L13 - Mineral and Organic Components of Soils

Question 1

What determines mineral composition of soil?

Answer 1

Weathering (physical, chemical and biological)

Question 2

Loamy soils

Answer 2

good drainage, sufficient water holding capacity, good for agriculture

Question 3

Coarse sandy soils

Answer 3

heat up quickly, good drainage (leaching), low water holding capacity (WHC)

Question 4

Clay soils

Answer 4

hold on to nutrients, poor drainage, waterlogged under wet conditions, solid under dry conditions

Question 5

How do silicates form chains?

Answer 5

By sharing O atoms

Question 6

Ortho-silicates

Answer 6

isolated

Question 7

Ino-silicates

Answer 7

single/double chain

Question 8

Phyllo-silicates

Answer 8

layered sheet

Question 9

Tekto-silicate

Answer 9

3D framework

Question 10

What climate are 2:1 clay particles typically found?

Answer 10

Temperate climates

Question 11

What climate are oxides typically found?

Answer 11

Humid, tropical climates. Soils here are highly weathered and contain mainly kaolinite (1:1) and iron/aluminium oxides

Question 12

What are clay minerals?

Answer 12

Layered silicates (phyllosilicates) formed by tetrahedrons and octahedrons. Connected by Si-O-Al/Si bridges

Question 13

Elementary layer

Answer 13

succession of T and O (TOT) atoms in a clay mineral layer. Can be split by weathering

Question 14

Basal spacing

Answer 14

distance from one basal layer to the next

Question 15

How are nutrients held at exchange sites?

Answer 15

held in their cationic form by outer sphere complex formation. May easily be taken up by plant roots

Question 16

What determines the bond strength between elementary layers?

Answer 16

By electrochemical charge. This influences sensitivity to weathering and physical characteristics such as swelling and shrinking

Question 17

Isomorphous substitution

Answer 17

exchange of ions with lower charge during clay mineral formation (compensation by adsorbing cations). Allows for permanent charge in 2:1 and 1:1 clay particles between elementary layers
E.g. tetrahedron Si4+ -> Al3+

Question 18

What are negative effects of shrinkage cracks in soil?

Answer 18

Water may bypass root zone and be transported straight to subsoil (B). This results in inefficient sprinkler irrigation, nutrient shortage in plants and groundwater pollution. Also destruction of buildings, pavements and pipes in heavy clay soils

Question 19

What are positive effects of shrinkage cracks in soil?

Answer 19

Drainage of heavy clay soils where (K) increases. Also naturally restores soil compaction

Question 20

Cation exchange capacity (CEC)

Answer 20

total number of cations a soil can hold, or total negative charge.

Question 21

Importance of CEC?

Answer 21

Important for availability of macro and micro nutrients, and soil remediation in case of heavy metal pollution

Question 22

What influences CEC?

Answer 22

1. Equilibrium reaction
2. Electrochemical charge
3. pH - acidification can lead to Al toxicity and cation release. BS% increases with pH

Question 23

How can iron oxides retain nutrients?

Answer 23

By forming inner sphere complexes - such nutrients are not readily available to plants, and are stored in subsoil horizons

Question 24

How does organic matter influence soil fertility?

Answer 24

Source of nutrients, regulates pH and is the basis for soil fauna

Question 25

How does organic mater influence soil structure?

Answer 25

Allows for the formation of stable soil aggregates with mineral components, important for drainage and aeration, and reduces soil sensitivity to erosion

Question 26

Importance of organic matter in soil?

Answer 26

Fertility, structure, resistance to root penetration, increases WHC and CEC

Question 27

What is the primary source of organic matter?

Answer 27

Plant material. Composition depends on species, part, growth conditions and condition when cut off

Question 28

Mineralisation

Answer 28

transformation of organic molecules to inorganic molecules (including nutrients) by soil microbes.

Question 29

What determines the rate of mineralisation?

Answer 29

Composition of the organic matter, microbes present and the abiotic conditions (e.g. pH and moisture content)
Composition of the organic matter, microbes present and the abiotic conditions (e.g. pH and moisture content)

Question 30

Organisms involved in decomposition

Answer 30

Detritivores, Primary decomposer, Secondary decomposer