Different forms of Soil Moisture and factors responsible for soil water retention are discussed

1. Soil Water Plant Relationships
By
M. DHAKSHINAMOORTHY
Professor of soil Science

2. Constituents of soil
AIR 25%
MINERAL
MATTER
45%
WATER 25%
O
M 5%

3. Soil
Plant nutrients
Water/air
Anchorage
Soul
Of
Infinite
Life

4. NUTRIENTS FOR PLANT GROWTH
C
H Non limiting/naturally abundant
O
N Ca
P Major Mg Secondary
K S
Fe Si
Mn Na
Zn Cl
Co Va
Cu Se
Mo
B

5. Soil Water Plant Inter-
related
Soil – 3 phase complex solid, liquid & gas
in 50:25:25
Solid – made up of Minerals, Organic
Matter & Chemical Compounds
Liquid – Water dissolved Minerals &
sol. Organic Matter
Gas – O2 ,CO2 ,N2

6. Composition of Soil &
Atmospheric Air in Percentage
O2 Co2 N2
Soil Air 20.05 29.20
0.25
Atmospheri
c
20.97 0.03 78.03
Air

7. Why study Soil water
Anchorage for plants
Medium for Water & Air Circulation
Reservoir for Water & Nutrients
Space for beneficiary Micro Organisms
Inter relationship between soil pores
and its water holding capacity
Plant water absorption rate

8. Soil Properties
Texture
Definition: Relative proportions of various
sizes of individual soil particles
USDA classification
Very Coarse Sand: 2.0– 1.0 mm
Coarse Sand: 1.0– 0.5 mm
Medium Sand: 0.5 – 0.25 mm
Fine Sand: 0.25 – 0.1 mm
Very Fine Sand: 0.1 – 0.05 mm
Silt: 0.05 – 0.002 mm
Clay: <0.002 mm

9. Soil Texture Continued – International Classification
Coarse Sand: 2.0– 0.2 mm
Fine Sand: 0.2 – 0.02 mm
Silt: 0.02 – 0.002 mm
Clay: <0.002 mm
Textural triangle: USDA Textural Classes
Coarse vs. Fine, Light vs. Heavy
Affects water movement and storage

10. Importance of Texture
Stones & Gravel
<10%  checks evap., Impr. drainage, seepage .
>10%  soil too open, rapid drainage, less water &
nutrient intention
Sand
<40%  soil friable , drainage water & air circulation
optimum
>40%  rapid evap., percolation & water holding
capacity
Good Loamy Sand
30-40% silt
>40% silt  poor drainage
Clay
40-50%  good for dry crops
>50%  unsuitable for irrigated crops

11. USDA Textural
Triangle

12. Broad Textural Classification
Open or light textural soils: these are mainly
coarse or sandy with low content of silt and
clay.
• Medium textured soils: these contain sand,
silt and clay in sizeable proportions, like
loamy soil.
• Heavy textured soils: these contain high
proportion of clay.

13. Textural Classification
(US Bureau of Soils)
Tex. Group SAND % SILT % CLAY %
Sand 80-100 0-20 0-20
Sandy loam 50-80 0-50 0-20
Loam 30-50 30-50 0-20
Silt loam 0-50 50-100 0-20
SCL 50-80 0-30 20-30
Silt C L 0-30 50-80 20-30
Clay loam 20-50 20-50 20-30
Sandy clay 50-70 0-20 30-50
Silty clay 0-20 50-70 30-50
Clay 0-50 0-50 30-100

14. Significant of Soil texture
Characters Sand Loam Silt Clay
Feel Gritty Gritty Silky Cloddy
Internal drainage Excessive Good Fair Fair to Poor
Plant Av. water Low Medium High High
Draw bar pull Light Light Medium Heavy
Tillage Easy Easy Medium Difficult
Run off potential Low Low-Med. Med - High High
Water Detachability High Medium Medium Low
Water Transportability Low Medium High High
Wind erodability High Medium Low Low

15. Soil Structure
Affects root penetration and water intake
and movement

16. Arrangement of soil particles in-situ
Orientation of sand, silt, and clay
Prismatic, columnar, granular and laminar (platy)
Single, massive, aggregate
Affect mechanical properties
Affected by mans action

