Soil Science & Analysis Flashcards

Which organic soil horizon is composed primarily of slightly decomposed plant material that retains recognizable fiber structure?

Answer: Oi (fibric)

The Oi horizon (fibric material) consists of slightly decomposed organic matter retaining over two-thirds recognizable plant fibers.

Capillary rise in soils is inversely proportional to:

Answer: Pore radius

Capillary rise height is inversely proportional to pore radius; smaller pores produce greater capillary rise according to the capillary equation h = 2T cosθ / (ρgr).

The National Cooperative Soil Survey (NCSS) hydric soils list is primarily used to:

Answer: Identify soils that formed under conditions of saturation sufficient to support wetland vegetation

The NCSS hydric soils list identifies soils formed under prolonged saturation, anaerobic conditions, and reducing environments that support obligate wetland plants.

Which test is used in the field to quickly evaluate whether a soil horizon qualifies as 'strongly contrasting' particle size in soil survey?

Answer: Ribbon test (hand texturing)

The ribbon test (hand texturing) allows rapid field estimation of soil texture by evaluating plasticity and ribbon length formed when moist soil is extruded between thumb and forefinger.

In the context of Green-Ampt infiltration model, what does the term 'wetting front suction (ψf)' represent?

Answer: The capillary suction (matric potential) at the advancing wetting front

The wetting front suction (ψf) in the Green-Ampt model is the matric potential (suction head) at the sharp wetting front as water infiltrates into initially dry soil.

A soil described as having 'prismatic' structure most commonly occurs in which horizon?

Answer: B horizon (subsoil)

Prismatic structure—vertical columns with flat tops—is most characteristic of B horizons, especially those with clay accumulation (Bt horizons).

The 'effective porosity' used in groundwater calculations differs from total porosity because it:

Answer: Excludes pores that do not contribute to fluid flow (isolated or dead-end pores)

Effective porosity excludes isolated, dead-end, and very fine pores that do not transmit water under normal gradients, giving the fraction contributing to actual flow.