Chapter 2.pdf

2.Soil-Water-Plant Relationships
Ceng5082
Mengistu .Z (MSc in Hydraulic Engineering )
Lecturer @ Hydraulic and Water Resources Engineering department
Mizan Tepi university
Email: mengistu.zantet@gmail.com or
mengistuzantet@mtu.edu.et
P.O.Box: 260
Tepi, Ethiopia
03-May-22
lecturer@ Hydraulic and water
resources Engineering Department
1

General aspects of Soil-Water-Plant Relationships
2.1 Introduction
2.2 Soil-Water Potential
2.3 Moisture Stress of Plants
2.4 Soil Moisture and Plant Growth
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2.1 Introduction
Soils are the storehouse of water, nutrients and air
which are necessary for plant growth.
 Plants need water – soil stores this water –
atmosphere provides some amount of energy for the
water to be withdrawn by plants.
 The water stored in the soil pore constitutes the soil
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Soils are the storehouse of water
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Soils are the storehouse of nutrients
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Soils are the storehouse of air
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Soil is the natural material that covers the land surface of
the earth. Soils have profiles.
They are formed by a combination of natural processes
under the interrelated influences of climate, vegetation,
relief (including hydrology), parent material, and time.
 Soil is a three-phase system constituting solid, liquid and
gases.
 The minerals and organic matters in soil together
constitute the solid phase
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Three phase diagram of a soil profile
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Air space
Soil particles
Water film
Figure 2.1: Diagram showing cross section of soil

Soils can be classified in many ways
1) On the basis of size
a) gravel
b) sand
c) Silt and
d) Clay
2) On basis of geological process of formation (or origin)
a) Residual soils:
b) Alluvial soils:
C) Aeolian soils:
d) Glacial soils:
e)Colluvial soils: f)Volcanicoil:
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Classifications On the basis of size
gravel
sand
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Silt Clay
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On basis of geological process of formation (or origin)
Residual soils: Alluvial soils:
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Aeolian soils:
Glacial soils:
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The fraction of total soil volume
occupied by solid soil and that occupied
by the pore space has significant effect
in limiting ability of soil to store water.
A typical silt loam soil contains about
50% soil solids, 30% water and 20% air.
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silt loam soil
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2.2 Moisture Stress of Plants
 Plants absorb water mainly through their roots and use only
1.0 to 1.5% of the volume of water absorbed for building
their vegetative structures and performing various
physiological and biochemical activities.
 Then where does the rest 98.5 % - 99% of water absorbed
goes …???
 Study of the process of water transport in soil, into plants
and from soil and plants to the atmosphere are the basics of
irrigation practice
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Important Question while thinking of Irrigation is
when to irrigate and how much to irrigate ?
Thus the knowledge of soil –water –atmosphere
relationship required.
Both excess and deficit soil water affects plant
growth and hence result in yield reduction.
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On the basis's agricultural considerations, soil
has the following characteristics are
1) Physical properties of soil,
2) Chemical properties of soil, and
3) Soil-water relationships.
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Soil physical Characteristics
Moisture retention
Storage, and transport
Availability to plants as well as mechanisms of
water absorption
Conduction and transpiration by plants.
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1.Physical Properties of Soil
The permeability of soils with respect to air, water, and
roots are as important to the growth of crop as an
adequate supply of nutrients and water.
The permeability of a soil depends on the porosity and
the distribution of pore spaces which, in turn, are
decided by the texture and structure of the soil.
1) Soil Texture
2) Soil Structure
3) Depth of Soil
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1) Soil Texture
 Soil texture refers to the relative size of soil particles in a given
soil (sand, silt and clay ).
 Most soils contain a mixture of sand size ranging from (0.05 to 1.00
mm in diameter), silt (0.002 to 0.05 mm) and clay (smaller than 0.002
mm)
 The texture of a soil affects the flow of water, aeration of soil,
and the rate of chemical transformation all of which are important
for plant life.
 The texture also determines the water holding capacity of the
soil03-May-22
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Soil Texture
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Size limits of soil separates
Soil separates Particle diameter (mm)
USDA ISSS
Very course sand 2.0 - 1.0 -
Course sand 1.0 - 0.5 2.0 - 0.2
Medium sand 0.50 - 0.25 -
Fine sand 0.25 - 0.10 0.20 - 0.02
Very fine sand 0.10 - 0.05 -
Silt 0.05 - 0.002 0.02 - 0.002
Clay < 0.002 < 0.002
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Fine sand
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Moisture Status: 25-50% 50-75% 75-100%
Loam soils (sandy clay loam , loam , silt loam)

