Soil Quality Information Sheet
Soil Quality Resource Concerns:
Available Water Capacity
USDA Natural Resources Conservation Service
What is available water
Available water capacity is the amount of water that a soil
can store that is available for use by plants.
It is the water held between field capacity and the wilting
point adjusted downward for rock fragments and for salts
in solution. Field capacity is the water retained in a freely
drained soil about 2 days after thorough wetting. The
wilting point is the water content at which sunflower
seedlings wilt irreversibly.
Why be concerned?
In areas where drizzle falls daily and supplies the soils
with as much or more water than is removed by plants,
available water capacity is of little importance. In areas
where plants remove more water than the amount sup-
plied by precipitation, the amount of available water that
the soil can supply may be critical. This water is neces-
sary to sustain the plants between rainfall events or
periods of irrigation. The soil effectively buffers the plant
root environment against periods of water deficit.
How is available water
Available water is expressed as a volume fraction (0.20),
as a percentage (20%), or as an amount (in inches). An
example of a volume fraction is water in inches per inch
of soil. If a soil has an available water fraction of 0.20, a
10 inch zone then contains 2 inches of available water.
Available water capacity is often stated for a common
depth of rooting (where 80 percent of the roots occur).
This depth is at 60 inches or more in areas of the western
United States that are irrigated and at 40 inches in the
higher rainfall areas of the eastern United States. Some
publications use classes of available water capacity.
These classes are specific to the area in which they are
used. Classes use such terms as very high, high, medium,
Soil properties affect available
Rock fragments reduce the available water capacity in
direct proportion to their volume unle