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