3. Soil
Soil is a mixture of both mineral and organic matter
Maintained by microscopic lifeforms
Soil Profiles Erosion: eluviation
Pedon, Polydedon Transport: leaching
Deposition: illuviation
Horizons
O: Organic matter: litter and humus: soil moisture retention,
mobilization of nutrients, provision of nutrients
A: Humus and clays (water retention of humus limits eluviation)
E: Sand and silt (clays and other oxides eluviated down to B)
B: Clays and oxides illuviated from E deposited here
C: Weathered bedrock, exclusive of the bedrock, not affected by
biota
R: Bedrock
4.
5.
6.
7. Soil Properties
Color
Soils rich in organic matter are dark or black
Iron and Aluminum oxides give soil a reddish color
Texture
Gravel
Sand 2mm
Silt 50 μm
Clay 2 μm
Structure
Consistency (wet, moist and dry)
Porosity:
affects inflitration
improved by biotic action
harmed by compaction (increases runoff)
Soil Moisture
10. Soil Moisture
Soil moisture related to
Organic matter content
Texture (esp. to fine particles)
Loss of organic matter can lead to
Loss of soil moisture retention
Loss of A horizon to E horizon
Eluviation of fine particles from E
Illuviation of fine particles to B
Further loss of soil moisture retention
13. Soil Chemistry
Soil Atmosphere (N2, CO2)
Nutrients take the form of ions
Cations: Na+, K+, Mg2+, Ca2+, NH4+
Anions: NO3-
Colloids acts as nutrient storage
soil particles (especially clay)
organic matter
Soil moisture acts as a transport agent by
solution
Root hairs have a slight negative character
and attract positive ions
Cation Exchange Capacity (CEC) is a measure
of how easily nutrients pass between colloids
14.
15. Acidity and pH
Acidity refers to the concentration of
Hydrogen ions (H+)
H+ ions occupy colloidal spaces, crowding
out other nutrients and lowering fertility
pH = -log[H+]
pH < 7, acidic
pH = 7, neutral, water
pH > 7, basic (alkaline)
16. Soil Biology
Critical for cycling nutrients
Performed largely by bacteria
Fungae, nematodes, springtails, earthworms, etc. (form
food chains)
Nitrogen cycle, Carbon cycle
Sources of Nitrogen
Atmosphere (N2)
Litter,
Through decomposition, also biological, provides NH4+ from organic
compounds
Inorganic fertilizer (NPK)
Utilizability of Nitrogen
Nitrogen gas, inert, unusable by plants
Nitrite, toxic to plants
Nitrate and Ammonium utilizable by plants
17.
18.
19.
20. Nitrogen Cycle
Plants must rely on soil microbiota to process
nitrogen into a form it can use
Nitrogen fixation: bacteria change atmospheric N
into various nitrogen compounds
legumes
Nitrification: Ammonium changed into nitrite by
“Type I” organisms (nitrosomona); nitrite turned
into nitrate by “Type II” organisms (nitrobacter)
Denitrification: nitrogen compounds turned back
into nitrogen gas by bacteria (20 – 40% of
fertilizer is wasted this way)
Plants return nitrogen to the cycle through
leaf litter and decomposition
A diverse soil biota is necessary for proper
functioning
21. Carbon Cycle
Plants take in carbon dioxide, add it to
their tissues
Other nutrients, including N, are taken in
Tissues becomes deposited in soil
through leaf litter
Builds up soil organic matter
Acts as a form of carbon storage
Contributes N to the soil
22. Soil Formation
Dynamic factors
Climatic – weathering of parent material; texture;
pedogenic regimes
Biologic – organic matter additions
Passive factors
parent material: affects nutrient availability
topography and relief: accumulation and depth
time
Lots of it (hundreds to thousands of years), under stable
conditions
Paleosols
Human factor
Rapid loss of soil
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24.