17. Soil - Types of Structure
Single Grained
} Rapid
Granular, Crumb
Blocky
} Moderate
Prismatic, Cloddy

18. Soil Structure in relation to water movement

19. Role of Structure in Irrigation
Management
Vital role in Soil Air & Water system
In surface soil str., associated with soil tilth,
permeability of Water Air & penetration of
roots
Soil porosity bulk density etc…
Promotes all plant growth factors

20. Bulk Density (ρb) Ms
ρb =
Vb
ρb = soil bulk density, g/cm3
Ms = mass of dry soil, g
Vb = volume of soil sample, cm3
Typical values: 1.1 - 1.6 g/cm3
Ms
Particle Density (ρp) ρp =
Vs
ρP = soil particle density, g/cm3
Ms = mass of dry soil, g
Vs = volume of solids, cm3
Typical values: 2.6 - 2.7 g/cm3

21. Porosity (φ)
volume of pores
φ=
volume of soil
 ρb 
φ = 1 − 100%
 ρp 
Typical values: 30 - 60%

22. Soil Classification
Alluvial soils
 Formed by successive deposition of silt
transported by rivers during floods, in the
flood plains and along the coastal belts.
Alluvial soils textures vary from clayey loam
to sandy loam.
 The water holding capacity of these soils is
fairly good and is good for irrigation.

23. Black soils
 Weathering of rocks such as basalts, traps, granites
 and gneisses.
Found in Maharashtra, MP, AP, Gujarat and TN
Heavy textured with the clay content varying from 40 to 60 %
High water holding capacity but poor in drainage.
Red soils
Formed by the weathering of igneous and metamorphic rock
comprising gneisses and schist’s.
Found in Tamil Nadu, Karnataka, Goa, Daman & Diu,
south-eastern Maharashtra, Eastern Andhra Pradesh, Orissa and
Jharkhand.
The red soils have low water holding capacity and hence
well drained.

24. Laterites and Lateritic soils
 Laterite is a formation peculiar to India and some other tropical countries,
with an intermittently moist climate.
Found in Karnataka, Kerala, Madhya Pradesh, Eastern Ghats of Orissa,
Maharashtra, West Bengal, Tamilnadu and Assam.
 These soils have low clay content and hence possess good drainage
Desert soils
Found in Western Rajasthan, Haryana, and Punjab,
Poor soil development.
Light textured sandy soils and react well to the application of irrigation water.

25. Problem soils
Cannot be used for the cultivation of crops
without adopting proper reclamation measures.
Highly eroded soils, ravine lands, soils on
steeply sloping lands etc. constitute one set of
problem soils.
Acid, saline and alkaline soils constitute
another set of problem soil.

26. Soil Water
Micro Pores Macro Pores
Water retained by
Adhesion [ Solid surface (soil mass)
to Liquid surface (soil water) ]
Cohesion - between Liquid Molecules
Surface Tension - total force acting
in solid liquid air- force pulling
tangentially along the surface of the
liquid

27. Water in Soils
Soil water content
Mw
θm =
Ms
Mass water content (θm)
θm = mass water content (fraction)
Mw = mass of water evaporated, g
(≥24 hours @ 105oC)
Ms = mass of dry soil, g

28. Volumetric water content (θv)
Vw
θv =
Vs
θV = volumetric water content (fraction)
Vw = volume of water
Vs = volume of soil sample
At saturation, θV = As θm
As = apparent soil specific gravity = ρb/ρw
(ρw = density of water = 1 g/cm3)
As = ρb numerically when units of g/cm3
are used

29. (cm3)
Equivalent Depth
(g) (g) (cm3)

30. Coarse Sand Silty Clay Loam
Dry Soil
Gravitational Water
Water Holding Capacity
Available Water
Unavailable Water

31. Soil Water Potential
Description
Measure of the energy status of the soil water
Important because it reflects how hard plants must
work to extract water
Units of measure are normally bars or atmospheres
Soil water potentials are negative pressures (tension
or suction)
Water flows from a higher (less negative) potential
to a lower (more negative) potential

32. Soil Water Potential
Components
ψt = ψ g + ψ m + ψ o
ψt = total soil water potential
ψg = gravitational potential (force of gravity pulling
on the water)
ψm = matric potential (force placed on the water
by the soil matrix – soil water “tension”)
ψo = osmotic potential (due to the difference in
salt concentration across a semi-permeable
membrane, such as a plant root)
Matric potential, ψm, normally has the greatest
effect on release of water from soil to plants

