Transcript Document

Physical properties – Texture
• Texture
–
proportions of sand, silt, and clay
–
Determines water holding capacity, water
availability, nutrient supply capacity
Soil Texture
• Proportions of sand, silt, and clay
• Not OM – nonetheless important
• Not coarse fragments – nonetheless important
Relative Size Comparison of Soil
Particles
“Big”  smaller  really small
Sand  silt 
clay
Loam:
Unequal proportions,
Equal properties
The small arrows indicate the proper direction in which to draw the lines.
Texture
• Surface area per unit volume
– 1 g sand ~ 0.1 m2
– 1 g silt ~ 1 m2
– 1 g clay ~ 10-1000 m2
lowest
highest
•
•
Large surface area means more charge so greater
ability to hold water and nutrients
Coarse textured soils larger pores vs. fine textured soils
greater total pore space (volume)
• particle surface area
• pore volume
• nutrient supply capacity
• plasticity and cohesion
• swelling
• pore size
• infiltration rate
• drainage rate
• aeration
Influence of Texture
Water-holding
capacity
Aeration
Drainage
Nutrient
retention
Sand
Silt
Clay
Low
Medium
High
Good
Medium
Poor
High
Slow
Very slow
Low
Medium
High
Physical properties
• Density
– particle density: mass per unit volume (no air)
– bulk density: mass per unit volume (with air)
Both: no water
• Porosity
– the volume percentage of the total bulk soil
NOT occupied by solids
But soil properties greatly influenced by –
• Pore size range  Particle heterogeneity
& Aggregation
– Finer pores – water unavailable, poor aeration, little
waterflow,
– Finest pores – too small for microbes
• Pore network  Aggregation
Aggregation influenced by
• Coarse scale – biotic:
– Roots, Burrowing animals (mammals, earthworms)
– Sticky networks: root hairs, fungi
• Fine scale – physical/chemical:
–
–
–
–
Clay properties: Flocculation, bridging (multivalent cations)
Clay/humus/cation complexes
Cementing: Iron oxides (Ultisols & Oxisols)
Volume changes in clays: shrink/swelling, freeze drying