1. The document discusses soil-water-plant relationships and various concepts related to how water moves through and is stored in soil. 2. Key concepts covered include the classification of soil water, soil water constants like field capacity and permanent wilting point, and how physical properties of soil like texture and structure influence water movement and retention. 3. Diagrams and equations are provided to illustrate volume and mass relationships of water, solids, and air in soil.

1. CHAPTER 2
SOIL-PLANT-WATER
RELATIONSHIP
1Irrigation Engineering

2. Learning Objectives
At the end of this chapter the students will be able
to:
 Describe the Soil –water relationship
 List and discuss about the Classification of soil
water
 List and explain the Soil water constants
 Define and Explain about Infiltration
2Irrigation Engineering

3. Contents of the chapter
2. Soil-water-plant Relationship
2.1 Introduction
2.2 Soil –water relationship
2.3 Classification of soil water
2.4 Soil water constants
2.5 Infiltration
3Irrigation Engineering

4. 2.1 Introduction
 Soil plant water relationships relate to the properties of soil
and plant that affect the movement, retention and use of
water.
 Soil serves as a storehouse of water.
 Irrigation water and rain water become available to plants
through the soil.
 The water stored in the soil pores within the root zone
constitutes the soil water.
 An understanding of the relation ship between soils and
water is essential to make the most efficient use of water in
crop production.
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5. 5Irrigation Engineering

6. Soil – A system
 Soil is a three-phase system consisting of solid, liquid and gases.
 The minerals and organic matters in soil constitute the solid phase.
 Water forms the liquid phase
 The soil air forms the gaseous phase
 Soil serves as a medium of plant growth.
 Soil components when exists in proper amounts offer a favorable
condition for plant growth
 Uses of soil
 Uses of water
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7. Soil – A system
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8. Soil Physical Properties Influencing Soil – Water
Relationship
 The important physical properties of soil affecting the soil-
water relationship relate to soil characteristics that governs
 entry of water in to the soil during irrigation or rain,
 water movement through the soil,
 retention of water by the soil and
 availability of water to crop plants.
 The two main physical properties of soil influencing soil-
water relationship are
 soil texture
 soil structure
8Irrigation Engineering

9. Soil Physical Properties Influencing Soil –
Water Relationship
 Soil texture refers to the relative sizes of soil particles in a
given soil.
 The sizes of particles making up a soil determine its texture.
 Soil structure refers to the manner in which soil particles are
arranged in groups or aggregates.
 The structure of soil is dynamic and it changes constantly
with soil management practices.
9Irrigation Engineering

10. Volume and Mass Relationships of Soil Constituents
 Soil has solids, liquid and air and their relative masses and
volumes are required for proper soil and crop management.
 A schematic diagram of soil shown below may be useful to
define the volume and mass relationship of the three soil
phases.
 The diagram shows the presence of the three phases in
relative proportions both in masses and volumes
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11. Volume and Mass Relationships of Soil Constituents
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12. Cont…
12
Air
Water
Solids
Va
Vw
Vp
Vt
Vs
Mt
Ma
Mw
Ms
Volume Relations Mass Relations
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13. Dry Bulk Density
 Dry Bulk Density is the weight of oven dry soil per unit
volume of soil.
 Typical values: 1.1 - 1.8 g/cm3
13
3
/ cmgin
V
M
t
s
dry 
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14. Apparent Specific Gravity
 Apparent Specific gravity refers to the ratio of dry
bulk density of soil to that of density of water.
 It is dimensionless /unit less quantity/.
14
w
dry
WaterofDensity
DBDsoilofDensityBulkDry
Asg



,
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15. Particle Density
 Particle density denotes the mass of soil solid per unit
volume of soil solids.
 It is also called true density or true specific gravity of soil.
 Typical values: 2.6 - 2.7 g/cm3
Average :2.65 g/cm3
 What is the main difference b/n DBD and PD
15
s
s
p
V
M

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16. Porosity
 Porosity can be defined as the ratio of the volume of
pores/voids to the total volume.
 It is influenced by texture and structure of the soil.
 The more finely divided are the individual soil particles, the
greater is the porosity
 Typical values: 30 - 60%
 R/n b/n Porosity and SMC
16
100*)1(1
st
s
t
st
swa
wa
t
p
D
DBD
v
v
V
VV
VVV
VV
V
V
n 





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17. Void Ratio
 Void ratio refers to the ratio of the volume of
pores to the volume of soil solids.
17
1




s
t
S
st
s
wa
s
p
V
V
V
VV
V
VV
V
V
e
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18. Soil Wetness
 Soil wetness refers to the relative water
content in the soil.
 It is expressed on
I. Weight basis (Mass Wetness),
II. Volume basis (volume wetness) and
III. Depth basis.
18

