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Soil and Agriculture
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I used to think my home town (Malone, NY) was the most isolated flyspeck on the face of the
Earth…
Strategically located - right in the middle of nowhere!
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One day a magazine arrived in the mail…
On the cover was the intrepid explorer Thor Heyerdal and his amazing Kon-Tiki raft
He had crossed the South Pacific from South America to Easter Island
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I was wrong about my home town…
Easter Island is the most remote scrap of land on the entire planet
Size = 64 mi2, located >1,400 miles from the nearest island (Pitcairn Island), and >2,000 miles
from the nearest continent (South America)
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Easter Island was one of the greatest archeological mysteries of all time
Heyerdal showed how pioneers from South America or distant islands could have reached
Easter Island in primitive native boats
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What happened to their civilization?
What was the hidden meaning behind the 200 colossal and mysterious statues they left behind?
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First Polynesian settlers reached the island ~1,600 years ago
Colonizers would have found a tropical paradise
Mild South Pacific climate, rich fertile soils
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Pollen from soil cores shows dense tropical vegetation for at least 30,000 years
Large trees and shrubs with a dense understory of shrubs, ferns, grasses
80 ft. palm trees, species known to be a rich source of timber, nuts, sugar, syrup, honey and
wine
Ancient garbage heaps filled with bones of porpoises, thousands of seabirds of at least 25
different species
Easter Island may have been one of the most important nesting grounds in the Pacific for
seabirds
Bones of owls, parrots, herons…
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Easter Island today is a barren wasteland
Only 47 species of plants, none larger than a shrub, mostly grasses and ferns…
No native animals larger than an insect
No birds, no lizards, no snails...only mammals are introduced rats etc…
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Easter Island may be the most thorough destruction of an ecosystem in recorded history
Having cut down every scrap of wood, the islanders could not even sail away to escape their fate
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After eating every living thing, they turned to the only available source of protein - one another
- they became cannibals
Ultimate insult was “the flesh of your mother sticks between my teeth”
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Complex civilization, culture of priests, chieftains, bureaucrats
Quickly devolved into anarchy, warring tribes that toppled one another’s statues
By the early 1800’s, destruction was complete
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Easter Island is a microcosm of planet Earth
Living lesson of the fruits of environmental destruction
“Why didn’t they look around, realize what they were doing, and stop before it was too late? What
were they thinking when they cut down the last palm tree?”
Jared Diamond
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Easter Island is not the only culture in Earth’s history to collapse under the weight of its own
ecological recklessness
Mayan civilization also ate itself to death…
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Mayan civilization thrived in Central America for nearly three millennia
During Classical Mayan period, population numbered in the millions
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Masters of agriculture, used swidden agriculture (slash and burn), small scale garden plots of
mixed species that grew well together
Managed secondary succession in the nearby tropical forest to maximize useful species
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Large terraced fields, orchards
Artificial rain forest gardens with up to 29 useful species of trees, vines, herbs, roots
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Traditional agriculture could not support their rapidly growing population
Swidden agriculture gave way to large scale intensive agriculture (like American Midwest
today)
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In the blink of an eye in the 9th Century, the entire civilization collapsed
Urban populations plunged from 600 people per km2 to less than 1/10th that number
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Ruins of Tikal, up to 90% of the houses were abandoned after the collapse of the Mayan Empire
Great mystery…only recently has evidence cast light upon this disaster
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Key to understanding the Mayan collapse is buried in its soils
Soils under the Mayan Empire were very fertile, unusual for tropical soils
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Sediment cores and other evidence point to widespread deforestation, massive erosion of
topsoil, large-scale depletion of soil nutrients
Inputs of phosphorous to ancient lakes shows exponential increase due to:
> Deforestation
> Sewage and other wastes
> Agricultural runoff
By early Classic Mayan times, vast rainforest was gone - present rainforest is all secondary
growth!
Evidence suggests a vicious cycle due to overpopulation
Increasing population:
> Increasingly intensive agriculture, progressive clearing of forest for larger and larger fields
> Ever-shortening fallow period
> Rapid loss of soil nutrients
Final straw was severe drought during the 8th and 9th Centuries
Mass starvation, violence, social unrest, political upheaval…
“Those who cannot remember the past are condemned to repeat it”
George Santayana
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Soil is not high on our list of daily concerns
But it should be…
Without the soil, we are nothing
> We cannot sustain ourselves
> We cannot feed ourselves
If we lose the soil, we lose ourselves…
Everywhere on Earth, our soils are in trouble
The only “stable” soils remaining (yellow on map) lie under the ice in Siberia and the Northwest
Territories!
We will consider:
> How soil ecosystems function
> How soils are formed and structured
> How soils can be damaged by harmful agricultural practices
Soil is not merely handfuls of dirt - soil is a complex ecosystem, with its own pattern of energy
flow
Much of the energy flow in the soil passes through animals called detritivores
Detritivores feed on detritus - dead and decaying organic matter
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Soil ecology depends on detritivores
Nitrogen, phosphorous, other critical nutrients cycle naturally through the environment, but
cycles are very long
Detritivores quickly break down organic matter and recycle nutrients back to the soil
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Formation of healthy topsoil is complex
Interaction of both biotic and abiotic factors (soil chemistry ex.)
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Decay of organisms provides constant influx of new organic matter to the soil
Adds to the parent material, minerals that are naturally weathered from rocks
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Detritus nibbled by countless bacteria, fungi, protists
These in turn help feed the larger detritivores - insects, pill bugs, earthworms, millipedes…
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Consider the lowly earthworm…
Darwin was the first to appreciate their value
Darwin estimated that 22-40 metric tons per hectare per year passed through the guts of
earthworms
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Detritivores like earthworms serve two important functions:
> Break up detritus into smaller particles for bacteria and fungi to decompose
> Constantly aerate the soil as they tunnel through it
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Soil consists of many layers, called soil horizons – a specific layer in the soil which differs from
the layers above it and below it
Soil profile refers to all the layers visible when you take a section through the soil (the sequence
of layers)
I learned about soil horizons the hard way - digging a hole to China in the backyard!
