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TO SCREW UP (AND FIX)
THE SOIL

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http://www.nbclearn.com/portal/site/learn/changing-planet
Withering Crops
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Bad results: Mineralization
• If detritus is lost, soil organisms starve
• Soil will no longer be kept loose and nutrient-rich
• Humus decomposes, breaking down the clumpy aggregate
structure of glued soil particles
• Water- and nutrient-holding capacities, infiltration, and
aeration decline
• Mineralization: loss of humus and collapse of topsoil
• All that remains are the minerals (sand, silt, clay)
• Topsoil results from balancing detritus and humus
additions and breakdown

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The importance of humus to topsoil
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Bad results: Soil degradation
• Turnover of plant material produces detritus
• When humans cut forests, graze livestock, or plant crops, the soil is
managed or mismanaged.
• Soil degradation: occurs when key soil attributes required
for plant growth or other ecosystem services deteriorate.
• Some reports on soil degradation are incorrect or outdated
• 75% of the land in Burkina Faso was said to be degraded
• But agricultural yields have increased due to soil and water
conservation

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Bad Soil: Erosion
1. Erosion: the process of soil and humus particles being
picked up and carried away by water and wind
1.
Occurs any time soil is bared and exposed
2. Soil removal may be slow and gradual (e.g., by wind) or
dramatic (e.g., gullies formed by a single storm)
• Vegetative cover prevents erosion from water
• Reducing the energy of raindrops
• Allowing slow infiltration
• Grass is excellent for erosion control
• Vegetation also slows wind velocity

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Splash, sheet, and gully erosion
• Splash erosion: begins the process of erosion
• Raindrops break up the clumpy structure of topsoil
• Dislodged particles wash between other aggregates
• Decreases infiltration and aeration
• Sheet erosion: the result of decreased infiltration
• More water runs off, carrying away fine particles
• Gully erosion: water converges into rivulets and streams
• Water’s greater volume, velocity, energy remove soil
• Once started, erosion can turn into a vicious cycle
• Less vegetation exposes soil to more erosion
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Causes of erosion: overcultivation
• Plowing to grow crops exposes soil to wind and water
erosion
• Soil remains bare before planting and after harvest
• Plowing causes splash erosion
• Destroying soil’s aggregate structure
• Decreasing aeration and infiltration
• Tractors compact soil
• Reducing aeration and infiltration
• Increasing evaporative water loss and humus oxidation
• Rotating cash crops with hay and clover is sustainable

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Erosion
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The other end of the erosion problem
• Water that does not infiltrate enters streams and rivers
• Causing flooding
• Sediment: eroded soil carried into streams and rivers
• Clogs channels, intensifies floods, fills reservoirs
• Kills fish and coral reefs
• Damages streams, rivers, bays, estuaries
• Excess sediments and nutrients from erosion are the greatest
pollution problem in many areas
• Groundwater is depleted
• Rainfall runs off and does not refill soil or ground water

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Desert pavement
• Another devastating feature of wind and water erosion:
differential removal of soil particles
• Lighter humus and clay are the first to be carried away
• Rocks, stones, coarse sand remain
• The remaining soil becomes coarser
• Deserts are sandy because wind removes fine material
• Desert pavement: occurs in some deserts
• Removal of fine material leaves a thin surface layer of
stones and gravel
• This protective layer is easily damaged (e.g., by
vehicles)

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Formation of desert pavement
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Drylands and desertification
• Clay and humus are the most important parts of soil
• For nutrient- and water-holding capacity
• Their removal results in nutrients being removed
• Regions with sparse rainfall or long dry seasons support
grasses, scrub trees, and crops only if soils have good waterand nutrient-holding capacity
• Erosion causes these areas to become deserts
• Desertification: a permanent reduction in the productivity of
arid, semiarid, and seasonally dry areas (drylands)
• Does not mean advancing deserts

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Desertification
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Drylands
• Desertification is a process of land degradation
• Due to droughts, overgrazing, erosion, deforestation,
overcultivation
• It is extremely serious because it is permanent
• Dryland ecosystems cover 41% of Earth’s surface
• They are defined by precipitation, not temperature
• They receive minimal rainfall
• Droughts are common—they can last for years
• Rainfall causes vegetation to return so drylands are
not desertified

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Preventing soil degradation
• Several farming strategies to prevent soil
degradation:
• • Crop rotation
• • Contour farming
• • Intercropping
• • Terracing
• • Shelterbelts
• • Conservation tillage

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Reducing soil erosion
• Contour strip cropping: plowing and cultivating at right
angles to contour slopes
• Shelterbelts: protective belts of trees and shrubs planted
along plowed fields
• The U.S. Natural Resource Conservation Service
(NRCS)
• Established in response to the Dust Bowl
• Regional offices provide information to farmers and
others regarding soil and water conservation practices
• U.S. soil erosion has decreased through conservation
• Windbreaks, grassed waterways, vegetation to filter runoff

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Recent soil conservation laws
• The U.S. has continued to pass soil
conservation legislation in recent years:
• • Food Security Act of 1985
• • Conservation Reserve Program, 1985
• • Freedom to Farm Act, 1996
• • Low-Input Sustainable Agriculture Program, 1998
• Internationally, there is the UN’s “FAR”
program in Asia.