33. Soil Water Release Curve
Curve of matric potential (tension) vs. water
content
Less water → more tension
At a given tension, finer-textured soils retain
more water (larger number of small pores)

34. Matric Potential and Soil Texture
The tension or suction created by small capillary tubes
(small soil pores) is greater that that created by large
tubes (large soil pores). At any given matric potential
coarse soils hold less water than fine-textured soils.
Height of capillary
rise inversely related
to tube diameter

35. Soil Moisture Tension
1 Atmosphere = 1036 cm Water Column
(or)
76.39 cm of Hg
1 Bar = 1023 cm Water Column

36. Soil Moisture Tension
Relationship
Height Water Atmosphere pF
(Schofield)
Colm. in cm. (Bars)
1 1/1000 0
10 1/100 1
100 1/10 2
346 1/3 2.54 (F.C)
1000 1 3
10000 10 4
15849 15 4.2 (P.W.P)
31623 31 4.5 (H.COEFF)

37. Classification of Soil Water
Gravitational water
– Excess water in soil pores
– drains out due to gravitational force
– Not available for plant growth
Capillary water
– Water left out in capillary pores after excess water has drained
– Held by surface tension – cohesive force 1/3-15 atmp.
– Available to plants
Hygroscopic water
– 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 atmp., water not available – permanent wilting
point

39. Soil water constants
Soil water proportions which dictate whether the water
is available or not for plant growth.
Saturation capacity: Water content of the soil when all the pores
of the soil are filled with water. (Maximum water holding capacity)
Soil moisture tension almost equal to zero.
Field capacity: Water retained by an initially saturated soil
against the force of gravity.
At field capacity, the macro-pores of the soil are drained off, but water is
retained in the micropores.
Soil Moisture tension at field capacity varies from 1/10 (for clayey soils)
to 1/3 (for sandy soils) atmospheres.

40. •Field Capacity (FC or θ fc)
–Soil water content where gravity drainage becomes
negligible
–Soil is not saturated but still a very wet condition
–Traditionally defined as the water content corresponding
to a soil water potential of 2.54 (PF)
•Permanent Wilting Point (WP or θ wp)
–Soil water content beyond which plants cannot recover
from water stress (dead)
–Still some water in the soil but not enough to be of use to
plants
–Traditionally defined as the water content corresponding
to -15 bars of SWP (pF 4.2)

41. Permanent wilting point
As the Plants extract water, the moisture content
diminishes and the negative (gauge)
pressure increases. At one point, the plant cannot extract
any further water and thus wilts.
Temporary wilting point:
this denotes the soil water content at which the plant
wilts at day time, but recovers during night or when
water is added to the soil.
Ultimate wilting point:
The plant wilts and fails to regain life even after
addition of water to soil.

42. Available Water
Definition
Water held in the soil between field capacity and
permanent wilting point
“Available” for plant use
Available Water Capacity (AWC)
AWC = θfc - θwp
Units: depth of available water per unit depth of
soil, “unitless” (in/in, or mm/mm)
Measured using field or laboratory methods

43. Field capacity - θ

44. Permanent wilting point - θpwp

47. Fraction available water depleted (fd)
 θfc − θv 
fd =  
 θfc − θwp 
(θfc - θv) = soil water deficit (SWD)
θv = current soil volumetric water content
Fraction available water remaining (fr)
 θv −θwp 
fr =  
 θfc −θwp 
(θv - θwp) = soil water balance (SWB)

48. Total Available Water (TAW)
TAW = (AWC) (Rd)
TAW = total available water capacity within the
plant root zone, (inches)
AWC = available water capacity of the soil,
(inches of H2O/inch of soil)
Rd = depth of the plant root zone, (inches)
If different soil layers have different AWC’s, need
to sum up the layer-by-layer TAW’s
TAW = (AWC1) (L1) + (AWC2) (L2) + . . . (AWCN)
(LN)
- L = thickness of soil layer, (inches)
- 1, 2, N: subscripts represent each successive soil
layer

50. Range of available water
holding capacity of soil
% moisture based on Depth of
dry wt. of soil available water
Soil texture
FC PWP cm per meter
depth of soil
Sand 6-12(9) 2-6 (4) 6-10(8)
Sandy loam 10-18(14) 4-8 (6) 9-15(12)
Loam 18-28(22) 8-12 (10) 14-20(17)
Clay loam 23-31(27) 11-15 (13) 17-22(19)
Silty clay 27-35(31) 13-17 (15) 18-23(21)
Clay 31-39(35) 15-19 (17) 20-25(23)

