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 m 2 –1 g silt ~ 1 m 2 –1 g clay ~ m 2 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 SandSiltClay Water-holding capacity Aeration Drainage Nutrient retention LowMediumHigh GoodPoor Medium SlowVery slowHigh LowMedium

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