Water in soil can be classified into three types based on how tightly it is held: 1) Capillary water held by surface tension in small pores. 2) Gravitational water that drains freely under gravity. 3) Hygroscopic water tightly bound to soil particles. Soil water content is measured using concepts like field capacity, wilting point, and moisture tension. Water moves through soil via saturated, unsaturated, or vapor flow depending on soil moisture levels. Infiltration rate depends on soil properties and moisture conditions.

1. Classification of Water
Prof. S.R. Suryavanshi,
Asst. Professor of Agronomy,
Dr. D.Y. Patil College of Agriculture,
Talsande

2. Water is retained by the soil-particles(on their surfaces)
especially colloidal particles and the pore spaces by the force of
adhesion and cohesion.
 Such water present in the soil and is called as soil-water.
Classification ofSoil Water
Gravitational water
Capillary water
Hygroscopic water
Physical classification Biological classification
Available water
Unavailable water
Super available water
(Drainage water)

4. When is water added to a dry soil either by rain
or irrigation, it is distributed around the soil particles
where it is held by adhesive and cohesive forces; it
displaces air in the pore spaces and eventually fills
the pores.
When all the pores, large and small, are filled,
the soil is said to be saturated and is at its maximum
retentive capacity.

5. Adhesion:
It is the attraction of solid surfaces for
water molecules.
Adhesion is operative only at the solid-
liquid interface and hence the film of water
established by it is very thin.
Cohesion:
It is the attraction of water molecules for
each other.
This force makes possible a marked
thickness of the films of water established by
hydration until they attain microscopic size.

6. Classification of Water
The following re the three main classes of soil water:
A) Capillary water:
Capillary water is that part, in excess of hygroscopic
water, which exists in the pore space of the soil by
molecular attraction.
 Water held by forces of surface tension and
continuous films around soil particles and in the
capillary spaces.
Water left out in capillary pores after excess water
has drained– Held by surface tension – cohesive force
1/3-15 atmp.– Available to plants

7. B) Gravitational water:
 Gravitational water is that part in excess
of hygroscopic and capillary water
which will move out of the soil if
favorable drainage is provided.
 Excess water in soil pores– drains out
due to gravitational force– Not available
for plant growth.
 Water that moves freely in response to
gravity and drains out of the soil.

8. C) Hygroscopic water:
 When an oven dried sample is kept open in the
atmosphere, it absorbs some amount of water
from the atmosphere.
 This is not capable of movement by the action of
gravity or capillary forces.
 Water held tightly to the surface of soil particles
by adsorption forces.
 Water absorbed by a oven dry soil when exposed
to a moist air.
 Held at high tension - tightly held by adhesion
force – water of adhesion 10000-31 atm p., water
not available – permanent wilting point

9. Water contents present in soil under certain standard
conditions
 It represents definite soil moisture relationship and
retention of soil moisture in the field.
The soil moisture tension is measured with
“TENSIOMETER”
Soil Water Content Soil Moisture Content

10.  Maximum water holding
capacity
Field capacity
Maximum Capillary capacity
Moisture equivalents
Permanent wilting point
Hygroscopic coefficient
Available soil-moisture
Air capacity
Total pore volume
While studying soil water and discussing its availability or other
wise to plant, some specific terms called as soil moisture
constants are used and they are as follows :

11. I. Field Capacity (FC):
It is the non-saturated but still very wet soil condition.
Where gravity drainage becomes negligible and only micropores
retain water.
 Factors affecting FC :
a.)Soil-texture
b.)soil structure
c.)type of clay
d.)organic matter content
e.)soil-compaction
f.) Impedition layer

12. I. Permanent Wilting point (WP or 0wp ): Also known as “wilting coefficient”. It
is the soil moisture content at which the plants can no longer be able to meet their
transpiration requirement, become water-stressed .There is still some water in the
soil but not enough to be used by the plants. WPis of two types :
• Temporary wilting point.
• Ultimate wilting point
FACTORS AFFECTING PWP
Soil properties Plant properties
Soil texture
&
structure
Types of clay Organic matter content

