of sedimentary rock.
Rock weathering is crucial for the release of biochemical
elements that have no gaseous form: Ca, K, Fe, and P.
Soil Erosion
• Main forces are wind and flowing water
• “rivers are the gutters down which flow the ruins of continents”
•sheet erosion occurs when
surface water moves across the
landscape in a wide, even flow.
•in rill erosion, surface water
forms small rivulets that flow at
high velocity through miniature
valleys.
•in gully erosion, the rivulets join
together to form larger paths of
high velocity flow.
Erosion by Wind
High winds can blow away
loose soils from flat or hilly
terrain.
During May 1934, the entire
eastern half of the U.S. was
blanketed by a massive cloud
of topsoil blown off the Great
Plains. Ships 200 miles out
in the Atlantic Ocean
received noticeable deposits
of Great Plains top soil.
Erosion by Water
Water and soil splashed during a raindrop impact.
Source: http://www.eci.ox.ac.uk/ld/ ldintro.htm
Soil erosion occurs both
incrementally, as a result
of many small rainfall
events, and more
dramatically, as a result of
large but relatively rare
storms.
Erosion due to small,
common events may
appear insignificant on the
field, but the cumulative
impact can be equally
severe.
Soil Erosion and Renewal
• Soil, especially topsoil or AHorizon, is classified as a
renewable resource
• Provided: average rate of
erosion is not greater than
average rate of renewal
• renewal or soil generation rate
= 1 inch per 500 years (range:
220 - 1,000 yrs)
• annual erosion rate is 18 - 100
x greater
• we cope (for awhile) with
fertilizers
Soil Erosion Worldwide
Source: http://www.eci.ox.ac.uk/ld/ ldintro.htm
Accelerated
erosion is a result
of humankind's
unwise actions,
such as
overgrazing or
unsuitable
cultivation
practices, which
leave the land
vulnerable during
times of erosive
rainfall or
windstorms.
Erosion’s Impacts - “On-site”
Eritrea, NE Africa.
Source: http://www.eci.ox.ac.uk/ld/ ldintro.htm
Erosion's most serious
impact may well be its
threat to the sustainability of agricultural
productivity, which
results from the the
'on-site' damage which
it causes. Crops are
particularly reliant on
the upper horizons of
the soil, which are the
most vulnerable to
erosion by water and
wind.
Erosion’s Impacts - “Off-site”
Sediment plume in the Yangtse River Delta, China.
NOAA image. Source: http://www.eci.ox.ac.uk/ld/ ldintro.htm
‘Off-site' impacts occur
when eroded sediment
and agricultural
chemicals (which move
with the eroded
sediment) move into, and
pollute, downstream
watercourses and water
bodies.
Where inputs of
agricultural chemicals are
high - as in the more
affluent nations - costs of
removing such pollutants
from drinking water can
be considerable.
Soil Erosion in
Affluent Countries
The damaging on-site effects of
erosion, in terms of decreased
agricultural productivity, are well
known in the developing countries of
Africa and Asia.
In erosion-prone areas of the more
affluent countries, productivity may
be maintained in the short to medium
term by increased fertilizer input. The
effects of erosion thus receive less
attention by farmers in developed
countries.
Farming in Mediaeval times. Source:
http://www.eci.ox.ac.uk/ld/ ldintro.htm
Dust Bowl
A consequence of
ploughing the prairies and
removing the tall grasses
The Dust Bowl
U.S.:
1930’s Dust bowl caused by
plowing the prairies (previously
the topsoil was retained by
long-root grasses)
Soil Conservation Service
(SCS) established in 1935
Great Plains: lost one-third of
its topsoil in 150 years
Iowa: lost half of its topsoil
California: topsoil eroding 80 Global:
times faster than it can be
 topsoil is eroding faster than it is
regenerated
replaced over one-third of the world’s
Each day: topsoil eroded
croplands
would fill a line of dump trucks  Each year we must feed 90 million
3,500 miles long! Costs
more people, with ~24 billion metric
estimated a $125B/yr!
tons less topsoil
Soil Conservation
• Various practices serve to conserve soil, mainly by keeping
the soil covered with vegetation
• conservation-tillage: disturb soil less, plow in spring
rather than fall, leave cover plants
• contour farming and terracing on gently and more severely
sloping land
• strip cropping alternates crop and cover crop
• gully reclamation and windbreaks
• a long-term study of Coon Creek in Wisconsin found soil
erosion was reduced to 6% of its dust bowl peak, based on
sediment accumulation