Soil components

Question 1

percentage of mineral matter in soil?

Answer 1

45%

Question 2

percentage of air in soil?

Answer 2

25%

Question 3

percentage of water in soil?

Answer 3

25%

Question 4

percentage of organic matter in soil?

Answer 4

5%

Question 5

four major components in soil?

Answer 5

mineral
organic
water
air

Question 6

mineral components

Answer 6

• inorganic materials derived from rocks

* extremely variable in size and composition.

Question 7

primary minerals

Answer 7

• 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.

Question 8

secondary minerals

Answer 8

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

Question 9

most common elements in soil

Answer 9

• 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

Question 10

elemental composition of soils

Answer 10

• 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%

Question 11

weathering

Answer 11

• 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).

Question 12

see pp for

Answer 12

weathering processes and responses

Question 13

chemical weathering consists of what 5 things

Answer 13

hydration
hydrolysis
dissolution
carbonation
oxidation-reduction

Question 14

hydration - chemical weathering

Answer 14

water molecules bind to a mineral e.g. transforming hematite into ferrihydrate
see pp for equation

Question 15

hydrolysis - chemical weathering

Answer 15

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

Question 16

dissolution - chemical weathering

Answer 16

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

Question 17

carbonation - chemical weathering

Answer 17

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

Question 18

oxidation-reduction - chemical weathering

Answer 18

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.

Question 19

biological weathering effects

Answer 19

• 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)

Question 20

particle density

Answer 20

• () 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.

Question 21

bulk density

Answer 21

• (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

Question 22

size of soil particles

Answer 22

• 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).

Question 23

see pp for

Answer 23

comparison of soil particles

Question 24

particle size determination

Answer 24

• 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 %

Question 25

sedimentation

Answer 25

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.

Question 26

hydrometer

Answer 26

* 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

Question 27

see pp for

Answer 27

hydrometer diagram

Question 28

see pp for

Answer 28

soil texture key

Question 29

see pp for

Answer 29

triangular graph of soil texture

Question 30

types of soil aggregates - characterised in terms of what 3 things

Answer 30

- shape (or type) - size (fine, medium, or coarse) - distinctness (or strength, e.g. strong, moderate, or weak).

Question 31

types of soil aggregates -soil structure is enhance by...

Answer 31

- 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

Question 32

surface area

Answer 32

* 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

Question 33

moisture content

Answer 33

``` • 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) ```

Question 34

soil porosity

Answer 34

• Total pore space of the soil • Sand has less than clay see pp for equation

Question 35

usual soil porosity %

Answer 35

* Generally, between 30-60%. | * High porosity indicates lack of compaction and good soil condition

Question 36

see pp for

Answer 36

porosity diagrams

Question 37

see pp for

Answer 37

permeability diagram

Question 38

soil air

Answer 38

much higher in CO2 than normal air

Question 39

CO2 in soil is produced by what

Answer 39

- Respiration of living organisms - Root respiration - Decay of organic matter - Humification where C  CO2 by soil microorganisms