1. Soil and the Soil Ecosystem1

2. Why Study Dirt?
90% of the world’s food comes from land based agriculture
22% of the land used for crops, grazing, and forestry has become degraded during the past fifty years (See Table 8-1 p.187). 2

3. http://www. globalchange. umich 3

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6. Areas of the planet threatened by soil erosion6

7. Loss of Farmland to Development is Something We Experience Locally7

8. No healthy topsoil = No us.8

9. Add to this air, water, soil nutrients, salts, and pH.
What is Healthy Soil?
Add to this air, water, soil nutrients, salts, and pH. 9

10. Soil Formation—Mineral Soil
Soil building begins with the physical and chemical fragmentation of the Parent Material.
Mechanical Weathering
Temperature change
Abrasion
Chemical Weathering
Acid deposition
Oxidation
Type of parent material and the climate determine the type of mineral soil. 10

11. Soil Formation—Detritus
Detritus is Decaying plants and animals.
Supplies nutrients for plants
Lowers soil pH
Makes inorganic nutrients available to plants by increasing their solubility
Typical range pH optimum for biological activity in most soil ecosystems.
Creates a loose texture that increases aeration and water infiltration / water holding capacity. 11

12. Soil Formation—Detritus
Optimum organic matter content for most non- wetland soil ecosystems is 3-5%
Detritus that has undergone most of its decomp. Is referred to as humus
humus-detritus that has been almost completely digested by the detritus feeder food web.
humus will 2%-5% of vol./yr. 12

13. Soil Formation—Detritus Food Web
Microflora (the decomposers) help to unlock the nutrients stored in detritus and make them available to the rest of the DFW.
Burrowing animals aerate the soil and mix the organic and inorganic components of the soil.
1 ha of healthy soil may contain 500k earthworms that can process 9 metric tons (20 k lbs) of soil/year.
See handout for other denizens of the soil 13

14. Bacteria are the most important organisms in the detritus food web
Bacteria are the most important organisms in the detritus food web. They are the first heterotrophs in any food chain.
15 tons/ acre 14

15. Rate of Topsoil Formation
Varies depending on the conditions of the area, but primary sources list a range from 1” every 50 to 500 years.
That’s a long time to wait for a meal. 15

16. Soil Characteristics Soils differ greatly from one area to another.
Even on relatively small plots of land, soil types may differ.
Soils are generally studied according to their
Profile
Texture
Structure
Color 16

17. Soil Profile
page 188 17

18. Soil Texture
Page 189 18

19. Soil Texture Sand are the largest particles and they feel "gritty."
Silt are medium sized, and they feel soft, silky or "floury."
Clay are the smallest sized particles, and they feel "sticky" and they are hard to squeeze. 19

20. Loam is common and generally considered ideal for plant growth.
Properties of Soil Particles Worth Noting:
large particles have large spaces separating them
small particles have a larger surface area to volume ratio
Nutrient ions and water molecules cling to surfaces
Workability (Tilth)-the ease with which a soil can be cultivated 20

21. Soil Consistence
Loose
Friable
Firm
Extremely Firm
Source 21

22. Soil Structure
Prismatic: Vertical columns of soil that might be a number of cm long. Usually found in lower horizons.
Blocky: Irregular blocks that are usually cm in diameter.
Granular: Resembles cookie crumbs and is usually less than 0.5 cm in diameter. Commonly found in surface horizons where roots have been growing.
Single Grained: Soil is broken into individual particles that do not stick together. Always accompanies a loose consistence. Commonly found in sandy soils.
Platy: Thin, flat plates of soil that lie horizontally. Usually found in compacted soil. 22

23. Soil Structure
Columnar: Vertical columns of soil that have a salt "cap" at the top. Found in soils of arid climates.
Massive: Soil has no visible structure, is hard to break apart and appears in very large clods. 23

24. Soil Structure
Soil Structure-the “feel” of the soil (clumpy, soft, loose, etc.)
Soil Structure vs. Soil Texture 24

25. Soil Color
Quantified by horizon and inclusions using the Munsell Soil Color Charts. 25

26. Soil Color 26

27. Soil and Plants Plant considerations pertaining to soil:
nutrients
water
air pH
salinity
These factors together affect the fertility “tilth” of the soil. 27

28. Nutrients Nitrogen (N2, NO3-, NO2-, NH3) Phosphate (PO43- )
Potassium (K+) 
Calcium (Ca2+)
These are added to the soil by weathering of parent material, fixation by bacteria from the atmosphere, and through decomposition of detritus. 28

29. Nitrogen Needed for chlorophyll synthesis. Leafy growth above ground.
Hasten maturity of crops.
Enhance fruit development.
Optimum amounts present in soil depend 29

30. Phosphorus Encourages cell division Encourages root development
50 – 75 lbs / acre typically available to plants for growth. 30

31. Potassium Enhances disease resistance
Activates enzymes needed for development
Cuticle development
Regulates turgor pressure
Necessary for protein metabolism 31

32. Other Soil Properties leaching nutrient-holding capacity
ion-exchange capacity
Organic vs. Inorganic Fertilizer 32

33. Water Infiltration Water Content What it is Why it matters
How to measure it
Water Content 33

34. Water Water Holding Capacity Evaporative Water Loss What it is
Why it matters
How to measure it
Evaporative Water Loss 34

35. Air
the ability of the underground organs of plants to exchange gases with the outside environment is essential for their survival.
Roots need air to grow.
Many things affect the aeration of soil.
mineral particles
OMC
Actions of Detritus Feeders and other animals
Actions of plants
Weather 35

36. pH
from water
from detritus
from mineral soil
evaporative qualities 36

37. Salinity
Plants must maintain an osmotic balance with their external environment.
addition of salt to the soil prevents plants from taking up water into their roots 37

38. Soil As an Ecosystem Nebel p.19238

39. Loss of humus in topsoil is called mineralization
There is an 85% to 95% decline in GPP when plants are grown on subsoil compared to when they are grown on topsoil.
Loss of humus in topsoil is called mineralization 39

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