Physical properties Texture

1. Physical properties – Texture Texture

2. Soil Texture Proportions of sand, silt, and clay Not OM – nonetheless important Not coarse fragments – nonetheless important Cons: Don’t add pore space, get in the way (plant roots, for example) Pros: Mineral soil source, rocky soils can allow deep percolation ?water source for deep rooted plantsCons: Don’t add pore space, get in the way (plant roots, for example) Pros: Mineral soil source, rocky soils can allow deep percolation ?water source for deep rooted plants

3. Relative Size Comparison of Soil Particles

5. Texture Surface area per unit volume 1 g sand ~ 0.1 m2 1 g silt ~ 1 m2 1 g clay ~ 10-1000 m2

6. Teton Dam failure story: used silt instead of clay as dam’s core, and it failed, killing 11 people and wiping out thousands of homes.Teton Dam failure story: used silt instead of clay as dam’s core, and it failed, killing 11 people and wiping out thousands of homes.

8. Influence of Texture

9. Physical properties Density Porosity

10. 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

11. 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