Soils and Environment
WEATHERING
Physical
and
Chemical
Effects
WEATHERING, EROSION,
TRANSPORTATION
• Weathering- Physical disintegration and
chemical decomposition of rocks
• Erosion- Physical removal
• Transportation- Movement of eroded particles
• Chemical vs. Physical Weathering
• Effects of weathering
– Surface alteration of outcrops
– Spheroidal weathering
– Differential weathering
Mechanical Weathering
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Freeze-Thaw Weathering
Salt Weathering
Wetting and Drying
Insolation Weathering
Pressure Release
Stress Corrosion Cracking
CHEMICAL WEATHERING
• Decomposition of rock to form new substances
– Changes in Equilibrium
• Water
– Congruent solution (limestones) vs incongruent
solution (clay minerals)
• Carbon Dioxide- changes in pH change
solubility of minerals
• Role of Oxygen
– Fe in ferromagnesian minerals becomes oxidized
• Hematite and Limonite
Chemical Weathering
• Solution of ions and molecules
• Production of new materials
– clay minerals
– oxides
– hydroxides
• Release of residual unweathered materials
– quartz and gold
Chemical Weathering of
Silicates
Interlayer Cations
hydrolysis
Na, K
Ca, Mg
Solutions of Na+, K+, Ca2+, Mg2+
Hydration, solution
Aluminosilicate sheets (e.g. as part of feldspars)
solution
Al 3+ & Si 4+
Silicic acid (H4SiO4)
Hydrolysis, hydration
Secondary minerals, e.g. clays
Brucite and alumina sheets and incorporated ions, e.g. Fe2+
Oxidation, hydrolysis
Brucite
Alumina
Fe2+
Hydrous oxides, e.g. FeO(OH)
hydration
hydrolysis
chelation
Chelate complexes
Results of Weathering: Clay
Mineral
• Clay minerals give information about
weathering conditions
• Kaolinite: humid, acid conditions, alteration
of K-Feldspar
• Illite: weathering of feldspars and micas
under alkaline conditions where leaching of
mobile K does not occur
• Montmorillonite: weathering of basic igneous
rocks under alkaline conditions with a deficit
of K+ ions
Clays
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Kaolinite
Illite
Montmorillonite
Chlorite
Mixed-layer clays
Soils: Definitions
• Loose unconsolidated material composed of
regolith and partially decayed organic matter,
water and gases
• A soil profile is a vertical face of soil that can be
exposed and includes all the layers (horizons)
from the surface to the parent rock (bedrock)
• The solum is that part of the profile that is
influenced by plant roots
• A pedon is a 3-D representation, the smallest
volume that can be called a soil
Soils and Food Production
• Roots of Agriculture
– Middle East (Iraq) origins
• Roman techniques of soil fertility
• Terrace building in Meso-America and
South East Asia
• Increasing world population reliance on
pesticides and fertilizers
Useful Properties of Soils
• Provides water, nutrients and anchorage for
vegetation
• Provides habitat for decomposers, essential in
carbon cycle and mineral cycling
• Acts as a buffer for temperature changes and
for the flow of water between atmosphere and
groundwater
• Because of its cation exchange properties,
acts as a pH buffer, retains nutrients and
other element loss by leaching and
volatilization
Soils as part of the Ecosystem
• An ecosystem is a community of interacting
organisms and their physical-chemical environment
that function as a self sufficient whole
• Soils are an essential part of the Carbon cycle due to
the effects of microorganisms
Atmospheric CO2
Primary
Producers
Decomposers
Organic Compounds
Soils and Geologic Time
• Soils could only exist after the colonization of land
by organisms, in particular vegetation
• First land plants in the Ordovician (450 my)
• By the Devonian the land had been colonized (370
my)
• By the Carboniferous (300 my) extensive forest
habitat generated soils similar to today
• Properties of soils determined by climate,
organisms, relief, parent material and time, thus
we can extrapolate the conditions that formed
paleosols
Soils and Humans
• Cultivation of soils began about 10,000 BP in
Mesopotamia (Tigris and Euphrates rivers of Iraq)
• The land was a porous friable silt loam that required
irrigation.
