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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
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)
Influence of Texture
Sand Silt Clay
Water-holding capacity
Aeration
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