Transcript Agricultural Ecology - University of Oklahoma

Agricultural Ecology
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
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
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).
Cation Exchange Capacity
• Remember the macro and micronutrients
needed for living organisms?
– List them…a 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.
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.
Cation Exchange Capacity
Plant root hair
Silt
H+
Sand
Sand
Clay
Mg++ -
-
Na+
- Ca++
- K+
Fe+3
More nutrient and water holding
capacity
Silt
Sand
Clay
Organic matter
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?
Other ways water is held in soil
Clay
Sand
Silt
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 doesn’t drain well
and the soil can become ANOXIC.
Soil Horizons
A horizon
B horizon
C horizon
A horizon: where most roots
are, most weathered, lots of
organic matter
B horizon: material leaches
down from A
C horizon: weathered parent
material, i.e. broken down
bedrock
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!
Agricultural ecosystems: nutrient
cycles
• The most limiting nutrient to most types of
agricultural production (crops, grazing,
agro-forestry) is Nitrogen.
• So, here’s the nitrogen cycle…
N2 (in atmosphere) N N
NO3
N2O
Haber-Bosch Process
High temp. &
Nitrogen fixation
Pressure
enzymes
NH4
N-fixing bacteria
NH4
Plants
Nitrifying bacteria
NO3
Consumers
Decomposers
Denitrifying
bacteria
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.
How current agriculture affects N
cycle
Positive 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.
Negative Effects
• 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.
Agricultural Food Webs
Agricultural Food Webs
Food Web # 1
Food Web # 2
Used by Humans
Consumed by predator
Harvested
Crop Plant
Waste? Consumed by “pest”
How do we maximize #1 and
minimize #2?
• Through the use of pesticides
• Through the use of plowing
• Through the use of IPM?
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.
Half-life
100
A
m
o
u
n
t
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%?
50
25
12.5
6.25
0
1
2
Time
3
4
5
Biological Amplification
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?
Key terms to know & understand
•
•
•
•
•
Contour plowing
Strip cropping
Mulching
Monoculture
Target organism