14. SATURATION FIELD CAPACITY WILTING POINT
100% Moisture in soil
pores (both macro and
micro)
When water is no longer
drained by gravity
When plants have
extracted as much water
as they can
ombine toCapillarity and surface attraction c
pull more strongly than gravity on:
1) water in “micropores” and
2) water close to the “soil skin”
Fig. : Diagrammatic representation of saturation, field capacity and wiltingpoint

15. This characteristic
curve describes the
relationship between
water tension and
water content for a
specific soil.

16. Appearance of
soil
Type of Soil Soil Moisture
Constant
Moisture Tension
in Atmosphere(in
bar)
Wet soil Gravitational water Maximum water 0.00 (~ 0.001)
Moist soil Available water Field capacity 0.33 (1/3)
Water held in micro
pores
Wilting point 15
Dry soil
Unavailable water
tightly held to the soil
particles
Hygroscoic
coefficient
31
Air dry 1000
Oven dry 10,000
Moisture tension of soil moisture constants

17. Saturated
flow
Unsaturated
flow
Water-vapour
movement
 The major principle of movement of soil-water is that it
is “along the gradient”.
•Wet soil to Dry Soil
•low soil moisture tension to high SMT
•high soil water potential to low soil water potential

18. • water move in the macropores since all of the pores are filled.
• Saturated flow is water flow caused by gravity’spull.
• This water moves at water potentials larger than – 33 kPa.
• Factors affecting saturated flow :
1. Texture
2. Structure
3. Amount of organic matter
4. Temperature
5. Depth of soil to hard pan
6. Pressure
7. Amount of water in the soil
Saturated flow due
to gravity

19. •Macropores full of air
•Micropores = water + air
•Moisture tension gradient creates unsaturated flow
•It is flow of water held with water potentials < -1/3bar.
•Factors Affecting the UnsaturatedFlow
Distribution of
pores
i. Nature of soil Size of pores
i. Soil moisture content : The higher the percentage of water in the moistsoil,
the greater is the suction gradient and the more rapid is thedelivery.
Unsaturated flow
Micropores
Macropores

20. Movement from
The movement of water vapour from soils takes place in twoways:
(a) Internal movement
(b) External movement
There are mainly two soil conditions that affect the water vapourmovement.
i. Moisture regimes
ii. Thermal regime

21.  Method of downward entry or
movement of water into the soil surface
Determines the part of the
precipitation that would become the
surface runoff.
Infiltration is governed by two forces:
a.) Gravity
b.) capillary action

22. Rate at which water enters the soil at the surface.
The rate of infiltration can be measured with Infiltrometer.
It is measured in inches per hour or millimeters perhour.
occurs when supply of water at surface is
not limited.
accumulated depth of water infiltrated
during a given period of time.

24.  Type of soil and its properties
- Porosity and hydraulic conductivity, soil-texture and
structure, soil-temperature
 Moisture content of soil
 Condition of soil surface and its vegetative cover
 rainfall intensity

28. Soil moisture characteristic curves:
Soil moisture characteristic curves (moisture
extraction curves), which are plot of moisture content versus
moisture tension show the amount of moisture a given soil
holds at various tensions.

29. Soil moisture characteristic curves:
 Soil moisture characteristic curve is more strongly
affected by soil texture.
 Greater the clay content, the greater the water content at
any particular suction and more gradual the slope of the
curve.
 In a sandy soil, most of the pores are relatively large and
once these large pores are emptied at a given suction,
only a small amount of water remains

30. Hysteresis:
The relationship between matric potential
and soil moisture can be obtained in two ways:
(1)Desorption, by taking an initially saturated
sample and applying increasing suction to
gradually dry the soil while taking successive
measurements of water content at various
suctions and
(2) Sorption, by gradual wetting an initially dry soil
while reducing the suction.