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Dug down through several feet, lighter color than the top layer
Another two feet, came to a third layer, tan color
What lay beneath that I’ll never know…
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We were so close…
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Breakdown of organic matter by detritivores in the soil is not complete
Leftover small organic particles in the soil are called humus
Humus forms the very top layer of soil, gives soils a very loose soil structure
Soil structure refers to the physical characteristics of the soil (ex. particle size, pore spaces, how
the grains clump together etc.)
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The humus makes up the “O horizon”
Just below the humus lies the topsoil, the “A horizon”
“A horizon” contains some humus carried down by tunneling detritivores, minerals leached
from above by rainwater
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Below the A horizon is the E horizon…
E horizon is lighter, very little humus - zone of leaching
Mixture of minerals leached from above by rain water, together with minerals already in the soil
that resist leaching
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Under the E horizon lies the B horizon or subsoil
Leached materials from above accumulate in the B horizon, give it a distinct color - red or
yellow
Color is from oxides of minerals like iron or aluminum
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Lowest layer is the C horizon
Basic mineral soil, no organic matter - called parent material
Parent material is the starting point in the formation of new soil
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Parent material can form in many ways
> Volcanic deposition
> Glacial deposition
> Natural weathering of bedrock
Below the C horizon is CHINA !!
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Soil, plants, and animals are interdependent components of a dynamic system
Plant cover helps prevent erosion - roots hold the soil in place
Vegetation also reduces water loss from the system by evaporation
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Loss of topsoil is a major global change, one of our most serious environmental problems
Good topsoil takes hundreds of years to form, but can be lost almost overnight
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Reasons for soil degradation are complex and inter-related
Primary causes of soil degradation are
> Deforestation
> Overgrazing
> Overcultivation
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Biggest problem in America is overcultivation
Large-scale intensive agriculture is very hard on soils
Modern “agribusiness” is replacing the traditional small family farm
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Deforestation, overgrazing, and overcultivation all expose the soil to erosion
Removes the protective layer of plants that both nurture the soil and hold it in place
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Traditional plowing destroys the soil by churning it up, exposing the rich topsoil to direct
erosion by wind and water
New practice is “no-till” agriculture
“No till” relies on special plowing disks - one set opens the soil, seed is dropped in, last set zips
the soil back up!
Deforestation, overgrazing, and overcultivation lead to desertification
Desertification means that healthy soils are turned into sterile soils, with no organic content, no
soil structure
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Loss of topsoil is a double whammy…
Soil loses most of its organic content and nutrients when humus is removed
Underlying soil can no longer hold water
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Topsoil acts like a sponge, holds water long enough for plants to absorb it through their roots
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Without topsoil, water passes quickly through soil, beyond the reach of plants
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Topsoil should be treated as a non-renewable resource - only renewable on a time span of
generations or centuries
Once topsoil is gone, for all practical purposes it is gone forever…
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Additional cause of soil degradation is salinization
Increase in soil salinity, caused by irrigated agriculture
During the last 20 years, irrigated land has increased by over 35%
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There is no such thing as fresh water
Even the freshest water has some salt in it
When fresh water evaporates, leaves minute amount of salt behind
Salt gradually builds up in irrigated fields, eventually can no longer grow crops
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Despite efforts by a host of local, state and federal agencies, problem continues to grow
Due to bad agricultural practices, US has already lost about one third of our topsoil
Roughly 200 million acres of fertile land lost to erosion!
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If we are not careful we may lose it all
We have already stood on the edge of this particular cliff….
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Up until World War I, vast areas of the Midwest were devoted to cattle ranches
Healthy rangeland stretched across the Texas Panhandle, up into Kansas, Colorado, New
Mexico and Oklahoma - 150,000 mi2
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In the 1930’s most of the American Midwest dried up and blew away
They called it the Dust Bowl…
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Higher grain prices during World War I convinced farmers that wheat was more lucrative than
cattle
Plowed up healthy rangeland to plant wheat
Just in time for a prolonged drought
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Exposed topsoil all blew away…
They called it the Black Blizzard…
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Sand dunes piled up by endless dry winds reached 30 feet or more
Covered farms, houses, even entire villages, buried the land under a thick blanket of sand
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Dust storms sent dust five miles into the air
Dust storms reached all the way to the Atlantic Coast
Thousands of people were driven from their homes - one of the biggest refugee crises of modern
times
Population in affected counties dropped 60% or more
Massive federal effort undertaken to restore the land
> Rows of trees planted as windbreaks to slow down erosion
> Contour plowing helped hold remaining topsoil in place
Massive federal effort undertaken to restore the land
> Vast areas reseeded to native prairie grasses
> Farmers told to leave half their land fallow each year, until the soil was restored
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By 1941, balance was restored, but remains a fragile system
Bad farming practices could easily lead to another Dust Bowl
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Dustbowl was a watershed in American culture
Made Route 66 into a legend, direct route through the Midwest to the promised land - California
California was ill prepared to cope with the flood of refugees escaping the Dust Bowl
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Plight of the Dust Bowl victims was immortalized in song and story
Woody Guthrie, America’s greatest folk musician, wrote many songs about the dustbowl
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John Steinbeck immortalized the events in his American classic The Grapes of Wrath
Later made into a movie by John Ford, starring Henry Fonda - one of the greatest films of all
time…