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Overview of Chapter 15. What is soil?Soil PropertiesMajor Soil OrdersSoil ProblemsSoil Conservation Soil Reclamation. Soil. Uppermost layer of earth's crust that supports plants, animals and microbesSoil Forming FactorsParent Material Rock that is slowing broken downTimeClimateOrganismsC
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1. Chapter 15Soil Resources
2. Overview of Chapter 15 What is soil?
Soil Properties
Major Soil Orders
Soil Problems
Soil Conservation
Soil Reclamation
3. Soil Uppermost layer of earth’s crust that supports plants, animals and microbes
Soil Forming Factors
Parent Material
Rock that is slowing broken down
Time
Climate
Organisms
Carbon dioxide from plants helps form carbonic acid to break down rock. Lichens produce acids too.
Topography
Regions surface features
4. Soil Composition Mineral Particles (45%)
Weathered rock
Provides essential nutrients for plants
Varies from soil to soil
Older soil mineral deficient
Younger soil mineral rich
Organic Material (5%)
Litter, animal dung, dead remains of plants and animals, humus (picture)
Humus: black or dark brown material that remains after decomposition
Helps to bind mineral ions
Holds water in soil
Water (25%)
Air (25%)
5. Soil Composition Pore space
50% of soil
Soil air- good for aeration
Soil water- provides water to roots
6. Soil Composition Soil water
Originates as precipitation
Water not bound to soil particles or absorbed by roots leaches through various layers and carries minerals
Deposition of leached material is illuviation
Leached materials can reach groundwater
7. Soil Air
Contains many of the same gases found in atmospheric air
Gases present in different proportions
More carbon dioxide than oxygen
result of cellular respiration of soil organisms
Carbon dioxide forms carbonic acid in water
Important weathering processes Soil Composition
8. Soil Horizons (distinctive horizontal layers) Soil Profile is a vertical section showing horizon layers
O-horizon
Rich in organic material
May be absent in deserts
A-horizon
Topsoil
E-horizon
Not present in all soils
B-horizon
Lighter colored subsoil
nutrients
C-horizon
Weathered parent material
Below extent of most roots
Saturated with groundwater
9. Soil Organisms There are millions of microorganisms in 1 tsp of fertile agricultural soil
Bacteria are most numerous
10. Soil Organisms Soil organisms provide ecosystem services
Important environmental benefits that ecosystems provide
Examples
Decaying and cycling organic material
Breaking down toxic materials
Cleansing water
Soil aeration (especially done by earthworms)
11. Soil Organisms Earthworms
Transport soil components via castings (bits of soil passed through the gut)
Tunnels aerate soil
Ants
Tunnels aerate soil too
Some food collected decomposes increasing organic matter
Carry seeds
Fungi and roots of vascular plants
Mycorrhizae
Enhannce plant growth
12. Nutrient Cycling Nutrients are cycled between plants, organisms and soil
Example
Bacteria and fungi decompose plant and animal wastes
They are transformed into CO2, soil nutrients and water
13. Soil Properties Soil Texture refers to relative proportion of different sized particles
Particles bigger than 2 mm in diameter not considered (why?)
Relative proportion of sand, silt and clay
Sand: 2mm-0.05mm (can see with naked eye)
Silt: 0.05mm-0.002mm (need EM to see)
Clay: >0.002mm
14. Soil Properties Soil texture affects soil properties
Coarse textured soil (sandy)
Will not hold water well- flows through easily
Fine textured soil (high in clay)
Poor drainage
Low oxygen levels in soil
15. Soil Properties Minerals
Often present in charged forms (ions)
Magnesium and potassium are positively charged ions needed for plant growth
Clay has negative charge
Attracts magnesium and potassium
17. Soil Properties
18. Soil Properties Soil Acidity
Measured using pH scale
0-7 = acidic
7 = neutral
7-14 = basic
pH of most soils range from 4-8
Affects solubility of certain plant nutrients
Optimum soil pH is 6-7
This is the pH where plant nutrients are most available to plants
Soil amendments (ex: lime) can be used to achieve this pH
19. Acidic Soil (low pH)
Example: Pygmy Forest, in Mendocino County, CA (pH 2.8-3.9)
Aluminum and manganese more soluble and may be toxic
Potassium leaches more readily
Basic Soil
Calcium phosphate less available (needed for plants)
Decomposition of organic matter generally decreases pH
Soil Properties
20. Major Soil Groups Variations in soil forming factors cause variation in soils around globe
Soil Taxonomy
Separates soils into 12 orders
Subdivided into more than 19,000 soil series that vary by locality
