soils

Question 1

Soil consist of four major components

Answer 1

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

look at ppt

Question 2

Mineral components

Answer 2

• inorganic materials derived from rocks

- extremely variable in size and composition.

Question 3

Primary minerals

Answer 3

• 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 4

Secondary minerals

Answer 4

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

Question 5

Most common elements in soils

Answer 5

• 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

look at ppt

Question 6

Elemental composition of soils

Answer 6

• 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 7

Weathering

Answer 7

• 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).

look at ppt

Question 8

Chemical weathering - Hydration

Answer 8

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

look at ppt

Question 9

chemical weathering hydrolysis

Answer 9

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

look at ppt

Question 10

chemical weathering dissolution (or solution)

Answer 10

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

look at ppt

Question 11

chemical weathering carbonation

Answer 11

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

look at ppt

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

Question 12

chemical weathering - oxidation- reduction

Answer 12

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 13

Biological weathering effects

Answer 13

• 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 14

Particle density

Answer 14

• () 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 15

Bulk density

Answer 15

• (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 16

Size of soil particle

Answer 16

• 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).

look at ppt

Question 17

Comparison of sizes of soil particles

Answer 17

look at ppt

Question 18

Particle size determination

Answer 18

• 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 19

sedimentation

Answer 19

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 20

Hydrometer

Answer 20

• 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

look at ppt

Question 21

Soil texture key

Answer 21

look at ppt

Question 22

Triangular graph of soil texture

Answer 22

look at ppt

Question 23

Types of soil aggregates, characterized in terms of

Answer 23

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

Question 24

Soil structure is enhanced by

Answer 24

• 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

look at ppt

Question 25

Surface area

Answer 25

- 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 look at ppt

Question 26

Moisture content

Answer 26

look at ppt

Question 27

soil porosity

Answer 27

- Total pore space of the soil - Sand has less than clay look at ppt

Question 28

typical values of porosity

Answer 28

look at ppt

Question 29

permeability

Answer 29

look at ppt

Question 30

soil air

Answer 30

- 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