Soil, Water and Plant Relationship (Day-11, 18 July 2020)

Soil-water-plant relationships
Er. UMARFAROOQUE MOMIN
Scientist (Agricultural Engineer)
AICRP for Dryland Agriculture
Regional Agricultural Research Station
Vijayapur-586101
University of Agricultural Sciences, Dharwad

Sunday, July 19, 2020
Introduction to hydrological cycle
Interaction of water with soil, plants, atmosphere
Source: https://www.egr.msu.edu/hydrology/

Source: https://www.warrenswcd.com/education-connection-a-blog/soils-sustain-life
Soil: The soil mass is a three phase system consisting of
solid particles (called soil grains), water and air.
Composition of Soil

Distinct soil layers are called
Horizons.
O- Composed of organic matter
A- organic matter mixed with
inorganic products of
weathering
B- Fine material accumulated
and enriched with calcium
carbonate
C- Soil parent material.
A Typical Soil Profile
Composition of Soil

Texture:
 Relative proportion of various
sizes of individual particles of soil.
Modification of soil texture may
be possible by organic matter, clay
minerals and their associates.
Properties of Soil
Soil Particle sizes (sand, silt, & Clay)
Source: https://theconstructor.org/building/soil-types-sand-silt-clay-loam/25208/

Classification of soil based on
texture:
USDA Textural triangle is used to
determine the textural classes
such as, Coarse, Fine, Light and
Heavy.
Importance of Soil Texture:
Affects movement of water and its
storage.
Properties of Soil
Example-1: What is the textural class, if a soil consists of sand (60%),
Silt (30%), and Clay (10%).
https://www.warrenswcd.com/education-connection-a-blog/soils-sustain-life
Textural Triangle

Analysis of Coarse grained soil
using sieve method (> 0.05
mm size)
Apparatus used: sieve shaker
Analysis of fine grained soil
(<0.05 mm Size) is done by
sedimentation method
Apparatus used: Hydrometer or
by pipette method
Properties of Soil
Particle size analysis:

Textural Analysis by Sieve methods:
Properties of Soil
Source: https://pavementinteractive.org/reference-desk/testing/aggregate-tests/sieve/

Sample Calculation by sieve methods
Properties of Soil
Source: http://www.basiccivilengineering.com/wp-content/uploads/2017/06/?SD

Sunday, July 19, 2020 Grain size distribution curve
Properties of Soil
Source: https://www.aboutcivil.org/Sieve-analysis-and-soil-classification.html

Textural analysis by pipette method:
Collect sample after 5 minute
(% silt and clay)
Collect sample after 5 hours
(% clay)
Weighs 20 g of soil
sample
Add 5 ml of
(NaPO3)6
Add distilled water
blend for 5 minutes
Make volume of
1000 ml
Pipetteout 0.25 ml at 5 minutes
and 5 hours for calculation of % of
sand, silt, and clay
Properties of Soil

Textural analysis by pipette method:
Properties of Soil

Textural analysis by hydrometer method:
Weighs 20 g of soil
sample
Add 5 ml of
(NaPO3)6
Add distilled water &
blend for 5 minutes
Make volume of
1000 ml
Properties of Soil
Record hydrometer reading at
40 sec and 20 minutes
Calibration of hydrometer

Textural analysis by hydrometer method:
Properties of Soil

Textural analysis (RELMA 2005):
Source: Regional Land Management unit/ World Agro-forestry Center (2005)
Add soil up to 1/3rd full bottle and water up to 2/3rd full.
Add pinch of salt shake for 1 min and leave it for 1 hour
Shake again, and leave it for 4 hours and measure the soil layers
thickness.
Calculate % of each soil fraction
Properties of Soil

Soil Textural Classification:
Classification of soil by United State Department of
Agriculture (USDA) and by International Soil Science
Society (ISSS) given below
Soil fraction Particle diameter (mm)
USDA ISSS
Gravel >2 >2
Very coarse sand 1-2 -
Coarse sand 0.5-1 0.2-2
Medium sand 0.25-0.5 -
Fine sand 0.1-0.25 0.02-0.2
Very fine sand 0.05-0.1 -
Silt 0.002-0.05 0.002-0.02
Clay <0.002 <0.002
Properties of Soil

Soil Structure:
Arrangement of soil particles in soil.
Affects root penetration, water intake, and movement.
Soil structure together with soil texture affects pore size, pore
distribution, and porosity of soil.
Properties of Soil

Soil Structure in Relation to Movement of Water
Properties of Soil

“Defined as the bulk mass (M) of the soil per unit volume
(V), also known as moist density.”
Bulk density of Soil (ρb):
V
M
b 
s
s
s
V
M

