1. Agricultural Ecology

2. 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

3. 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 mm

5. 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).

6. 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.

7. 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.

8. Cation Exchange Capacity
Plant root hair
Silt H+
Sand
Sand - -
Mg++
Na+ - -
Ca++ - - K+ - -
Clay
Fe+3

9. More nutrient and water holding capacity
Silt
Sand
Clay
Organic matter

10. 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?

11. Other ways water is held in soil
Sand
Clay
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.

12. 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

13. 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!

14. 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…

15. 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

16. 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.

17. 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.

18. Agricultural Food Webs

19. Agricultural Food Webs
Used by Humans
Consumed by predator
Harvested
Waste?
Consumed by “pest”
Crop Plant

20. How do we maximize #1 and minimize #2?
Through the use of pesticides
Through the use of plowing
Through the use of IPM?

21. 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.

22. 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? %?
Amount 50 25
12.5
6.25
Time

23. Biological Amplification27 3 1 9 1 3 3 1

24. 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?

25. Key terms to know & understand
Contour plowing
Strip cropping
Mulching
Monoculture
Target organism