Clay soils
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Moisture Status: 25-50% 50-75% 75-100%

 The relative proportion of soil separates is
determined by mechanical analysis-Sieve
Analysis.
 Triangular classification is then utilized to
differentiate the soil texture.
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USDA Triangular soil Textural classification Chart
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Fig. 2.3: USDA textural triangle
72%
3%
25%
Sandy Clay Loam

2) Soil Structure
 Arrangement and organization of soil particles in the soil
and the tendency of individual soil particles to bind together
into aggregates.
 The arrangements of soil aggregates give soil its structure.
 Grouping of particles into structural units occur in all soils.
 However, the strength of the bonds, the size and the shape
of the structural units and the proportion of the soil particles
involved in the units differ considerably among soils.
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Soil Structure
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Soil Structure development is influenced by:
 Amount and type of clay,
 exchangeable ions on the clay.
 Amount and type of organic matter.
 Presence of iron and aluminum oxides (cementing agents)
 Binding between organic and inorganic
 compounds (aluminum oxides, cations, clays).
 Vegetation: produces OM, roots act as
 holding soil together, and protects soil surface.
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The overall quality of the soil structure
may be evaluated in terms of its:
Porosity,
Aggregation,
 Cohesiveness,
Permeability for water or air.
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The overall quality of the soil structure may be evaluated in terms
of its:
Aggregation,
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The overall quality of the soil structure may be evaluated in terms
of its:
Cohesiveness, Permeability for water or air.
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3) Depth of Soil
The importance of having an adequate depth of soil
for storing sufficient amount of irrigation
water and providing space for root penetration
cannot be overemphasized.
Shallow soils require more frequent irrigations and
cause excessive deep percolation losses when
shallow soils overlie coarse-textured and highly
permeable sands and gravels.
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Depth of Soil
deep soils shallow soils
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Cont.…
On the other hand, deep soils would generally require less
frequent irrigations, permit the plant roots to penetrate
deeper, and provide for large storage of irrigation water
As a result, actual water requirement for a given
crop (or plant) is more in case of shallow soils than in deep soils
even though the amount of water actually absorbed by the crop (or
plant) may be the same in both types of soils.
This is due to the unavoidable water losses at each irrigation
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2.Chemical Properties of Soil
For satisfactory crop yield, soils must have sufficient plant
nutrients, such as nitrogen, carbon, hydrogen, iron, oxygen,
potassium, phosphorus, Sulphur, magnesium, and so on.
Nitrogen is the most important of all the nutrients.
Nitrogenous matter is supplied to the soil from barnyard
manure or from the growing of legume crops as green
manures, or from commercial fertilizers.
Plants absorb nitrogen in the form of soluble nitrates
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soils must have sufficient plant nutrients
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SOIL–WATER RELATIONSHIPS
Any given volume V of soil consists of :
1)volume of solids Vs ,
2)volume of liquid(water) Vw, and
3) volume of gas (air) Va.
Obviously, the volume of voids (or pore spaces)
Vv =Vw+ Va
For a fully saturated soil sample,
Va= 0 and Vv= Vw.
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Likewise,
for a completely dry specimen, Vw= 0 and Vv= Va.
The weight of air is considered zero compared to the
weights of water and soil grains.
The void ratio e, the porosity n, the volumetric
moisture content w, and the saturation ,S are defined
as
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Volume and mass relationships
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a
w
s
t V
V
V
V 


a
w
s
t M
M
M
M 



Note
It should be noted that the value of porosity, n
is always less than 1.0.
But, the value of void ratio ,e may be less,
equal to, or greater than 1.0.
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Cont..

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Bulk density -
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 Bulk density (gm/cm3) while apparent specific
gravity (dimensionless).
 Bulk density is normally expressed on a dry
weight basis : 1.0 – 1.8 gm/cm3 for mineral soils.
 Particle density - Thus the particle density of any
soil is constant and does not vary with the amount of
space between the particles. .
 It is defined as the mass (weight) per unit volume of
soil particles (soil solids).
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The specific weight (or the unit weight)
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Cont..
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Soil Water Content
Soil water content is expressed on mass basis or volume basis.
It is measured using gravimetric, neutron scattering, gamma ray,
capacitance method, time domain reflectometer.
1.Gravimetric method (on mass or volume basis)
Mass basis:
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100
1
3
3
2




W
W
W
W
w
W1 = weight of empty aluminium box, gm
W2 = weight of box + moist soil sample, gm
W3 = weight of box + dried soil sample, gm

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 
f
s
w
t
w
V
V
V
V
V




w
b
v w


 
 The gravimetric method is still the most widely used
technique to determine the soil water content and is often
taken as a standard for the calibration of other methods
 A disadvantage is that it is laborious, because samples in
duplicate or in triplicate are required to compensate for errors
and variability.

