Hydric soil
Encyclopedia
A hydric soil is a soil that formed under conditions of saturation, flooding, or ponding
long enough during the growing season to develop anaerobic
conditions in the upper part.
This term is part of the legal definition of a wetland
included in the US Food Security Act of 1985
(P.L. 99-198). The US Natural Resources Conservation Service
maintains the official list of hydric soils.
s are aerobic. This is important because plant
roots respire (that is, they consume oxygen
and carbohydrates while releasing carbon dioxide
) and there must be sufficient air -- especially oxygen -- in the soil to support most forms of soil life
. Air normally moves through interconnected pores
by forces such as changes in atmospheric pressure
, the flushing action of rainwater, and by simple diffusion
.
In addition to plant root
s, most forms of soil microorganisms need oxygen to survive. This is true of the more well-known soil animals as well, such as ants, earthworms and moles
. But soils can often become saturated with water due to rainfall and flooding. Gas diffusion in soil slows (some 10,000 times slower) when soil becomes saturated with water because there are no open passageways for air to travel. When oxygen levels become limited, intense competition arises between soil life forms for the remaining oxygen. When this anaerobic environment continues for long periods during the growing season, quite different biological and chemical reactions begin to dominate, compared with aerobic soils. In soils where saturation with water is prolonged and is repeated for many years, unique soil properties usually develop that can be recognized in the field. Soils with these unique properties are called hydric soils, and although they may occupy a relatively small portion of the landscape, they maintain important functions in the environment and play a role in soil.
Hydric soils have water holding capacity and also store more organic carbon.Thus it is suitable for rice (wetland) cultivation.
Ponding
Ponding is a term that refers to the unwanted pooling of water, typically on a flat roof.-Cause:Most flat roof systems are designed with a slight pitch to shed water off the sides, usually into a scupper system or into an internal drainage system. When a scupper or drain is clogged or fails for...
long enough during the growing season to develop anaerobic
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
conditions in the upper part.
This term is part of the legal definition of a wetland
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....
included in the US Food Security Act of 1985
Food Security Act of 1985
The Food Security Act of 1985 , a 5-year omnibus farm bill, allowed lower commodity price and income supports and established a dairy herd buyout program. This 1985 farm bill made changes in a variety of other USDA programs...
(P.L. 99-198). The US Natural Resources Conservation Service
Natural Resources Conservation Service
The Natural Resources Conservation Service , formerly known as the Soil Conservation Service , is an agency of the United States Department of Agriculture that provides technical assistance to farmers and other private landowners and managers.Its name was changed in 1994 during the Presidency of...
maintains the official list of hydric soils.
Overview
Most soilSoil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s are aerobic. This is important because plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
roots respire (that is, they consume oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and carbohydrates while releasing carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
) and there must be sufficient air -- especially oxygen -- in the soil to support most forms of soil life
Soil life
Soil life or soil biota is a collective term for all the organisms living within the soil.-Overview:In balanced soil, plants grow in an active and steady environment. The mineral content of the soil and its heartiful structure are important for their well-being, but it is the life in the earth that...
. Air normally moves through interconnected pores
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
by forces such as changes in atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
, the flushing action of rainwater, and by simple diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
.
In addition to plant root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s, most forms of soil microorganisms need oxygen to survive. This is true of the more well-known soil animals as well, such as ants, earthworms and moles
Mole (animal)
Moles are small cylindrical mammals adapted to a subterranean lifestyle. They have velvety fur; tiny or invisible ears and eyes; and short, powerful limbs with large paws oriented for digging. The term is especially and most properly used for the true moles, those of the Talpidae family in the...
. But soils can often become saturated with water due to rainfall and flooding. Gas diffusion in soil slows (some 10,000 times slower) when soil becomes saturated with water because there are no open passageways for air to travel. When oxygen levels become limited, intense competition arises between soil life forms for the remaining oxygen. When this anaerobic environment continues for long periods during the growing season, quite different biological and chemical reactions begin to dominate, compared with aerobic soils. In soils where saturation with water is prolonged and is repeated for many years, unique soil properties usually develop that can be recognized in the field. Soils with these unique properties are called hydric soils, and although they may occupy a relatively small portion of the landscape, they maintain important functions in the environment and play a role in soil.
Hydric soils have water holding capacity and also store more organic carbon.Thus it is suitable for rice (wetland) cultivation.