Post on 11-Feb-2016
description
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
“Big” smaller really small
Sand silt clay
Relative Size Comparison of Soil Particles
The small arrows indicate the proper direction in which to draw the lines.
Loam: Unequal proportions, Equal properties
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 TextureSand Silt Clay
Water-holding capacityAeration
Drainage
Nutrient retention
Low Medium High
Good PoorMedium
Slow Very slowHigh
HighLow Medium
Physical properties
• Density–
–
• Porosity–
particle density: mass per unit volume (no air)
bulk density: mass per unit volume (with air)
Both: no water
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