• The civilization ended due to wars, floods, infilled
irrigation channels, erosion (gullying), salinization,
loss of food production and famine
• Other centers of agriculture in the fertile Nile valley,
Indus and the river valleys of China
• In Europe soil erosion instigated colonization of other
lands and remains the worst problem facing humans.
• In other areas terracing became the primary farming
technique (Southeast Asia, Peru)
The Green Revolution: An Idea of
the 1960’s
• Increase world population demands the increase of food
production
• Increases in land under cultivation, more intensive agriculture
(mechanization) or both
• The introduction of fertilizers, pesticides, irrigation, varietal
seeds (seed banks), Population growth 2% WHILE food
growth 4%.
• In 1970, Norman Borlaug received the Nobel Peace Price
• However, not a panacea >> potential realized, need for
irrigation,constant inputs of fertilizers, pesticides, and energy
intensive mechanized labor, benefit large land holders,
detrimental to most 3rd world countries
Soils
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Pebbles, gravel and sand particles
Aggregates (mm to cm) of clays
Roots
Partially decayed to totally decayed
vegetation (Humus)
• Organisms (earthworms, arthropods)
• Pore spaces filled with air and gases
• Water
SOIL Texture
• Texture: Relative proportions of sand, silt
and clay
–Dominant size fraction as a descriptor [clay,
sandy clay, silty clay]
–If no dominant fraction then >> Loam {40%
sand; 40% silt; 20% clay}
• Clay-sized particles vs. clay minerals
– type of clay not just % clay
• Texture is an indicator of other properties
(ease of cultivation)
Soil Textural Classification
Soil Structure
• Arrangement of soil particles into
cemented aggregates
• Aggregates are secondary units or
granules composed of many soil particles
held together by organic substances, iron
oxides, carbonates, clays and/or silica
• Natural aggregates are called PEDS
• A CLOD is a coherent mass of soil broken
apart by artificial means
Bulk Density
• Density of soil minerals ranges between 2.6
to 2.7 g/cm3
• When dry, the bulk density is about half the
above value, because voids are filled with air
• Defined as rb = M/V;
• Commonly 1.0-1.6 g/cm3
• Varies over small distances due to weather,
cultivation, compression by animals
• Increases with depth
Core sampler for determination of bulk density. The sampler
yields a core of a fixed volume. The core is dried and weighed
The weight divided into volume gives the bulk density of soil
Porosity
• Calculated from the dry bulk density and
the particle density
– e = 1 - (rb / rs) x 100 = % porosity
• Where rs is usually between 2.6-2.7 g/cm3
• The pore space is occupied by water and
air
• Transmission pores >50mm
• Storage pores 0.5-50 mm
• Residual pores <0.5 mm
Relationship Between Texture
Bulk Density, and Porosity
Textural
Class
Sand
Bulk
Porosity
Density
1.55 g/cm3 42%
Sandy Loam
1.40 g/cm3 48%
Loam
1.20 g/cm3 55%
Silt Loam
1.15 g/cm3 56%
Clay Loam
1.05 g/cm3 59%
SOIL
• Various definitions
– Unconsolidated material above bedrock
– weathered material & organic matter
• supports plant life [air, water, organic matter &
mineral material]
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Loam {40% sand; 40% silt; 20% clay}
Clay-sized particles vs. clay minerals
Soil Horizons
Residual Soil (on bedrock)
Transported Soil (alluvium)
SOIL
• Parent Rock, Time, and Slope
• Organic Activity
• Soils and Climate
– Pedalfer- aluminum and iron rich clays
– Pedocal- calcium rich
– Hardpan- crusts (Fe and caliche)
– Laterites- tropical soils
• Bauxite- Principal ore of Al
• Buried soils
Soil Horizons
• Identified, named, by symbols consisting of
upper and lower case letters
• Each symbol recognizes a formational
property
– O- organic material
– A (A1)-accumulation of decomposed org. matter
