1. Soil

2. Soil Composition

3. Soil
Uppermost layer of earth’s crust that supports plants, animals and microbes
Soil Forming Factors
Parent Material
Time
Climate
Vegetation & Organisms
Topography

4. 1. Parent Material
Refers to the rock and minerals from which the soil derives.
The nature of the parent rock has a direct effect on the soil texture (think igneous vs sedimentary rocks)
Parent material may be native or transported to area by wind , water or glacier.

5. 2. Time After enough time, soil reaches maturity.
Depends on several factors
On average it takes 500 years to form 1 inch of soil.

6. 3. Climate
Two most important factors that determine climate are Temperature and Moisture, they affect
Weathering processes
Microenvironmental conditions for soil organisms
Plant growth
Decomposition rates
Soil pH
Chemical reactions in the soil

7. Formation of soil occurs faster in a climate with:
higher temperatures
(increase the speed of the reaction)
more rainfall
(provides water for chemical reactions)

8. Weathering
Physical
includes temperature changes (freezing and thawing), crystal growth, pressure, plant roots, burrowing animals
causes disintegration of parent material and facilitates chemical weathering
Chemical
always in water
includes hydration, hydrolysis, oxidation.
examples :
oxidation of Fe to form limonite

9. 4. Vegetation & Organisms
A single teaspoonful of soil is home to more than a billion organisms.
Bacteria and protozoans –
around 1 billion
Nematode worms—300
Springtails—15
Mites—20
Pseudoscorpions—1

10. Larger Soil Organisms
Worms – digest materials that make up humus and produce castings (amazing worm poo) and aerated soils with tunnels
Ants – aerate soils with tunnels and chambers, forage for food on surface and leave underground to decompose, bury seeds

11. Ecosystem Services Soil organisms provide ecosystem services
Important environmental benefits that ecosystems provide
Examples
Decaying and cycling organic material
Preventing soil erosion
Breaking down toxic materials
Cleansing water
Soil aeration (especially done by earthworms)

12. Symbiotic Relationships
Mycorrhizae – fungus on the roots of some plants
Rhizobium – Soil bacteria that fix Nitrogen
(remember them?... )
Found on Legumes (peas, clover, peanuts etc…) NOT ON potatoes!
Helps plants absorb essential nutrient minerals from the soil
Mycelium – threadlike body of the fungus (extends beyond the roots)

13. 5. Topography
Physical characteristics of location where soil is formed.
Drainage
Slope direction
Elevation
Wind exposure

14. Soil Composition Mineral Particles (45%) Weathered rock
Provides essential nutrients for plants
Organic Material (5%)
Litter, animal waste, dead remains of plants and animals, humus (picture)
Water (25%)
Air (25%)

15. Texture of Soil Sand Silt Clay
Relative proportion of sand, silt and clay determines TEXTURE
Soil textures are produced by the combination of
Sand
Silt
Clay

16. Sand - Basketball
Biggest
Can feel individual particles
Silt - softball
Clay- tip of your pen
Smallest
Can NOT feel individual particles
(think baby powder)

18. Spaces in the Soil
40-60% of volume of soil is normally pore space (space in between the grains)
Spaces allow water and air to travel through soil.
Plant roots growing in the soil need air

19. Permeability
The rate at which water (and air) moves from upper to lower soil layers.
Amount of room between grains (pore spaces) determines this
Larger pore spaces drain water quickly
Smaller pore spaces tend to hold water and drain much slower

20. Sand Silt Clay “Large” “Medium” “Small” Water Water High permeability
Figure 3.25
Natural capital: the size, shape, and degree of clumping of soil particles determine the number and volume of spaces for air and water within a soil. Soils with more pore spaces (left) contain more air and are more permeable to water than soils with fewer pores (right).
High permeability
Low permeability

21. Loam Soils made of a mixture of: Sand Silt Clay ~40% sand ~40% silt

22. Soil Textural Triangle
What type of soil am I?
55% sand
5% silt
40% clay

23. Soil Textural Triangle
What type of soil am I?
20% sand
75% silt
5% clay

24. Soil Properties Soil texture affects soil properties
Coarse textured soil (sandy)
Large Pore Spaces
Will not hold water well- flows through easily
Fine textured soil (high in clay)
Small Pore Spaces
Poor drainage
Low oxygen levels in soil
Due to negatively charged surface, able to hold onto important plant nutrients (K+, Ca2+, NO2-)

26. Soil Nutrients & Layers

27. Soil pH
The pH of most soils ranges from 4.0 to 8.0.
But, the soil of the Pygmy Forest in California is extremely acidic ( ) and in Death Valley, California, it is very basic (10.5).
Proper pH directly affects the availability of plant food nutrients

28. pH Too acidic or basic will not Allow compounds to dissolve
Allow presence of certain ions
If soil is too acidic, add ground limestone
If soil is too basic, add organic material like cow manure

29. Soil Nutrients Macronutrients
Larger in atomic structure. Ex. Nitrogen (N), Phosphorus (P) & Potassium (K).
Micronutrients
These are smaller in atomic structure. Plants need them in small amounts.
Ex. Selenium, Zinc & Iron.