19. 1. Mass Wetness
 It is the ratio of mass of water to mass of soil
solids.
 It is commonly called gravimetric soil
moisture content on weight basis.
19
Ms
Mw
solidofMass
waterofMass
WetnessMass m  
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20. 1. Mass Wetness
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21. 2. Volume Wetness
 It is the ratio of volume of water to total volume of soil
 Volume wetness, = Mass wetness. x Apparent specific
gravity
21
  100100 )
VV
V
(x
V
V
v
vSMC
,%basis
volumeoncontentwaterSoil
PS
w
t
W
v








  ASG
v
vSMV mv * 
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22. Cont…
22
hrsoilofVolumeCoreofVolume 2

t
s
dry
V
M

w
dry
Asg



    Asg
w
wSMC
v
vSMC
svolumebasion
contentwaterSoil
*
,%






whr
WW
 2
21 

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23. Cont…
23
hrsoilofVolumeCoreofVolume 2

t
s
dry
V
M

w
dry
Asg



    Asg
w
wSMC
v
vSMC
svolumebasion
contentwaterSoil
*
,%






whr
WW
 2
21 

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24. Equivalent depth of water
 Equivalent depth of water is the volume of water per unit
land area.
 It refers to the depth of water formed if the water existing in
the soil is squeezed and collected without affecting the soil
structure.
 The soil water exists distributed in the soil pores in a given
volume of soil.
24
L
A
AL
d v
v



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25. Cont…
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 Solved problems

26. Cont…
26Irrigation Engineering
 Solved problems

27. 1.Soil Water
Definition:
–Water spread over the soil
• irrigation or rainfall ….. absorbed by soil pores:
• soil water or
• soil moisture
• The water above the water table
Classification of soil water
a. Gravitational: moves freely in response to gravity
b. Capillary: held by surface tension in the pore spaces
c. Hygroscopic: held tightly to the surface of the grains
27Irrigation Engineering

28. a. Gravitational/Free water
 It is the water in the soil macro pores that moves down
ward freely under the influence of gravity.
 Gravitational water is not available to plants because of
the rapid disappearance of the water from the soil.
 The upper limit or maximum level of gravitational water is
when the soil is saturated.
 For coarse sandy soil gravitational water will drain in one
day but for fine clay soil it will drain with in 2 to 3 days.
 The water tension at this stage is 1/3 atmosphere or less.
28Irrigation Engineering

29. b. Capillary Water
 Capillarity water refers to water retained by soil after
cessation of the down ward movement of water
(gravitational water).
 It is water held by forces of surface tension and
continuous film around soil particles and in capillary
spaces.
 The water is held at a tension of 1/3 to 31 atm. and
much of it is in fluid state.
 The capillary water supplies the whole or largest part
of water available to plants.
29Irrigation Engineering

30. c. Hygroscopic Water
 Hygroscopic water refers to the soil water held tightly to
the surface of soil particles by adsorption forces.
 It occurs as a very thin film over the surface of soil
particles.
 Held tenaciously at a tension of 31 atmospheres or
above.
 The water is held by adhesive force.
 Much of it is non-liquid and moves as vapor.
 It is unavailable water to plants.
30Irrigation Engineering

31. Figure: Diagrammatic Representation of Kinds of Soil Water
31
Tension of thinnest film
about 10000 atm
Soil Solids
Solid-liquid interface
Hygroscopic Water
(Water of Adhesion)
Capillary
Water
(Water of
Cohesion)
thinnest film
about 10000
atm
Zone of progressive thickening
of water film
Tension of thickest
film around 1/3 atm
Gravitational water
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32. 2. Soil Water Constants
 Soil water content/ soil moisture varies constantly under
natural conditions.
 In order to describe the soil water status under certain
conditions of water equilibrium some terms referred to as
soil water constants are used.
 The soil water constants include:
1. Saturation Capacity
2. Field Capacity
3. Permanent wilting point
4. Oven dry soil
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33. a. Saturation Capacity
 Saturation Capacity is the percentage water content
of a soil fully saturated with water & all its pores
completely filled with water under restricted
drainage.
 It is also called maximum water holding capacity.
 Complete saturation occurs in surface soils
immediately after heavy irrigation or rainfall.
 The soil water is in free state and the tension at this
stage is zero.
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34. Saturation Capacity
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35. b. Field Capacity
 Field capacity of a soil is the moisture content after
gravitational water has:
-drained off and/or has become very slow and
-the moisture content of the soil become more stable.
 This stage is reached when the excess water from a
saturated soil after irrigation or rainfall has fully
percolated down.
 Refers to the moisture content of a soil 1 to 2/3 days after
heavy rainfall or irrigation depending up on the soil
texture.
 Soil water tension at field capacity ranges from 0.1 to 0.33
atm
 It is the highest point of available water range
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36. Field Capacity
36Irrigation Engineering

