Soil components Flashcards
percentage of mineral matter in soil?
45%
percentage of air in soil?
25%
percentage of water in soil?
25%
percentage of organic matter in soil?
5%
four major components in soil?
mineral
organic
water
air
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 soil
- 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
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).
see pp for
weathering processes and responses
chemical weathering consists of what 5 things
hydration hydrolysis dissolution carbonation oxidation-reduction
hydration - chemical weathering
water molecules bind to a mineral e.g. transforming hematite into ferrihydrate
see pp for equation
hydrolysis - chemical weathering
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
see pp for equation
dissolution - chemical weathering
water can dissolve many minerals by hydrating the cations and anions e.g. dissolution of gypsum
see pp for equation
carbonation - chemical weathering
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
see pp for equation
oxidation-reduction - chemical weathering
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 particles
• 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).
see pp for
comparison of soil particles
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
see pp for
hydrometer diagram
see pp for
soil texture key
see pp for
triangular graph of soil texture
types of soil aggregates - characterised in terms of what 3 things
- shape (or type)
- size (fine, medium, or coarse)
- distinctness (or strength, e.g. strong, moderate, or weak).
types of soil aggregates -soil structure is enhance 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
surface area
- The large interfacial area per unit volume in soils give rise to liquid-solid interaction and adsorption of water and chemicals.
- The soil clay fraction dominates soil specific surface area
moisture content
• An indicator of the amount of the water present in soil • Moisture concent, w(%) see pp for equation • ASTM 2216 (conventional oven method) • ASTM D 4643 (Microwave oven method) • 3 minutes at 50% power (mass=50g)
soil porosity
• Total pore space of the soil
• Sand has less than clay
see pp for equation
usual soil porosity %
- Generally, between 30-60%.
* High porosity indicates lack of compaction and good soil condition
see pp for
porosity diagrams
see pp for
permeability diagram
soil air
much higher in CO2 than normal air
CO2 in soil is produced by what
- Respiration of living organisms
- Root respiration
- Decay of organic matter
- Humification where C CO2 by soil microorganisms
