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Weathering and Soils - New Jersey City University

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									Soils and Environment
• 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
•   Freeze-Thaw Weathering
•   Salt Weathering
•   Wetting and Drying
•   Insolation Weathering
•   Pressure Release
•   Stress Corrosion Cracking
• 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
Interlayer Cations
   Na, K
   Ca, Mg          Hydration, solution
                                            Solutions of Na+, K+, Ca2+, Mg2+

 Aluminosilicate sheets (e.g. as part of feldspars)
                                               Silicic acid (H4SiO4)
   Al 3+ & Si 4+    Hydrolysis, hydration
                          Secondary minerals, e.g. clays
 Brucite and alumina sheets and incorporated ions, e.g. Fe2+
                           Oxidation, hydrolysis
                                                   Hydrous oxides, e.g. FeO(OH)
        Fe2+              hydrolysis               Chelate complexes
   Results of Weathering: Clay
• 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
•   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
• 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
• Because of its cation exchange properties,
  acts as a pH buffer, retains nutrients and
  other element loss by leaching and
  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                       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
• 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
              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
• 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
• 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
• 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
• 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
• Transmission pores >50mm
• Storage pores 0.5-50 mm
• Residual pores <0.5 mm
Relationship Between Texture
 Bulk Density, and Porosity
• Various definitions
    – Unconsolidated material above bedrock
    – weathered material & organic matter
        •   supports plant life [air, water, organic matter &
            mineral material]
•   Loam {40% sand; 40% silt; 20% clay}
•   Clay-sized particles vs. clay minerals
•   Soil Horizons
•   Residual Soil (on bedrock)
•   Transported Soil (alluvium)
• 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
  – O- organic material
  – A (A1)-accumulation of decomposed org. matter
  – E (A2)- mineral layer, loss of silicate, eluviation (leaching)
  – 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
• 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.
             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)
• 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, low-
     moderate 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
• 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,
• 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
• Air pollution: fine particles can reduce solar
  radiation and affect chemical processes in the
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
• 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: 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 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
• 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:
• 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 NO2-
     in 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
   – 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 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 Spring-
  Rachael Carson)
• DDT was banned in US in the early 1970’s
• 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
• 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 half-
• 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
  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
• The more nature is bent abnormally by more
  and more people, the more catastrophic will
  be the results, whenever we lose control
• 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

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