37. c. Permanent Wilting Point (PWP)
 It refers to the soil moisture content at which plants do not
get enough water to meet the transpiration demand and
remain wilted unless water is added to the soil.
 It is the moisture content of the soil when plants growing
on that soil starts to show signs of wilting due to moisture
stress.
 Permanent wilting point is considered as the lowest limit of
available water range.
 Soil water tension at PWP ranges from 7 to 32 atmosphere
depending on several factors
37Irrigation Engineering

38. Permanent Wilting Point (PWP)
38Irrigation Engineering

39. d. Oven Dry Soil
 Oven dry soil is used to describe the soil water status when
a soil sample is dried at 1050 c in a hot air oven until
sample loses no more water i.e., for 24 hrs.
 The equilibrium tension of soil water at this stage is 10,000
atmosphere.
 All estimations of soil water content are based on the oven
dry weight of the soil and the soil at this stage is
considered to contain zero amount of water.
39Irrigation Engineering

40. Oven Dry Soil
40Irrigation Engineering

41. Fig Schematic Representation of Soil Water Constants
and Soil water Ranges
41
Oven dry/Absolute wilting
Permanent Wilting Point
Field Capacity
Saturation
Gravitational Water
Capillary Water
Hygroscopic Water
Unavailable
Water
Available Water
Unavailabl
e Water
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42. Determination of Field Capacity (FC)
1.Gravimetric Method – Field Method
2.Pressure plate – Laboratory Method
1. Gravimetric Method – Field Method
a. The soil surface is cleaned of weeds to
-prevent the possible transpiration loss
-spreading a black polythene sheet over the area prevents surface
evaporation.
b. Make the soil fully saturated.
c. allowing the water to drain for few days depending on the soil class
d. Soil Samples are taken from the desired layers and the water
content is determined
42Irrigation Engineering

43. Typical Values of Soil Moisture at FC and PWP
43Irrigation Engineering
Soil Textural
Class
ρb
in g/cm3
θsat
(Mass %)
θsat
(Vol %)
θFC
(Mass
%)
θFC
(Vol %)
θPWP
(Mass
%)
θPWP
(Vol %)
Sand 1.65 23.03 38 9.09 15 4.24 7
Sandy loam 1.5 30.67 46 14.00 21 4.00 6
Loam 1.4 33.57 47 22.14 31 7.14 10
Clay loam 1.35 32.59 44 29.63 40 19.26 26
Silty clay 1.3 39.23 51 32.31 42 19.23 25
Clay 1.25 43.20 54 36.00 45 21.60 27

44. 3.Soil Moisture Ranges
 The soil water ranges are the available water range and
unavailable water range.
Available Water
 The water held by soil between field capacity and permanent
wilting point.
 It is available to plants and is termed as available water.
 It comprises the greater part of capillary water.
44Irrigation Engineering

45. Cont…
 In-order to calculate the amount of available water the following
parameters must be known.
 the soil moisture content in weight basis at FC and PWP
 the dry bulk density of soil and apparent specific gravity
 the soil moisture content in volume basis at FC and PWP
 the effective root zone depth
45
  RZDASGvpwpVfcrzDAsgvPWPvFCAvailable
vpwpvfcvPWPvFC
wpwpwfcwPWPwFC
**))()((*%%
)()(%%
)()(%%






dAWbasis,depthinwater
v%AWbasis,volumeinwaterAvailable
w%AWbasis,weightinwaterAvailable
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46. Soil Moisture Ranges
46Irrigation Engineering

47. Cont…
Unavailable water
 There are two situations at which soil water is not
available to most plants
 When the soil water content falls below the
permanent wilting point.
 When the soil water above the field capacity and held
at a tension between zero and 1/3 atmosphere.
47Irrigation Engineering

48. Root Zone Depth
 Root zone depth is the maximum depth below the surface of soil
from which a particular crop derives water for use and develops its
root system.
 Crops uses water for its growth in different proportions from the
root zone depth.
 Root zone depth in irrigated fields are dependent on soil types,
crop types, distance of water table from the ground surface and the
amount of water applied during irrigation.
 In general crop plants develop most of their roots and derive most
of their moisture supplies from the upper portion of the root zone
depths.
48Irrigation Engineering

49. Root Zone Depth
49Irrigation Engineering

50. 4.Measurement of Soil Moisture Content
 Soil moisture content refers to the amount of water stored
and present in the soil at the time of measurement.
 The significance of measuring soil moisture content are as
under:
 For proper scheduling/design of irrigations
 For estimating the amount of water to apply in each
irrigation
50Irrigation Engineering