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Crop rotation
• Alternating the crop planted (e.g., between corn and
soybeans) can restore nutrients to soil and fight pests and
disease.
Figure 8.16a

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Contour farming
• Planting along contour lines of slopes helps reduce erosion on
hillsides.
Figure 8.16b

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Intercropping
• Mixing crops such as in strip cropping can provide nutrients
and reduce erosion.
Figure 8.16c

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Terracing
• Cutting stairsteps or terraces is the only way to farm extremely
steep hillsides without causing massive erosion. It is laborintensive to create, but has been a mainstay for centuries in
the Himalayas and the Andes.
Figure 8.16d

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Shelterbelts
• Rows of fast-growing trees around crop plantings provide
windbreaks, reducing erosion by wind.
Figure 8.16e

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Tillage
• Plowing has multiple desirable effects:
–Weeds and weed seeds are buried / destroyed.
–Crop residue is turned under - decays faster
and helps build soil structure.
–Leached nutrients brought closer to surface.
–Cooler, darker soil is brought to top and
warmed.

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Problem:
• Each trip over the field is an added
expense to the farmer, and at the same
time increases the amount of time the
soil is open to erosion via wind or water.

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No-till planting
• No-till agriculture: a technique allowing continuous
cropping while minimizing erosion
• Routinely practiced in the U.S.
• After spraying a field with herbicide to kill weeds
• A planting apparatus cuts a furrow through the mulch
• Drops seeds and fertilizer
• Closes the furrow
• The waste from the previous crop becomes detritus
• So the soil is never exposed
• Low-till farming uses one pass (not 6–12) over a field
• http://www.youtube.com/watch?v=I_7d0h2bSoY&feature
=related

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Apparatus for no-till planting
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Conservation tillage
• No-till and reduced-tillage farming leaves old crop residue on
the ground instead of plowing it into soil. This covers the soil,
keeping it in place.
• Here, corn grows up out of a “cover crop.”
Figure 8.16f

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Conservation tillage
• Conservation tillage is not a panacea for all
crops everywhere.
• It often requires more chemical herbicides (because
weeds are not plowed under).
• It often requires more fertilizer (because other plants
compete with crops for nutrients).
• But legume cover crops can keep weeds at
bay while nourishing soil, and green manures
can be used as organic fertilizers.

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Trade-Offs
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Conservation Tillage
Advantages
Reduces erosion
Saves fuel
Disadvantages
Can increase
herbicide use for
some crops
Cuts costs
Holds more soil
water
Reduces soil
compaction
Allows several crops
per season
Does not reduce
crop yields
Reduces CO2
release from soil
Leaves stalks that
can harbor crop
pests and fungal
diseases and
increase pesticide
use
Requires
investment
in expensive
equipment
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Waterlogging and salinization
• Overirrigation can raise the water table high
enough to suffocate plant roots with
waterlogging.
• Salinization (buildup of salts in surface soil
layers) is a more widespread problem.
• Evaporation in arid areas draws water up
through the soil, bringing salts with it. Irrigation
causes repeated evaporation, bringing more

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Fertilizers
• Supply nutrients to crops
• Inorganic fertilizers =
mined or synthetically
manufactured mineral
supplements
• Organic fertilizers =
animal manure, crop
residues, compost, etc.
Figure 8.18

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Global fertilizer usages
• Fertilizer use has risen dramatically in the past 50 years.
Figure 8.19b

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Inorganic fertilizer
• Can provide optimal amounts of nutrients efficiently
• But it lacks organic matter to support organisms and
build soil structure
• It can keep nutrient content high under intensive
cultivation (two or more cash crops/year)
• But mineralization and soil degradation proceed
• Additional fertilizer leaches into waterways
• Chemical fertilizers have a valuable place in agriculture
• Organic fertilizers may not have enough nutrients
• Growers must use each fertilizer as necessary

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Soil conservation & Sustainable Ag
• Sustainability means doing all we can to reduce erosion
• Soil conservation must be practiced at two levels
• Individual landholders can best preserve soil through traditional knowledge and
practices
• Public policies can lead to conservation or disaster
• Goals:
• Maintain productive topsoil
• Keep food safe and wholesome
• Reduce chemical fertilizers and pesticides
• Keep farms economically viable
• Sustainable options mimic past practices
• Contouring, crop rotation, terracing, little or no chemicals
• The U.S. Sustainable Agriculture Research and Education program
(SARE) (1988)
• Provides $5–12 million/year for building and disseminating knowledge about
sustainable agriculture