A soil auger is a helpful tool for sampling soils below the surface. A layer of clay-rich soil may be found under the top layer of organic soil in a vernal pool. A clay layer helps pools hold water longer. Credit: Sally Ray
Curls of pale yellowish clayey mineral soils were pulled up using a soil augur. They stand out against and dark organic rich soils found on the surface. Credit: Sally Ray
Vernal pools tend to have an impermeable layer that results in ponded water. The soil texture
(the amount of sand, silt, and clay particles) typically contains higher amounts of fine silts and
clays that lower percolation rates. Pools that retain water for a sufficient length of time will
develop hydric soils. Hydric soils form when the soil is saturated from flooding for extended periods of
time and anaerobic conditions (lacking oxygen or air) develop.
Hydric soils have a mottled appearance with bright red iron accumulations and light gray iron depletions.
These features may appear in the lower horizons of seasonal pools as a result of anaerobic conditions.
A soil may also be deemed "hydric" if it meets criteria set by the USDA-NRCS (http://soils.usda.gov/use/hydric/).
Typical hydric soils in Pennsylvania meet F3 Depleted Matrix or A2 Histic Epipedon criteria. F3 Depleted Matrix
soils have a grayish tint to the surface horizon while the A2 Histic Epipedon soils have an accumulation of
organic matter yielding a dark black color.
Most vernal pools have a layer of organic soil overlying the mineral soils. The amount of organic material
that can accumulate in the pool basin increases with the length of time a pool is typically inundated.