51. Depth of Available Water
 The available water can be expressed in weight basis volume
basis or as a depth of water
dw= depth of water
d= the soil depth
S =is the apparent specific gravity of the soil
51Irrigation Engineering
mdSdw **
w
ss
w
W
S



w
ss
w
W
S



soilofweight
waterofWeight
m 

52. Cont…
 Depth of water at (F.C)
 Depth of water at (PWP)
 So, depth of available water,
 The depth of available water per meter depth of soil
 The allowable depletion value (p) varies with the type of crop
and evaporative demand.
If plant is matured enough use p<0.5
If plant is at initial stage use p>0.5
52Irrigation Engineering
)(**).(** mfcrzrzfc dSCFdSd 
  )(**..** mpwprzrzpwp dSPWPdSd 
)(**)...*(* )()( mpwpmfcrzrzw dSPWPCFdSd  
  )(*..* )()( vpwpvfcw SPWPFCSd  
 PWPCFdSpRAWWaterofDepthAvailableadily ...**,Re 

53. Cont…
 If the water content of the soil at the lower limit of the
readily available water is Mo ,
 the readily available depth of water,
Where,
Or
The moisture content mo is also called the optimum moisture
content or critical point
53Irrigation Engineering
wateravailableadilyCFmo Re. 
  oMmfcrzdSMFCdS   )(0 (****
 PWPCFpCFmo ...*. 

54. 54
Cont…
Field Capacity MC
Available M.C(Capillary
Water)
Non- Available
MC(Hygroscopic water)
Optimum MC
Permanent wilting point MC
Oven dry level
Readily
Available
Water
Moisture Content
Of soil
Time
Irrigation Engineering

55. Cont…
 It is necessary to note that the soil moisture is not
allowed to be depleted up to the wilting point, as it
would result in considerable fall in crop yields.
 The optimum level up to which the soil moisture
may be allowed to be depleted in the root zone
without fail in crop yields has to be worked out with
experimentation.
55Irrigation Engineering

56. Cont…
 Irrigation water should be supplied as soon as the moisture
falls up to the optimum level (fixing irrigation frequency)
and its quantity should be just sufficient to bring the
moisture content up to its field capacity, making allowance
for application losses (fixing depth).
 The optimum soil water regime means the range of
available soil water in which plants do not suffer from water
stress and all the plant activities occur at an optimal rate.
56Irrigation Engineering

57. Cont…
 The optimum soil water range is also called Readily Available
Water, RAW.
 The readily available water is that portion of the total available
water, which can be easily extracted by plant roots. It differs from
one crop to another.
 It has been found in practice that about 20- 75% of the available
water is readily available .
 The optimum level or critical soil water level or allowable depletion
value (p) up to which the soil moisture may be allowed to be
depleted in the root zone with out fall in crop yield has to worked
out for every crop and soil by experimentation.
57Irrigation Engineering

58. Cont…
 The allowable depletion value (p) varies with the type of
crop and evaporative demand.
 Water will be utilized by the plants after irrigation and soil
moisture will start falling.
 It will be recouped or refilled by a fresh dose of irrigation as
soon as the soil moisture reaches the optimum level.
 This sequence of operation can be shown in the following
figure.
58Irrigation Engineering

59. Cont…
59
Readily
Available
Moisture
Available
Moisture
Irrigation Interval/
frequency
PWP level
/ Pwp Mc
Optimum MC
/critical levelMoisture
content of
soil
Field Capacity
level
Time
Irrigation Engineering

60. Infiltration of Water into Soils
 Infiltration is the entrance or movement of water from the
surface into the soil.
 It refers to the vertical entrance of water from the surface in
to the soil.
 The infiltration characteristics of the soil is one of the
dominant variables influencing irrigation.
 Infiltration rate is the soil characteristics determining the
maximum rate at which water can enter the soil under
specific conditions.
 Accumulated infiltration or cumulative infiltration is the total
quantity of water that enters the soil in a given time.
60Irrigation Engineering

61. Measurement of Infiltration
 Cylinder Infiltrometer
 Cylinder infiltrometer are metal cylinders which are formed of 2mm
rolled steel sheet metal.
 Two cylinders are mostly used, one outer and the other inner cylinder.
 The most commonly used cylinders are of the following dimensions.
 Inner Cylinder
 Diameter = 30cm
 Height = 25 cm
 Outer Cylinder
 Diameter = 60 cm
 Height = 25 cm
61Irrigation Engineering

62. Cont…
 In this method the infiltration characteristics of soils can be
determined by pounding water in a metal cylinder installed
on the field surface and observing the rate at which water
level is lowered in the cylinder.
 Since by definition infiltration is the vertical entrance of
water from the surface in to the soils, the lateral movement
of water should be minimized.
 This can be achieved by using double ring cylinder
infiltrometer. The lateral movement of water from the inner
cylinder is avoided or minimized by pounding water in an
outer/ guard cylinder of buffer area around the inner
cylinder.
62Irrigation Engineering

63. Cont…
63Irrigation Engineering

64. Cont…
64Irrigation Engineering