30. Nitrogen Content Stimulates above ground growth
Produces rich green color
Influences quality and protein content of fruit
A plant’s use of other elements is stimulated by presence of N
Replenished naturally by rhizobium on legume roots
Fertilizer from manure or Chemicals

31. Phosphorus for Growth Abundant in Strong root system
Increases seed yield and fruit development
Parts of root involved in water uptake (hairs)
Fertilizer is made from rock phosphate (remember the phosphorus cycle does not cycle through the atmosphere)

32. Potassium Content Potash Important in vigor and vitality of plant
Carries carbohydrates through the plant
Improves color of flowers
Improves quality of fruit
Promotes vigorous root systems
Offsets too much N
Found naturally in feldspar and mica rocks

33. Soil Horizons O - Rich in organic material A - Topsoil
E – Heavily leached (not always present)
B - Lighter colored subsoil
C - Weathered parent material

34. Organic Layer (O-horizon)
The uppermost layer; it is rich in organic material.
Plant litter accumulates in the O-horizon and gradually decays.
In desert soils the O-horizon is completely absent, but in certain organically rich soils it may be the dominant layer.

35. Topsoil (A-horizon)
It is dark and rich in accumulated organic matter and humus.
It has a granular texture and is somewhat nutrient- poor due to the loss of many nutrient minerals to deeper layers and by leaching.

36. Subsoil (B-horizon)
The light-colored subsoil beneath the A-horizon; it is often a zone of illuviation where nutrient minerals have leached out of the topsoil and litter accumulate.
It is typically rich in iron and aluminum compounds and clay.

37. Parent Material (C-horizon)
This contains weathered pieces of rock and borders the un-weathered solid parent material (R Layer) .
Most roots do not go down this deep and it is often saturated with groundwater.

38. Soil Problems

39. Soil Problems Soil Erosion
Wearing away or removal of soil from the land
Caused primarily by water and wind
Why a problem?
Causes a loss in soil fertility as organic material and nutrients are eroded
More fertilizers must be used to replace nutrients lost to erosion
Accelerated by poor soil management practices

40. Soil Erosion Moderate Severe Very severe Figure 13.11
Natural capital degradation: desertification of arid and semiarid lands is caused by a combination of prolonged drought and human activities that expose soil to erosion. QUESTION: What three things would you do to reduce desertification? (Data from UN Environment Programme and Harold E. Drengue)
Moderate
Severe
Very severe

41. Soil is eroding faster than it is forming on more than one-third of the world’s cropland.

42. Salinization & Waterlogging

43. Soil Salinization
Gradual accumulation of salt in the soil, usually due to improper irrigation techniques
Often in arid and semi-arid areas
The little precipitation that falls is quickly evaporated
Leaves behind salts
Salt concentrations get to levels toxic to plants

44. Desertification
Degradation of once-fertile rangeland, agricultural land, or tropical dry forest into nonproductive desert
Typically a human-induced condition
Change in vegetation changes climate, further decreasing precipitation levels (usually in a positive feedback loop scenario)
Asia and Africa – largest areas – many droughts

45. American Dust Bowl
Great Plains have low precipitation and subject to drought
severe drought
No natural vegetation roots to hold soil in place
Native vegetation replaced by annual crops
Winds blew soil as far east as NYC and DC.
Farmers went bankrupt

46. Soil Conservation Policies in US
Soil Conservation Act 1935
Authorized formation of Soil Conservation Service, now called Natural Resource Conservation Service (NRCS)
Assess soil damage and develop policies to improve soil
Food Security Act (Farm Bill) 1985
Required farmers with highly erodible soil had to change their farming practices
Instituted Conservation Reserve Program (CRP)
Pays farmers to stop farming highly erodible land

47. Soil Conservation Practices
Conservation Tillage
Residues from previous year’s crops are left in place to prevent soil erosion
Includes no till agriculture

48. Soil Conservation Practices
Crop Rotation
Planting a series of different crops in the same field over a period of years
Lessens pest and insect disease
Lets nutrients specific for certain plants naturally replenish

49. Soil Conservation Practices
Contour Plowing
Plowing around hill (following natural contour of land) instead of up-down
Decreases soil erosion

50. Soil Conservation Practices
Strip Cropping
Alternating strips of different crops along natural contours

51. Soil Conservation Practices
Terracing
Creating terraces on steep slopes to prevent erosion

52. Soil Conservation Practices
Agroforestry
Trees and crops are planted together to improve soil fertility in degraded soils
Trees grow much longer and provide many soil benefits:
- reduces soil erosion
- regulates water
- provides habitat for natural enemies of crop pests
- leaf litter regenerates soil
- shade
Example: Acacia trees and millet

53. Soil Reclamation Two steps Stabilize land to prevent further erosion
Restoring soil to former fertility
Best way to do this is to plant shelterbelts
Row of trees planted to reduce wind erosion of soil