Five common soil orders
Spodosols, alfisols, mollisols, aridosols, oxisols
21. Major Soil Groups Spodosols
Form under coniferous forests (cooler, good precip, good drainage)
O-horizon composed of needles (acidic)
E-horizon is ash-gray under A-horizon
Not good farmland- too acidic
22. Major Soil Groups Alfisols
Temperate deciduous forests
Brown to gray-brown A-horizon
Precipitation high enough to leach most organics and nutrients out of A-, E- and B-horizons
Soil fertility maintained by leaf litter
23. Major Soil Groups Mollisols
Found in temperate, semi-arid grassland
Very fertile soil
Thick, dark brown/ black A-horizon (humus)
Soluble nutrients stay in A-horizon due to low leaching (rain does not wash out)
Grain crops are grown
24. Major Soil Groups Aridosols
Found in arid regions of all continents
Low precipitation preclude leaching and growth of lush vegetation
Development of salty horizon possible
Need water to grow crops
25. Major Soil Groups Oxisols
Found in tropical and subtropical areas with high precipitation
Very little organic material accumulation due to fast decay rate
B-horizon is highly leached and nutrient poor
As plants and animals decay nutrients are taken up quickly
26. Soil Problems Sustainable soil use: wise use of soil resources, without reduction in fertility, so that it is useful for future generations
Soil Erosion
Defined as 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
Sediments can cause problems (How?)
Plant cover can reduce erosion
Accelerated by poor soil management practices
27. Soil Problems Problems in USA and world
Every 5 years Natural Resources Conservation Service measure soil erosion in USA
Soil erosion of croplands has declined (between 1982-1992)
Most serious in S. Iowa, N. Missouri, Texas, E. Tennessee
Water erosion is bad in midwestern grain belt and parts of California
Worldwide annual loss of as much as 75 billion metric tons of topsoil
28. Case in Point: American Dust Bowl Great Plains have low precipitation and subject to drought
1930-1937 severe drought
Area was overused (farming/grazing)
No natural vegetation roots to hold soil in place (removed)
Replaced by annual crops
Winds blew soil as far east as NYC and DC.
Rain caused severe erosion
Farmers went bankrupt
US Soil Conservation Service addressed the issues (how?)
29. Soil Problems Nutrient Mineral Depletion
When organisms die and microorganisms decompose them, the essential nutrient minerals are released into the soil
Farming and clearcutting disrupt cycle
30. Soil Problems Mineral depletion in tropical rainforest soils
Soils are somewhat nutrient-poor because the nutrient minerals are stored primarily in vegetation
When the forest is cleared, its efficient nutrient cycling is disrupted
Can recover but takes a long time
31. Soil Problems Soil Salinization
Def: gradual accumulation of salt in the soil, usually due to improper irrigation techniques
Often in arid and semi-arid areas (natural mineral salt conc. high)
The little precipitation that falls is quickly evaporated
Leaves behind salts
Salt concentrations get to levels toxic to plants
32. Soil Problems Desertification
Def: 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
Big problem in Asia and Africa (soil is overexploited)
33. Soil Conservation Conservation Tillage
Residues from previous year’s crops are left in place to prevent soil erosion
Includes no tillage
Leave soil undisturbed
34. Soil Conservation Contour Plowing
Plowing around hill instead of up-down
Decreases soil erosion
Strip Cropping (type of Contour Plowing)
Alternating strips of different crops along natural contours
Terracing
Creating terraces on steep slopes to prevent erosion
35. Preserving Soil Fertility Organic fertilizers
Animal manure, crop residue, bone meal and compost
Nutrient available to plants only as material decomposes
Slow acting and long lasting
Inorganic fertilizers
Manufactured from chemical compounds
Soluble
Fast acting, short lasting
Environmentally sound to limit use
Mobile- easily leach and pollute groundwater
No water holding capacity (like organic)
36. Compare Organic Fertilizers to Inorganic Fertilizers What are the pros and cons of both?
Which is better for the environment and why?
37. Soil Reclamation It is possible to fix soil erosion
EX: Dust Bowl in USA in 1930s and badly eroded land in N. China
Two steps
Stabilize land to prevent further erosion (planting of native grasses during the dustbowl)
Restoring soil to former fertility (use fertilizers)
Best way to reduce effects of wind on soil is shelterbelts
Row of trees planted to reduce wind erosion of soil
38. 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