The aquic (L. aqua, water) moisture regime signifies a reducing regime in a soil that is virtually free of dissolved oxygen because it is saturated by ground water or by water of the capillary fringe. Some soils at times are saturated with water while dissolved oxygen is present, either because the water is moving or because the environment is unfavorable for micro-organisms (e.g., if the temperature is less than 1°C); such a regime is not considered aquic.

It is not known how long a soil must be saturated to have an aquic moisture regime, but the duration must be at least a few days, because it is implicit in the concept that dissolved oxygen is virtually absent. Because dissolved oxygen is removed from ground water by respiration of micro-organisms, roots, and soil fauna, it is also implicit in the concept that the soil temperature is above biologic zero (5°C) for some time while the soil is saturated.

Very commonly, the level of ground water fluctuates with the seasons; it is highest in the rainy season, or in fall, winter, or spring if cold weather virtually stops evapotranspiration. There are soils, however, in which the ground water is always at or very close to the surface. A tidal marsh and a closed, landlocked depression fed by perennial streams are examples. Such soils are considered to have a peraquic moisture regime.

The distinction between the aquic moisture regime and the peraquic moisture regime is not closely defined because neither regime is used as a criterion for taxa. These terms can, however, be used in descriptions of taxa.

Some soils with an aquic moisture regime also have a xeric, ustic, or aridic (torric) moisture regime.

7/15/98