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HDSO Forensics > soils > Flashcards

soils Flashcards

1
Q

Soil consist of four major components

A
  • mineral (or inorganic)
  • organic,
  • water,
  • air.
  • relative proportions of these four soil components vary with soil type and climatic conditions.

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2
Q

Mineral components

A
  • inorganic materials derived from rocks

- extremely variable in size and composition.

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3
Q

Primary minerals

A
  • formed at high temperature and pressure, under reducing conditions without free oxygen.
  • mainly present in soils as sand (quartz), feldspar and silicates (discrete and chain)
  • crystallized from igneous rock formed from molten lava.
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4
Q

Secondary minerals

A
  • formed at low temperature and pressure through oxidation.
  • weathering product of primary minerals,
  • usually present in soil as clay particles.
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5
Q

Most common elements in soils

A
  • highest quantities are O, Si, Al, Fe, C, Ca, K, Na, and Mg.
  • Oxygen is the most prevalent - comprises about 47% of the Earths crust by weight and more than 90% by volume

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6
Q

Elemental composition of soils

A
  • Oxygen 49%
  • Silicon 31%
  • Aluminum 7.2%
  • Iron 2.6%
  • Carbon 2.5%
  • Calcium 2.4%
  • Potassium 1.5%
  • Sodium 1.2%
  • Other 2.6%
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7
Q

Weathering

A
  • Breakdown and changes in rocks by biological, chemical, and physical agents can result in synthesis of new (secondary) minerals of great importance in soil (e.g. clay minerals).

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8
Q

Chemical weathering - Hydration

A
  • Hydration - water molecules bind to a mineral e.g. transforming hematite into ferrihydrate

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9
Q

chemical weathering hydrolysis

A

water molecules split into their hydrogen and hydroxyl components and hydrogen replaces a cation from the mineral structure e.g. transformation of feldspar to kaolinite

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10
Q

chemical weathering dissolution (or solution)

A

water can dissolve many minerals by hydrating the cations and anions e.g. dissolution of gypsum

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11
Q

chemical weathering carbonation

A

When carbon dioxide dissolves in water (enhanced by microbial and root respiration) the carbonic acid produced hastens the chemical dissolution limestone into soluble hydrogen carbonate

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    • As CO2 is more soluble in water as temp decreases carbonation important in glacial weathering
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12
Q

chemical weathering - oxidation- reduction

A

minerals that contain Fe, Mn, or S are especially susceptible to oxidation-reduction reactions.
- Fe(II) oxidized to Fe(III) forming – colour change to red-brown and change in valence and ionic radius destabilizes crystal structure of the mineral.

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13
Q

Biological weathering effects

A
  • the break up of rock particles by roots
  • the transfer and mixing of materials by burrowing animals and
  • the formation of organo-mineral complexes (soil biological processes produce organic acids that can solubilize Al and Si ions)
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14
Q

Particle density

A
  • () is mass of solids (Ms) per volume of solids (Vs).
  • = Ms/Vs
  • mean particle density is 2.6-2.7 gm/cm3.
  • Soils with a high content of iron oxides and various heavy minerals have a particle density of 5.2-5.3 gm/cm3,
  • Soils with high organic matter content can have a particle density as low as 1.3 gm/cm3.
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15
Q

Bulk density

A
  • (b) is the mass of solids (Ms) per total soil volume (Vt).
  • b = Ms / Vt b= Ms / (Vs + Va + Vw)
  • Vs=volume of solids; Va=volume of air; Vw=volume of water
  • Bulk density is always smaller than particle density.
  • Since pores constitute half the volume, b is about half of , namely 1.3-1.35 g/cm3
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16
Q

Size of soil particle

A
  • mineral particles in soils vary enormously in size. Sieved for size fractions.
  • a) particles < 2-mm diameter holes - the fine earth (consisting of sand, silt, and clay particles),
  • b) that is retained on the sieve (> 2 mm) - the coarse fragments (gravel, cobbles, and stones).

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17
Q

Comparison of sizes of soil particles

A

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18
Q

Particle size determination

A
  • Sieving - Stack of sieves, larger mesh size at top
  • Soil on each sieve turned out onto paper.
  • Back of sieve brushed onto paper and weighed.
  • wire brush used (for coarse sieves) and nylon brush (for fine nylon sieves).
  • Results expressed as a %
19
Q

sedimentation

A

soil sample dispersed, dried, weighed then poured into column of water
Settling rate used as a standard method
Stokes Law 1851, settling rate ∝ diameter of particle
settling rate depends on particle diameter and density and density of the fluid.
Large grains settle out immediately, e.g. sand within 5 min,
silt after 8 hours and clay remains suspended for long time.
Very small particles (<1µm) never settle unless they are very dense.
Evaporate a 20cm3 sample withdrawn 10cm3 below surface. Weight of residue  solids still in suspension.

20
Q

Hydrometer

A
  • Measures density of liquid as particles settle out density drops and hydrometer sinks.
  • used to grade fine-grained soils, silts and clays too small for sieve analysis

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21
Q

Soil texture key

A

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22
Q

Triangular graph of soil texture

A

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23
Q

Types of soil aggregates, characterized in terms of

A
  • shape (or type)
  • size (fine, medium, or coarse)
  • distinctness (or strength, e.g. strong, moderate, or weak).
24
Q

Soil structure is enhanced by

A
  • physical and chemical processes e.g.wetting/drying, or freeze/thaw
  • Biotic processes, e.g. soil compression by roots, burrowing activities of earthworms, enmeshment of mineral particles by network of roots and fungal hyphae

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25
Q

Surface area

A
  • The large interfacial area per unit volume soils give rise to liquid solid interactions and adsorption of water and chemicals
  • The soil clay fraction dominates soil specific surface area

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26
Q

Moisture content

A

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27
Q

soil porosity

A
  • Total pore space of the soil
  • Sand has less than clay
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28
Q

typical values of porosity

A

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29
Q

permeability

A

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30
Q

soil air

A
  • Much higher in CO2 than normal air
  • O2 % CO2 %
  • Air in atmosphere 20.9 0.03
  • Soil air 15 – 20 0.25 - 4.5
  • CO2 in soil produced by:
  • Respiration of living organisms
  • Root respiration
  • Decay of organic matter
  • Humification where C  CO2 by soil microorganisms

HDSO Forensics (10 decks)

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