L14 - Mineral and Organic Components of Soils Flashcards
What determines mineral composition of soil?
Weathering (physical, chemical and biological)
Loamy soils
good drainage, sufficient water holding capacity, good for agriculture
Coarse sandy soils
heat up quickly, good drainage (leaching), low water holding capacity (WHC)
Clay soils
hold on to nutrients, poor drainage, waterlogged under wet conditions, solid under dry conditions
How do silicates form chains?
By sharing O atoms
Ortho-silicates
isolated
Ino-silicates
single/double chain
Phyllo-silicates
layered sheet
Tekto-silicate
3D framework
What climate are 2:1 clay particles typically found?
Temperate climates
What climate are oxides typically found?
Humid, tropical climates. Soils here are highly weathered and contain mainly kaolinite (1:1) and iron/aluminium oxides
What are clay minerals?
Layered silicates (phyllosilicates) formed by tetrahedrons and octahedrons. Connected by Si-O-Al/Si bridges
Elementary layer
succession of T and O (TOT) atoms in a clay mineral layer. Can be split by weathering
Basal spacing
distance from one basal layer to the next
How are nutrients held at exchange sites?
held in their cationic form by outer sphere complex formation. May easily be taken up by plant roots
What determines the bond strength between elementary layers?
By electrochemical charge. This influences sensitivity to weathering and physical characteristics such as swelling and shrinking
Isomorphous substitution
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+
What are negative effects of shrinkage cracks in soil?
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
What are positive effects of shrinkage cracks in soil?
Drainage of heavy clay soils where (K) increases. Also naturally restores soil compaction
Cation exchange capacity (CEC)
total number of cations a soil can hold, or total negative charge.
Importance of CEC?
Important for availability of macro and micro nutrients, and soil remediation in case of heavy metal pollution
What influences CEC?
- Equilibrium reaction
- Electrochemical charge
- pH - acidification can lead to Al toxicity and cation release. BS% increases with pH
How can iron oxides retain nutrients?
By forming inner sphere complexes - such nutrients are not readily available to plants, and are stored in subsoil horizons
How does organic matter influence soil fertility?
Source of nutrients, regulates pH and is the basis for soil fauna
How does organic mater influence soil structure?
Allows for the formation of stable soil aggregates with mineral components, important for drainage and aeration, and reduces soil sensitivity to erosion
Importance of organic matter in soil?
Fertility, structure, resistance to root penetration, increases WHC and CEC
What is the primary source of organic matter?
Plant material. Composition depends on species, part, growth conditions and condition when cut off
Mineralisation
transformation of organic molecules to inorganic molecules (including nutrients) by soil microbes.
What determines the rate of mineralisation?
Composition of the organic matter, microbes present and the abiotic conditions (e.g. pH and moisture content)
Organisms involved in decomposition
Detritivores, Primary decomposer, Secondary decomposer
