Physical properties – Texture

Physical properties – Texture • proportions of sand, silt, and clay • Texture 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.

lowest highest Texture • Surface area per unit volume • 1 g sand ~ 0.1 m2 • 1 g silt ~ 1 m2 • 1 g clay ~ 10-1000 m2 Large surface area means morecharge 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

Low Medium High Good Poor Medium High Slow Very slow Low Medium High Influence of Texture

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

Physical properties – Texture