soils Flashcards
Soil consist of four major components
- mineral (or inorganic)
- organic,
- water,
- air.
- relative proportions of these four soil components vary with soil type and climatic conditions.
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Mineral components
- inorganic materials derived from rocks
- extremely variable in size and composition.
Primary minerals
- 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.
Secondary minerals
- formed at low temperature and pressure through oxidation.
- weathering product of primary minerals,
- usually present in soil as clay particles.
Most common elements in soils
- 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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Elemental composition of soils
- 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%
Weathering
- 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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Chemical weathering - Hydration
- Hydration - water molecules bind to a mineral e.g. transforming hematite into ferrihydrate
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chemical weathering hydrolysis
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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chemical weathering dissolution (or solution)
water can dissolve many minerals by hydrating the cations and anions e.g. dissolution of gypsum
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chemical weathering carbonation
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
chemical weathering - oxidation- reduction
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.
Biological weathering effects
- 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)
Particle density
- () 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.
Bulk density
- (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
Size of soil particle
- 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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Comparison of sizes of soil particles
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Particle size determination
- 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 %
sedimentation
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.
Hydrometer
- 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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Soil texture key
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Triangular graph of soil texture
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Types of soil aggregates, characterized in terms of
- shape (or type)
- size (fine, medium, or coarse)
- distinctness (or strength, e.g. strong, moderate, or weak).
Soil structure is enhanced by
- 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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Surface area
- 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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Moisture content
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soil porosity
- Total pore space of the soil
- Sand has less than clay
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typical values of porosity
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permeability
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soil air
- 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