2.3.1) ROOT-ZONE SOIL WATER
Water serves the following useful functions in the
process of plant growth:
 Germination of seeds,
 All chemical reactions,
 All biological processes,
 Absorption of plant nutrients through their aqueous solution,
 Temperature control
 Tillage operations, and
 Washing out or dilution of salts.
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Germination of seeds
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All chemical reactions,
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All biological processes,
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Absorption of plant nutrients
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Temperature control
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Tillage operations
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Water Movement Between Soils and Plants
plant = solute + matric + pressure
Matric Forces: Water’s tendency to adhere to container
walls.
pressure is the reduction in water potential due to
negative pressure created by water evaporating from
leaves.
As long as plant > soil, water flows from the soil to the
plant.
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Types of soil water
1.Gravitational water
 When sufficient water is added to soil, water
gradually fills the pore system expelling air
completely from soil.
Water moving downwards through soil under
gravity.
 The water tension at this stage is 0.33 atm. or
less.
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2. Capillary water (Plant available water)
 With increasing supply of water, the water film held around
soil particles thickens.
 Water enters the pores until the soil-water tension equal to
the gravity force.
 This soil water tension is about 0.10 – 0.33atm.
 This water is available to plants.
 Factors that influence the amount of capillary water in the
soil are the structure, texture and organic matter content of
the soil.
 Optimum growth of plant takes place when the soil water
is maintained at near field capacity
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3. Hygroscopic water
The water that an oven dry soil absorbs.
 Water held tightly to the surface of soil
particles by adsorption forces.
 Occurs as a very thin film over the surface of
soil particles.
held tenaciously at a tension of 31 atm.
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Fig. 2.5: Illustration of soil water constants
Oven dry
Air dry
Hygroscopic coeff.
Permanent welting point
Field capacity
Saturation
Gravitational water
Capillary water (Plant
available water)
Hygroscopic water
Unavailable water
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• Cohesion water & adhesion water
Adhesion Water- water attracted to solid
surfaces
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 held by strong electrical
forces - low energy
 little movement- held tight
by soil
 exists as a film
 unavailable to plants
 removed from soil by drying
in an oven

Cohesion water & adhesion water
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Basic concepts of soil - water Dynamics
Forms of energy recognized in soil-water movement are:
1. The kinetic Energy- due to motion of water through soil layer.
-negligible due to the slow motion
2. The potential Energy –due to position of soil water within soil
body & internal conditions.
- responsible in determining soil- water status.
 The magnitude the force is the difference b/n soil water
potential at two different points.
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kinetic Energy
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potential Energy
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•The potential energy is defined w.r.t the reference state.
Components of soil-water forces
•Soil water is subject to several force fields – results in
deviation from the reference state (total soil water
potential).
•Total soil water potential:the amount of work that an
infinitesimal unit quantity of water at equilibrium is capable
of doing work when it moves (isothermally and reversibly) to
a pool of water at similar standard (reference)
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•The primary forces acting on soil water held within a rigid soil
matrix under isothermal conditions can be conveniently
grouped as:
I - Matric forces:
resulting from interactions of the solid phase with the liquid
and gaseous phases;
eg. adsorptive forces and capillary forces
- capillarity caused by liquid–gas interfaces
- adhesion of water molecules to solid surfaces
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Fig. 2.6: Schematic diagram of two
water molecules.
Adsorptive forces
Adsorptive forces cause water molecules adsorbed on clay particles

Cohesion water & adhesion water
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ii - Osmotic forces:
owing to differences in chemical composition of soil
solution;
 Soil water …dissolved salts + other solutes →soil
solution
 Presence of solutes in soil water decreases the potential
energy of water in the soil.
 The potential energy of water in the solution is lower
than the reference state (pure water).
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Osmotic forces:
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iii- Body/Gravitational forces:
induced by gravitational and other (e.g.,
centrifugal) inertial force fields.
Compare to matric & osmotic forces
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Soil-water potential Concept
The effect of a force on soil water may conveniently be
described by potential energy of soil water in a particular force
field.
The forces governing soil-water flow can be described by the
energy concept.
 According to this principle, water moves from points with
higher energy status to points with lower energy status.
 The energy status of water is simply called 'water potential'.
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• The total soil water potential can be written as:
t = total potential
m =matric potential
o= osmotic potential
g = gravitational potential
OR
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g
O
m
t 


 


g
O
m
t h
h
h
h 


•The matric head (hm) in unsaturated soil is negative

Measurement of soil water
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THE END
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