– E (A2)- mineral layer, loss of silicate, eluviation (leaching)
horizon
– B (B2)-Illuviated humus, silicate clay or hydrous oxides
– C (C)- mineral horizon above parent
– R (R)- Consolidated Bedrock
Soil Structure: Pan Structures
• Dense layers or pans
• Interference with root and water
penetration
• Produce shallow soils
• Due to compaction, filling of pores with
clays or chemical cements
• Very firm layers are called hardpans
Types of Pans
• Claypan
– Dense soil layers produced by downward migration of clay and
accumulation in subsoil as a B-horizon material
• Duripan
– Layers cemented by precipitates of silica, alone or in combination
with iron oxides or calcium carbonates
• Fragipan
– Fragilis (brittle), dense subsoil layers (50-60 cm beneath surface)
bonded into a hard, brittle form by clay
• Caliche
– Hard lime-cemented white crust in arid regions
• Plinthite
– Laterite, precipitated sesquioxides as cements. Weathered soils of
the tropics formed at depth
• Plowpan
– Artificially produced, due to compaction by plows.
Caliche
Soil Taxonomy
• Organization into 11 orders*, 54
suborders, 238 great groups, 1922
subgroups and then families and series,
each series subdivided into mapping units
called phases of series
– * including a tentative order andisols (soils
with over 60% volcanic ejecta)
Orders
• Most general category
• 5 of the orders exist in a wide variety of climates
– Histosols (organic soils); Entisols (undeveloped);
Inceptisols (slightly developed); Andisols (volcanic);
Vertisols (swelling-clay)
• 6 are the product of time and the microclimate in
which they develop
– Mollisols- naturally fertile, slightly leached, semiarid to
subhumid, grassland
– Alfisols- fertile soils in good moisture regimes
– Ultisols-leached, acidic soils, warm climates, lowmoderate fertility
– Aridisols-arid region soils
– Oxisols- infertile, hot humid tropics
– Spodosols- cool climate, acidic sandy
Soil Orders
• In the US Mollisols cover 25% of the land
• Worldwide distribution
– Aridisols 19%
– Alfisols 13%
– Inceptisols 9%
– Mollisols 8%
– Oxisols 8%
Soil Taxonomy
• Classification at 6 different levels
• Level of generalization relates to the range in
properties allowed in the different classes
• Soil Orders
– Suborders
• Great Groups
– Subgroups
» Families
» Series
Soil Orders
Engineering Properties
• Plasticity-water content of soil
• Soil Strength-ability to resist deformation
– Cohesion-ability of particles to stick together
– Friction-fnc. Density, size, shape of soil particles
– Sensitivity-measures changes in soil strength, clay
soils very sensitive to disturbance (liquifaction)
• Compressibility- soils tendency to consolidate
(decrease in volume), settling causes foundation
cracks
• Erodibility- ease of removal by wind and water
• Corrosion- function of soil chemistry
• Ease of Excavation
• Shrink-swell potential- gain or loose water
(expansive soils)
Soil Erosion
• Soil Erosion: Removal in part or whole of
soil by wind or water
• Natural process
• Erosion is slight from areas covered by
dense grasses or forest but increases
dramatically in exposed steep, poorly
covered soils
• Increased by human activity especially
poor agricultural practices
Soil Erosion
• Soil erosion has been documented as early as
10,000 ybp in Mesopotamia due to agriculture
that cleared the land (deforestation), overgrazed
the land by herbivores (sheep, goats)
• Erosion in Europe is believed to have occurred
5000 ybp with clearing of woodlands
• In the us during the 1930’s (Dust Bowl) wind and
water erosion left devastating effects
• On a positive note, erosion from Ethiopian
highlands generated the fertile sediment for
Egyptian agriculture for 1000’s of years
Erosion by water in 1961 Kentucky
Environmental Problem
• Human induced activities such as over cultivating can
deplete soils. Severe erosion can exceed 200
