Agricultural Ecology Agricultural systems The success of an

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Agricultural Ecology
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Agricultural systems

• The success of an agricultural system depends on
  its soil.
• The success of an agricultural system depends on
  its food web
• The success of an agricultural system depends on
  limiting factors

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Soil Properties

• Soil is a living, viable ecosystem dirt is
  what you get under your fingernails.
• Soil is classified according to its texture the
  amount of sand, silt and clay in it.
• Sand 0.2 mm
• Silt 0.02 mm
• Clay 0.002 mm

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Soil properties

• Soil with more clay in it has greater nutrient
  holding ability than soil with less clay in it.
• This is the process known as Cation Exchange
  Capacity (CEC).

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Cation Exchange Capacity

• Remember the macro and micronutrients needed for
  living organisms?
• List thema quick quiz
• Na, Cl, C, H, O, P, K, I, N, S, Ca, Fe, Mg
• Mo, B, Cl, Mn, Cu, Zn
• Remember that most of these nutrients follow a
  sedimentary nutrient cycle.

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Cation Exchange Capacity

• Nutrients that are in a sedimentary cycle, are
  make accessible to plants through the soil.
• In the soil, most of the nutrients become
  available after they dissolve in water. When a
  chemical dissolves in water, it becomes an ion
  a charged particle.
• Positively charged ions are called cations,
  negatively charged ions are called anions.

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Cation Exchange Capacity
Plant root hair
Silt
H
Sand
Sand
-
-
Mg
Na
-
Ca
-
-
K
-
-
-
Clay
Fe3
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More nutrient and water holding capacity
Silt
Sand
Clay
Organic matter
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What does organic matter do for the soil?

• Source of decomposable nutrients keeps nutrient
  cycles running
• Acts like a sponge helps hold water in the soil
• Also acts to hold anions in the soil
• What are the sources of organic matter in the
  soil?

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Other ways water is held in soil
Sand
Silt
Clay
Sand
Sand
Sand
Note the smaller the particle, the smaller the
pore space between particles and the more water
can be held. If the pore spaces are too small,
then water doesnt drain well and the soil can
become ANOXIC.
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Soil Horizons
A horizon where most roots are, most weathered,
lots of organic matter
A horizon
B horizon material leaches down from A
B horizon
C horizon weathered parent material, i.e. broken
down bedrock
C horizon
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Soil Summary of main pts.

• Soil is a complex, living ecosystem that takes
  millennia to build.
• Although some erosion is natural, accelerated
  erosion, will lose soil and nutrients.
• Soil texture and organic matter content are
  important. Soil pH is, too. More on this later.
• Take a look at Figure on page 308 of your text!

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Agricultural ecosystems nutrient cycles

• The most limiting nutrient to most types of
  agricultural production (crops, grazing,
  agro-forestry) is Nitrogen.
• So, heres the nitrogen cycle

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N2 (in atmosphere) N N
N2O
Haber-Bosch Process
High temp. Pressure
NO3
Nitrogen fixation
enzymes
NH4
N-fixing bacteria
NH4
Nitrifying bacteria
NO3
Denitrifying bacteria
Plants
Consumers
Decomposers
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Limitations on Nitrogen Cycle

• The enzyme for nitrogen fixation is destroyed by
  oxygen
• N-fixing bacteria can not tolerate acid pH.
• All of the bacteria require adequate water
  supplies, but not too much.

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How current agriculture affects N cycle
Positive Effects Negative
Effects

• When manures are used as fertilizer, can increase
  rates of N cycling.
• When mulches are used to manage water, can
  increase rates of N cycling.

• Plowing increases O2 content of soil, decreasing
  N fixation.
• Chemical fertilizers make soil acidic, decreasing
  N fixation.
• Accelerated erosion increases leakiness and
  washes away bacteria.

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Agricultural Food Webs
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Agricultural Food Webs
Food Web 1
Food Web 2
Used by Humans
Consumed by predator
Harvested
Waste?
Consumed by pest
Crop Plant
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How do we maximize 1 and minimize 2?

• Through the use of pesticides
• Through the use of plowing
• Through the use of IPM?

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Types of pesticides

• Chlorinated Hydrocarbons e.g. DDT, long
  half-life
• Organophosphate e.g. Malathion, shorter
  half-life
• Carbamates e.g. Sevin, shorter half-life
• Naturally-occurring pesticides e.g. pyrethrins,
  short half-life, more targeted.

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Half-life
100
So, the half-life on DDT is 30 years! How long
will it take to get down to 3.125 of its
original concentration? 1.625?
Amount
50
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12.5
6.25
0 1 2 3 4
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Time
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Biological Amplification
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3
1
9
1
3
3
1
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Pesticide Resistance Food webs

• If you sprayed the plant shown here, which
  population will recover first? What does that
  mean to future food webs in your field?

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Key terms to know understand

• Contour plowing
• Strip cropping
• Mulching
• Monoculture
• Target organism