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Agricultural Ecology

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.

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Agricultural Ecology

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  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 0.002 mm

  4. 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).

  5. 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.

  6. 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.

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

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

  9. 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?

  10. 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.

  11. 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

  12. 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!

  13. 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…

  14. 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

  15. 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.

  16. 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. How current agriculture affects N cycle Positive Effects Negative Effects

  17. Agricultural Food Webs

  18. Agricultural Food Webs Food Web # 1 Food Web # 2 Used by Humans Consumed by predator Harvested Waste? Consumed by “pest” Crop Plant

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

  20. 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.

  21. 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 25 12.5 6.25 0 1 2 3 4 5 Time

  22. Biological Amplification 27 3 1 9 1 3 3 1

  23. 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?

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

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