Classes and Availability of Soil Water (Soil Moisture Relationship) Gravitational water is that part in excess of hygroscopic and capillary water which will move out of the soil if favourable drainage is provided. Capillary water is that part in excess of hygroscopic water which exists in the pore space of the soil by molecular attraction. When an oven-dried sample is kept open in the atmosphere, it absorbs some amount of water from the atmosphere. This is known as hygroscopic water, and is not capable of movement by the gravity or capillary forces. Soil Moisture tension The force per unit area that must be exerted in order to extract water from the soil is known as soil moisture tension and is expressed in terms of atmosphere (atm). also known as Capillary potential, Capillary tension or force of suction. Soil moisture tension is inversely proportional to moisture content of a soil of given texture and structure. measured in the laboratory with the Help of various instruments such as centrifuge, tensiometer etc. Soil moisture stress Soil moisture stress is defined as the sum of the soil moisture tension and osmotic pressure of soil solution. Osmotic pressure is the increase in the force (or tension) caused by the salts present in the soil solution. The growth of plants is a function of both soil moisture tension as well as the osmotic pressure, and hence is a function of soil moisture stress. Soil moisture constants Saturation Capacity: amount of water required to fill all the pore spaces between soil particles by replacing all air held in pore spaces Field capacity: moisture content of the soil after free drainage has removed most of the gravity water Permeant wilting point: the soil water content at or below which plant roots cannot absorb water any more Available moisture: difference in water content of the soil between field capacity and permanent wilting point Readily available moisture: portion of the available moisture that is most easily extracted by plants. It is approximately 75% of the available moisture. Moisture equivalent: percentage of moisture retained in a small sample of wet soil 1 cm deep when subjected to centrifugal force 1000 times as greater as gravity, usually for a period of 30 min. Moisture equivalent = Field capacity = 1.8 to 2 permanent wilting point = 2.7 Hygroscopic coefficient Soil-Moisture deficiency (Field moisture deficiency): water required to bring the soil moisture content to its field capacity.

1. Classes and Availability of
Soil Water
(Soil Moisture Relationship)
T.E. CIVIL-B
Roll No.-B52

2. When an oven-dried sample is kept open in
the atmosphere, it absorbs some amount of
water from the atmosphere. This is known
as hygroscopic water, and is not capable of
movement by the gravity or capillary forces.
Capillary water is that part in excess of
hygroscopic water which exists in the pore
space of the soil by molecular attraction.
Gravitational water is that part in excess of
hygroscopic and capillary water which will
move out of the soil if favourable drainage
is provided.

3. Soil Moisture tension
1) The force per unit area that must be exerted in order to extract water
from the soil is known as soil moisture tension and is expressed in terms
of atmosphere (atm).
2) also known as Capillary potential, Capillary tension or force of suction.
3) Soil moisture tension is inversely proportional to moisture content of a
soil of given texture and structure.
4) measured in the laboratory with the Help of various instruments such as
centrifuge, tensiometer etc.
Soil moisture stress
1) Soil moisture stress is defined as the sum of the soil moisture tension and
osmotic pressure of soil solution.
2) Osmotic pressure is the increase in the force (or tension) caused by the
salts present in the soil solution.
3) The growth of plants is a function of both soil moisture tension as well as
the osmotic pressure, and hence is a function of soil moisture stress.

4. Soil moisture constants
1) Saturation Capacity: amount of water required to fill all the
pore spaces between soil particles by replacing all air held in
pore spaces
2) Field capacity: moisture content of the soil after free drainage
has removed most of the gravity water
3) Permeant wilting point: the soil water content at or below
which plant roots cannot absorb water any more
4) Available moisture: difference in water content of the soil
between field capacity and permanent wilting point
5) Readily available moisture: portion of the available moisture
that is most easily extracted by plants.
It is approximately 75% of the available moisture.

5. 6) Moisture equivalent: percentage of moisture retained in a
small sample of wet soil 1 cm deep when subjected to
centrifugal force 1000 times as greater as gravity, usually for
a period of 30 min.
Moisture equivalent = Field capacity
= 1.8 to 2 permanent wilting point
= 2.7 Hygroscopic coefficient
7) Soil-Moisture deficiency (Field moisture deficiency): water
required to bring the soil moisture content to its field capacity.