Unit 6: Soil and Food Resources
Section 2: Humans and Agriculture
Agriculture Arose about 10,000 years ago
• Earliest plant and animal domestication occurred in “Fertile
Crescent” – Present day Middle East
• Led to Stationary lifestyle – Establishment of cities and towns
• Traditional Agriculture – used hand tools and muscle power
• Also called sustenance or subsistence farming – farmed enough food to
survive
• Many crops grown together (polyculture)
• Led to Industrial Agriculture – uses large equipment driven by
fossil fuels to grow crops for profit
• Large fields of single crops – monoculture
• Use pesticides, fertilizer, and irrigation
Green Revolution
• Farming technology was
introduced to developing
countries
• Decreased starvation
• BUT, led to massive soil
degradation
Soil Degradation
• When mismanaged, soil use can lead to land degradation
• Caused by…
• Deforestation
• Cropland
• Overgrazing
• Degraded soil…
• Doesn’t hold water as well
• Is unable to keep high nutrient contents
• Often erodes faster than other soil
• This can lead to…
•
•
•
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Reduced biodiversity
Increased desertification
Soil salinization
Reduced efficiency of food production
Soil Erosion
• Erosion is driven by wind,
water, and chemical
substances
• Flowing water deposits eroded
soil when it slows
• Erosion targets topsoil (most
nutrient rich layer)
• Cover crops can protect soil
between plantings
The Dust Bowl of the 1930s
• Following introduction of industrial
agriculture, drought occurred and soil
eroded in massive dust clouds.
• Prairie grasses no longer present to
keep soil in place due to overgrazing
and farming.
• Lead to economic disaster as farmers
abandoned their farms.
The Soil Conservation Service pioneered
measures to address soil degradation
• In response to the Dust Bowl, the U.S. Congress created the Soil
Conservation Service
• Started in 1935, the Service works with farmers to develop conservation
plans for individual farms
• In 1994, the Service was renamed Natural Resources Conservation
Service (NRCS)
• Responsibilities expanded to also include water quality protection and pollution
control
• 1985 Food Security Act (Farm Act): farmers receive a subsidy for taking
highly erodible land out of production and replanting it with soil saving
plants for 10-15 years.
• The 2012 farm bill was never called for a vote in the House of
Representatives
• The future of conservation programs is unclear
Farmers protect soil in many ways
• Crop rotation = growing different crops from one year to the next
• Prevents erosion, reduces pests
• No-till farmers may alternate wheat or corn with nitrogen-fixing soybeans (to
increase natural nitrogen in soil)
• Contour farming = plowing furrows sideways across a hillside
• Sides of furrows trap water and prevent erosion
• Terracing = cutting level platforms into steep hillsides
• The steps of this “staircase” hold water
Farmers protect soil in many ways
• Intercropping = planting different crops in alternating bands or
mixed arrangements
• Increases ground cover, preventing erosion
• Replenishes soil
• Shelterbelts (windbreaks) = rows of trees planted along edges of
fields to slow the wind (therefore erosion)
• Conservation tillage = strategies that reduce the amount of tilling
• Leaves at least 30% of crop residues in the field (organic matter)
• No-till farming disturbs the soil even less
No-till farming has many benefits
Plant cover is the key to erosion control
• Farming methods to reduce erosion have one
goal—keep the plant cover in place
• Move livestock to prevent overgrazing
• Cut select trees in an area rather than clear-cutting
• Plant vegetation along riverbanks and roadsides
• China’s huge tree-planting program slows erosion
• However, the monocultures are not ecologically functioning forests
Overgrazing can degrade soil
• Grazing animals on rangeland can be
sustainable if the total number of
grazing animals is kept below the
rangeland’s carrying capacity
• Grazing above carrying capacity leads
to degradation
• Impedes plant regrowth
• Soil is exposed, allowing erosion, less
regrowth, and (positive feedback) more
erosion
• Trampling compacts the soil,
preventing water infiltration
Irrigation boosts productivity but can damage
soils
• Irrigation = artificially
providing water to support
agriculture
• Unproductive regions
become productive
farmland
• 70% of all fresh water
used by humans goes to
irrigation
• Irrigation Types:
• Center-pivot – most
effective
• Gravity flow – uses gravity
to release water down
sloped ditches
• Drip irrigation – uses the
least amount of water;
hoses drip water directly
on plants (plants use 90%)
Salinization and
Waterlogging
• Repeated irrigation can reduce
crop yields by causing salt
buildup in the soil and
waterlogging of crop plants.
• Easier to prevent these than to
fix them
• Irrigate efficiently
• Filter water
Fertilizers boost crop yields but can be
overapplied
•
Organic Fertilizers – animal manure, crop
residues, bone meal, and compost (best choices)
•
Inorganic Fertilizers – man-made from chemical
compounds
Benefits – exact compositions are known; they
are soluble & thus immediately available to the
plant
Costs – quickly leach away; this pollutes the
water; doesn’t help the water holding capacity
of the soil like organic fertilizers do.
Some policies worsen land degradation
• Governments spend billions on farm subsides that may be
unsustainable
• Some subsidies support growing water-intensive crops in desert regions
• Some encourage use of easily degraded land
• Grazing on federally owned land costs $1.35 per animal unit per
month
• Low cost encourages overgrazing
• Ranchers are now teaming up with environmental activists to prevent
loss of rangeland from development
Wetlands have been drained for farming
• Wetlands = swamps, marshes, bogs, river
floodplains
• Over 50% have been drained for agriculture in the U.S.
• Government policy encouraged draining
• Swamp Lands Acts (1849, 1850, 1860) = drained and
converted wetlands to control floods and malaria
• Wetlands Reserve Program = landowners are paid
to protect, restore, and enhance wetlands