Soils and Soil Science Fundamentals
Texture and erodibility
Texture is the sand–silt–clay fraction read from the textural triangle. For erosion work, remember: silt-size particles are the most water-erodible - low cohesion, easily detached, easily carried. Sand detaches readily but is heavy and settles fast. Clay resists detachment while dry and aggregated, but once dispersed it stays in suspension and is very hard to capture; dispersive clays erode through chemical action and are a classic failure cause in channels and embankments.
Structure, compaction, and infiltration
Granular surface structure and organic matter promote infiltration and reduce runoff. Compaction from construction traffic destroys structure, cuts infiltration, and increases both runoff volume and erosion. Organic matter binds aggregates; topsoil specifications commonly call for roughly 4–6% organic matter.
Two numbers candidates confuse
- K factor - the RUSLE soil-erodibility factor, reflecting texture, organic matter, structure, and permeability together.
- Permeability - the rate water moves through soil. Clay has high total porosity but low permeability; that pairing is a frequent wrong-answer trap.
Hydrologic soil groups
Group A = high infiltration (deep sands), D = low infiltration (clays, shallow bedrock, high water table). Traps reverse these, or ignore dual groups (A/D) that depend on drainage.
Classification systems
NRCS texture is for agronomic and erosion work; USCS and AASHTO are engineering classifications. They are not interchangeable.
Rule of thumb
The ribbon test: a ribbon longer than about 2 inches between thumb and forefinger indicates high clay content and likely low permeability - expect high runoff and poor infiltration.