51. Factors that change AWC
Organic matter content (increase 10 %)
Structure (+/- 10 %)
Good: granular, blocky, prismatic
Bad: platy, massive, single grain
Compaction (decrease 20 %)
Restrictive layers (increase above 10 %)
Depth (5 % per 30 cm depth)

52. Gravity vs. Capillarity
Horizontal movement
Vertical movement
due to capillarity
due largely to gravity

53. Water Infiltration
Def’n.: the entry of water into the soil
Influencing Factors
Soil texture
Initial soil water content
Surface sealing (structure, etc.)
Soil cracking
Tillage practices
Method of application (e.g., Basin vs. Furrow)
Water temperature

54. Cumulative Infiltration Depth vs. Time
For Different Soil Textures

55. Infiltration Rate vs. Time
For Different Soil Textures

56. Water Infiltration Rates and Soil Texture

57. Infiltration rate for different
soil textures
Soil Texture Basic infiltration rate (cm/hr)
Sand 2.5-25
Sandy loam 1.3-7.6
Loam
0.8-2.0
Clay loam
Sandy clay 0.25-1.5
Clay 0.03-0.5
0.01-0.1

58. Soil Infiltration Rate vs. Constant
Irrigation Application Rate

59. Soil Infiltration Rate vs. Variable
Irrigation Application Rate

61. Rooting Characteristic of
Plants
Shallow Mod. deep Deep Very deep
[60cm] [90cm] [120cm] [180cm]
Rice Wheat Maize Sugarcane
Potato Tobacco Cotton Citrus
Onion Castor Sorghum Grape vine
Cabbage Groundnut Tomato Sunflower
Cauliflower Chilli Pearl millet Tree crops

63. Water requirements of crops
Sl. Crop Duration (days) Water Req. (mm)
No.
1 Rice 135 1200
2 Groundnut 105 500
3 Sorghum 100 500
4 Maize 110 500
5 Sugarcane 365 2000
6 Ragi 100 400
7 Cotton 165 600

64. Points to remember
Cropped field acts as soil – water reservoir
Residual soil moisture and shallow water
table contributes to crop water need
Water added in excess lost as – deep
percolation - lead to nutrient loss, water
logging and salinity
Soils classified based on texture
Water retention capacity differ with soils

65. FC-upper limit of soil water
storage
Soil water content between FC and
PWP- is total ASW for plant growth
Crops differ in ability to withstand
diff. depletion of ASW
The growth stage and root
characteristics mainly contribute to
withstand S-W depletion

66. ET losses influenced by duration of
crops, rate of growth , Pl. popln. , Pl. ht
and moisture extrn pattern by roots
Rate of loss of water from cropped field
depends on climatic factor
Solar radiation , temp., humidity and
wind important climatic factors
influencing ET rate
Total ET value of crops varies based on
weather conditions

68. Soil Water Measurement
Gravimetric
Measures mass water content (θm)
Take field samples → weigh → oven dry →
weigh
Advantages: accurate; Multiple locations
Disadvantages: labor; Time delay
Feel and appearance
Take field samples and feel them by hand
Advantages: low cost; Multiple locations
Disadvantages: experience required; Not
highly accurate

69. Soil Water Measurement
Neutron scattering (attenuation)
 Measures volumetric water content (θv)
 Attenuation of high-energy neutrons by hydrogen
nucleus
 Advantages:
 samples a relatively large soil sphere
 repeatedly sample same site and several depths
 accurate
 Disadvantages:
 high cost instrument
 radioactive licensing and safety
 not reliable for shallow measurements near the soil
surface

70. Soil Water Measurement
Neutron Attenuation

71. Soil Water Measurement
Tensiometers
Measure soil water potential (tension)
Practical operating range is about 0 to 0.75 bar
of tension (this can be a limitation on medium-
and fine-textured soils)
Electrical resistance blocks
Measure soil water potential (tension)
Tend to work better at higher tensions (lower
water contents)
Thermal dissipation blocks
Measure soil water potential (tension)
Require individual calibration

72. Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in)
Based on Root Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)

73. Electrical Resistance Blocks & Meters