25. Classification of Soil Types
Pedogenic regimes
An earlier system of classification based on
climatic factors
Replaced earlier texture classification system with
one that was more focused on development
USDA Soil Taxonomy
Replaced pedogenic regimes as a system, but
includes them as a diagnostic feature
Uses diagnostic horizons to classify soils
epipedon: Upper most surface layers, exclusive of C and
R
diagnostic subsurface horizons: A and B horizons
Twelve orders and multiple suborders
26. Pedogenic Regimes
Relates soil
formation to
climate
Soil types are
closely related
to these
regimes
climate is not
the sole factor
in determining
soil type,
however
28. Oxisols
Tropical soils
Laterization
Iron and aluminum oxides (oxidation; reddish color)
Heavily leached A horizon (leaching occurs throughout the
year)
Loss of nutrients and colloidal material
poor fertility and low CEC
Exposure to air and water creates hardpan (plinthite,
laterite)
Rainforest soil
Can be farmed sustainably
slash and burn, with long fallow (low demand)
Vulnerable to erosion if activities not done right
(1000s of tons per sq. km per year)
29.
30. Aridisols
Desert soils (arid climates)
shallow horizons
poor colloidal structure (fewer fine
particles)
lacking in organic matter
poor water retention
contributes to flash flooding
leaching highly seasonal
High evapotranspiration
Salinization
Evaporation deposits salts near the soil
surface
31. Mollisols
Grassland soils
world’s richest soil type
deep and rich organic layer (humus) due to
deep grass roots
Calcification
More CO2 in the soil atmosphere causes
leaching of calcium and magnesium
carbonates
illuviation into B and C horizons
caliche, kunkur
32.
33.
34. Alfisols
Name taken from Aluminum and Iron
content (Al, Fe)
Moderately weathered forest soil
Considered to be a moist version of
mollisols
Note similarities to Oxisols
35. Ultisols
Highly weathered forest soils
Heavily leached A horizon with residual Fe
and Al oxides
Intermediate between Oxisols and Alfisols
36. Spodosols
Northern coniferous forest soils
Humid continental mild summer
cool, moist climates
Humus and fine particles eluviated from A
horizon
Uppermost horizon lacks clay and humus,
tends to have a sandy texture and has a light
color
Podzolization
Low pH (acidic)
leaching of fine particles and nutrients
37.
38.
39. Entisols
recENT soils
Not climate dependent
Insufficient time passage to develop
horizons
Insufficient weathering of parent material
lacking fine particles and nutrients
Occurence: areas of recent deposition
slopes, floodplains, tidal mudflats, dune
and desert sands
40. Inceptisols
Soils in earliest stages of development
Eluviation and illuviation are evident, but
distinct horizons have not yet formed
Low fertility
41. Gelisols
Cold and frozen soils
the presence of permafrost is a defining characteristic
Form very slowly due to low temperatures
slowing chemical and physical weathering
disturbance to these soils has long lasting effects
cryoturbation
freeze-thaw cycle of frost tends to churn the soil
42. Andisols
Named after the Andes
Volcanic parent material
pyroclastic material
Rapidly weather into colloidal material
high fertility, high CEC
Distributed around subduction zones
Pacific Ring of Fire especially
43. Vertisols
Expandable clay soils
At least 30% expandable clay (shrink and swell)
Climatic variables
Subhumid to semi-arid moisture, moderate to high
temperature
Regions with highly variable seasonal moisture
balances
Distinct dry season following wet season
Form under tropical and subtropical
grasslands and savanna
Disastrous for construction (don’t buy a house
built on vertisols)
47. Human-Environment
Interaction Issues with Soils
Irrigation and Salinization
More common in arid environments
The accumulation and evaporation of irrigated
water causes salts to build up in the soils, and
ultimately ruins its fertility
Drainage tiles
Groundwater issues, Colorado River Basin
issues
48. Soil degradation
results from various practices that destroy agriculturally
viable land
agricultural practices that leave soil vulnerable to erosion
overgrazing, vegetation removal, over exploitation
20% drop in yield over next 20 years at moderate erosion rates
Urbanization
3 – 5 million acres of US farmland are lost each year
38% decrease in world’s farmable land since 1950
1380 million acres
12 – 15 million acres per year
Agriculturally viable soil takes hundreds to thousands of
years to form
Soil as a non-renewable resource?
Agriculture as soil mining?
