Soil properties

Question 1

what are the divisions of particle sizes?

Answer 1

• gravel (>2mm diameter)
• sand (2-0.02 mm diameter)
• silt (0.02 to 0.002 mm diameter)
• clay (<0.002 mm or 2micromm diameter)

Question 2

why is soil texture important? (5 reasons)

Answer 2

• it influences the soil infiltration rate (thus, overland flow and erosion)
• influences soil permeability (thus, drainage)
• controls available water capacity of soils (ability to supply water to plants)
• influences soil structure (root growth and aeration)
• provides cation exchange capacity (nutrient supply to plants and buffering against acid rain)

Question 3

what is the difference between soil structure and texture?

Answer 3

• analogy of bricks as soil texture and walls as soil structure
• structure: spatial arrangement of particles to complex aggregates, pores and channels

Question 4

what are the three main influences of soil structure?

Answer 4

• water/air movement
• root growth
• movement of macro/meso fauna

Question 5

what are the 6 major structural forms in soil?

Answer 5

• prismatic
• columnar
• angular blocky
• subangular blocky
• platy
• granular

Question 6

what are 4 processes leading to the development of soil structure?

Answer 6

• expansion/contraction upon wetting and drying
• root penetration
• earthworms and ingestion/excretion by other soil organisms
• input and decomposition of soil organic matter

Question 7

what are 6 properties that maintain soil structure?

Answer 7

1. high organic matter content promotes bonding between particles
2. clay minerals -> large, reactive surfaces with positive and negative charges
3. presence of Fe and Al oxides and hydroxides as cementing agents
4. Si coating on minerals -> produces duripans in dry climates
5. CaCO3 deposits -> produce caliche in semi-arid soils
6. presence of flocculating cations (Ca2+ and Mg 2+)

Question 8

what are 6 processes leading to the destruction of soil structure?

Answer 8

1. loss of soil organic matter
2. raindrop impact on bare soils
3. reduction in root growth
4. trampling by stock and machinery
5. reduction in soil organisms
6. loss of flocculating cations (Ca2+ and Mg2+) and replacement by dispersing cations (Na+ and K+)

Question 9

what are the effects of ploughing on soil structure?

Answer 9

1. breakdown of weakened aggregates as soil surface
   1a) erosion removes fine particles and OM, surface crusting reduces seeding emergence
2. formation of pan at base of plough depth
   +
   compacted subsoil
   2a) impede root growth and water and air movement

Question 10

what are flocculating cations?

Answer 10

cations that make clay particles stick together (Mg+ and Ca+)