Mg/ha/yr (90 tons/acre/yr)
• Loss of soil to support growth of crops, grasslands,
forest
• Soil erosion destroys human-made structures
(reservoirs), lakes and rivers and badly damages land
• Deposition of sediment in rivers can cause them to
change course, variable seasonal flow and flooding
• The cost of dredging rivers and harbors each year is
15X the cost of holding the soil in place
Environmental Problem
• More than 1 million acre-feet of sediment settles
annually in reservoirs lowering their capacity
• Water can become polluted. 1 ton of soil
containing 0.2% N and 0.05% P will transfer 2kg
N and 0.5Kg P to rivers and lakes causing
eutrophication
• Air pollution: fine particles can reduce solar
radiation and affect chemical processes in the
atmosphere
Poorly Managed Construction Site
City of Ballinger, TX used
water from this reservoir from
1920-1952. By early 1970’s
soil erosion sediments filled
the reservoir to more than 35
feet destroying the dams ability
to hold water.
Dredging of a Sediment Filled Drainage Ditch
Eutrophication of Water Body due to Nutrient Loading
Environmental Problem
• This high rate of erosion has tried to be controlled
by laws past. In 1972, US Congressed passed
P.L. 92-500, The Federal Water Pollution Control
Act (FWPCA).
• The Clean Water Act amended in 1977 (Section
208 of FWPCA) required states to develop plans
to control ‘non-point sources’ such as sediment in
waterways
• In 1981 renewed effort that targeted areas having
the most severe erosion
• By 1986 some improvements were evident
Effects of Soil Loss
• Amount of erosion depends on erodibility of the soil,
characteristics of the land and land use management
• Universal Soil Loss Equation (USLE)
A = RKLSCP
A: annual soil loss
R: erosivity of rain
K: soil erodibility factor (easily detached particles);
reference soil plot obtained using standard plot 22.1m long on
9% slope, bare of vegetation, plowed up and down
L & S: length and angle of slope (in percent)
C: crop management factor and vegetative cover
P: practices for soil conservation (contour, terracing)
Agricultural Soil Erosion
Contour strip croping of hay and corn
Bench Terraces
Calkins sweep plow designed to
provide 90% stubble on soil as mulch
to reduce wind erosion
Field windbreak protecting a corn crop in North Dakota
Soil Erosion
Soils and Pollution
• Pollution vs. Contamination
• All chemicals are harmful in excess
concentrations
• Concentrations of chemicals are regulated
by law in most industrialized countries
• Natural soils can have chemicals in
excess concentrations (selenium,
molybdenum, lead)
• Soils are nature’s filters and a receptacle for burial
– Physical (sieve action)
– Chemical (adsorption and precipitation)
– Biological (decomposition of organic material)
• Soils are needed to:
– Grow crops for food, animal fodder and fibers, trees for
fuel and timber and to support natural ecosystems
• Increasingly human population explosion has
increased the amount of land in cultivation, so that
what remains is marginal or sub-economic land
• The question is sustainable development of resources
for our increasing population, estimated at 10 billion by
2050
• The answer of management is not easy since social,
political, economic and cultural conditions have to be
met just as the physical ones
Contamination by Nutrients:
Nitrates
• Nitrate is present in soils from microbial
breakdown of organic matter, manures and
plant residues; fertilizers; the microbial
oxidation of ammonium (NH4+) and additions
from the atmosphere as HNO3
• NO3- is not adsorbed by most soils because
of its negative charge. It remains in solution
until it is either taken up by plants, leached
out in drainage water or denitrified
• Loss of nitrate is undesirable because it is a
health hazard, it causes economic losses
(fertilizers); it causes eutrophication
