This document discusses soil water and defines key terms related to soil moisture content and movement. It defines classifications of soil water, soil moisture constants like field capacity and wilting point, saturated and unsaturated water flow, infiltration, and factors that influence soil water movement. Diagrams show the relationship between soil moisture tension and water content. Soil water is essential for plant growth and understanding these concepts helps determine moisture levels in soil.

1. SSC-411,4(0+4) : Village Attachment
DEPARTMENT OF HORTICULTURE,
INSTITUTE OF AGRICULTURAL SCIENCES,
RAJIV GANDHI SOUTH CAMPUS,
BANARAS HINDU UNIVERSITY,
BARKACHHA,MIRZAPUR

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 of Soil Water
Gravitational water
Capillary water
Hygroscopic water
Physical classification Biological classification
Available water
Unavailable water
Super available water
(Drainage water)

4. Water contents present in soil under certain standard
conditions
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

5.  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 :
 Maximum water holding
capacity
Field capacity
Maximum Capillary capacity
Moisture equivalents
Permanent wilting point
Hygroscopic coefficient
Available soil-moisture
Air capacity
Total pore volume
Continued.
.

6. I. Field Capacity (FC or 0fc):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
Table : Influence of three soil textures on soil moisture constants

7. 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. WP is 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

9. 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
Capillarity and surface attraction combine to
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 wilting point

10. Plant available water
Gravitational water
(Drainable)
10-30micron
Oven-dry
Air-dry
Wilting point Field capacity Saturation
Unavailable water
~0.2micron
Hygroscopic coefficient
-10000
bar
-1000
bar
-31 bar -15 bar -0.33bar -0.00 bar
Capillary waterHygroscopic
water
Increasing soil water constants
Increasing water potential
*-1000 bar to -300 bar depending on humidity

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

12. 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
Hygroscopic
coefficient 31
Air dry 1000
Oven dry 10,000
Table II : Moisture tension of soil moisture constants

13. 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

14. • water move in the macropores since all of the pores are filled.
• Saturated flow is water flow caused by gravity’s pull.
• 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

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

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

17.  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

18. 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 per hour.
occurs when supply of water at surface is
not limited.
accumulated depth of water infiltrated
during a given period of time.

20.  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

23.  Percolation
 Water intake
 Overland flow
 Interflow (or
Subsurface flow)
 Seepage
 Leaching
Permeability

25.  Soil water is very essential for the proper plant growth and
development.
 Soil moisture constants are necessary to determine the moisture present
in soil under any certain condition and at any instant of time.
 Soil water movement is mainly of three types viz. saturated, unsaturated
and water vapour movement.
 Infiltration and other modes of water entry into the soil contributes to
the formation of water reservoir in soil.
 Higher the rainfall rate, lesser the infiltration and higher the runoff.