Health Risks of N
• A health hazard from nitrite was first recognized in
1945 in Iowa- Methemoglobinemia (blue baby
syndrome), also affects the elderly and livestock
– Nitrate becomes toxic to any animal with a disrupted
digestive track that causes microbes to reduce NO3- to NO2in large amounts. The nitrite is absorbed into the
bloodstream where it oxidizes oxyhemoglobin to
methemoglobin thus suffocating the young animal, turning it
blue (cyanosis), when 70% of hemoglobin is changed, death
ensues
– Nitrate content of wells is regulated at 45ppm nitrate or
10ppm Nitrate-nitrogen; many rural wells exceed this by 2X
• Respiratory illnesses from PANS
(peroxyacetylnitrates)and other nitrogen oxides
• Cancer (gastric) from nitrosamines from NO2- and
secondary amines in food
Pesticides
• Pesticides are extensively used to control harmful
populations of insects
• Since Greek and Roman times some mixtures have
been used to control insect populations and fungi
(sulfur, arsenic and copper compounds)
• In 1930’s 2,4-D and DDT were found to kill weeds and
insects. It was the ‘magic-bullet’. Its ‘inventor’ was
awarded the Nobel Prize in Chemistry in 1948
• DDT is still an excellent insecticide (malaria control,
Chagas disease, typhus ect..). However, its half-life in
the environment is too long (10-25 years) and it
bioaccumulates in the fat of animals (Silent SpringRachael Carson)
• DDT was banned in US in the early 1970’s
Pesticides
• All pesticides are organic chemicals
(chlorinated hydrocarbons, organophosphorus
compounds, carbamates etc…)
• About 600 commercially important ones exist
and over 1500 registered for sale
• To be a ‘good pesticide’; it must be 1) short
lived in the environment, 2) not carcinogenic,
teratogenic or mutagenic and must 3) be
effective yet be able to be handled safely
Pesticides
• Most pesticides are adsorbed to soils
especially those of a high molecular mass
that form positively charged ions
• Some pesticides are volatilized and all are
eventually biodegraded by soil
microorganisms depending on their halflife
• Groundwater contamination is one of the
greatest threats we face today
Soil Degredation
• Erosion: greatest long term hazard to long term maintenance
of soil fertility
• Acidification: the soil pH of a weakly buffered soil in the
humid tropics can drop from 6.0 to 4.5 in 3 years when fertilized
by ammonium sulfate
• Salinization and sodification: particular problems that occur
in arid and semi-arid environments under irrigation
• Accumulation of toxic elements: from mining and industry
• Depletion of plant nutrients: harvesting of specific crops
• Reduction of soil organic matter content
• Compaction and crusting
• Waterlogging and drought: periods of wet/dry. Sahel has
been in a drought since the mid 1960’s and the onslaught of
desertification
The Ultimate Pollutant: People
• Pollution: the degredation of a substance or
system for people’s use
• Carrying capacity (limit of an ecosystem to
support organisms without causing a
catastrophe)
• The more nature is bent abnormally by more
and more people, the more catastrophic will
be the results, whenever we lose control
Degradation
• The American Farmland Trust stated that
unless California’s agricultural problems
were addressed in the next 10-20 years
the state farming industry would decline
– Agricultural land conversion to nonagricultural uses
• Over 17,00 ha/yr were converted to urban uses, >80% were
irrigated croplands
– Soil erosion
– Increasing salinity of soil and water
– Diminishing water supply and diversion for
nonagricultural uses
Parting Thought
AS IMPORTANT AS TECHNOLOGY, POLITICS,
LAW, AND ETHICS ARE TO THE POLLUTION
QUESTION, ALL SUCH APPROACHES ARE
BOUND TO HAVE DISSAPPOINTING
RESULTS, FOR THEY IGNORE THE PRIMARY
FACT THAT POLLUTION IS PRIMARILY AN
ECONOMIC PROBLEM, WHICH MUST BE
UNDERSTOOD IN ECONOMIC TERMS