A mass (Ms) of the dry soil
per unit volume (Vs) of dry
soil, also known as Particle
density.”
Properties of Soil

M1 M2 M3 M4
    ws
MMMM
MM
 



2314
12 )(
Particle density by Gay-Lussac specific gravity bottles /pyncometer:
Density bottle No. 1
Mass of bottle + Stopper M1
Mass of bottle + Stopper +
dry soil
M2
Mass of bottle + Stopper
+dry soil + water
M3
Mass of bottle + Stopper +
full Water
M4
Mass of dry soil used M2-M1
Mass of water used M3-M2
Mass of water to fill bottle M4-M1
Particle density, g/cm3 ρs
Properties of Soil

Basic Soil Water Relationships
Fig: Phase diagram of soil under different
condition (partially, fully saturated, and dry)
Depending upon the soil moisture content the soil system may also be a
two phase during saturation and dry condition of soil which shown in
figure.

A bulk undisturbed volume of soil (V) here:
V = Vs + Vw + Va
Weight or Mass W=Wd+Ww

s
s
s
V
M

aws
ss
b
VVV
M
V
M


Density of Solids (Particle density):
Mass of dry soil (g) per unit volume of
soil solids (cm3).
Typically values range: 2.6-2.7 g/cm3.
Dry bulk density:
Mass of dry soil (g) per unit total
volume of soil (cm3).
Typically values range: 1.1-1.6 g/cm3.
aws
sw
b
VVV
MM
V
M



Total (wet) bulk density:
Total mass of soil (g) per unit total
volume of soil (cm3).

Void Ratio: “ Defined as the ratio
of Volume of the voids to the volume
of solids.” Expressed as fraction
s
v
V
V
e 

Porosity: Defined as the ratio of
Volume of the voids to the total
volume of soil”. Expressed as
percentage.
V
V
or
V
V
n v
T
v


“Soil elements in terms of e” “Soil elements in terms of n”
n
n
V
V
e
s
v


1 e
e
V
V
n v


1

Specific gravity of Soil
Defined as the ratio of the density of solids to the density
of water .
w
b
G



Specific
gravity bottle
W1= Wt of
bottle + Water
W2= Wt of
bottle + Soil +
Water

Water Content in Soil
Soil water content (dry basis):
Ratio of mass of water (g) to the
mass of dry soil (g).
Soil water content (wet basis):
Ratio of mass of water(g) to the
mass of wet soil (g).
Volumetric water content:
Ratio of volume of water (cm3) to
the total volume of soil (cm3).
s
w
dm
M
M

sw
w
wm
MM
M


dms
w
V A
V
V
 

Water Content in Soil
Equivalent depth of water
Where,
d = equivalent depth of water in a soil layer.
L = depth (thickness) of soil layer.
L
A
LA
d V
V


 


Degree of Saturation
Fully saturated soil Fully dry soil
vw VV  1S 0wV 0S
Degree of Saturation: Defined as the
ratio of Volume of water present in a
given soil mass to the total volume of
voids.”
wa
w
v
w
VV
V
V
V
S



Water content & equivalent depth

w
bdm
V


 
 wm
vm
dm





1
 dm
dm
vm





1
 dm
b





1
 e
s
b


1


n
n
e


1
s
b
e
e
n




 1
1
Inter-relationships

Soil Water
Kinds of Water in Soil

Soil Water: General kinds of water present in soil are gravitation
water, capillary water, hygroscopic water.
Soil Water

Gravitational water:
Soil water between saturation (0 bar) and field capacity (1/3rd bar).
Held in macro pores & drained easily by gravitational force.
Not available to plants.
Soil Water

Capillary water:
Soil water between field capacity (1/3rd bar) and hygroscopic
coefficient (31 bar suction).
Two groups: Water available (1/3 to 15 bars), Not available (15 to
31 bar).
Moves easily in soils, but does not drain freely from soil profile.
Soil Water

Hygroscopic water:
Soil water above hygroscopic coefficient (at suction > 31 bar).
Not available to the plants.
Mostly held in soil colloids and moves at extremely slow rate in
vapor state.
Soil Water

Soil Water
Kinds of Water in the Soil and Differences in Avialable
Moisture content

Field Capacity (FC or θfc):
The amount of water held in the soil after the excess gravitational
water has drained away.
Source: WWW.Hydrogold.com
This is the upper limit of soil water
available to the plants.
Here gravity drainage becomes
negligible.
Soil is not saturated but still at a very
wet condition.
Traditional defined as the water content
corresponds to a soil water potential of -
1/10th to -1/3rd bar.
Soil Water

The moisture retained after whirling a saturated for 40 minutes
at an angular velocity subjected to centrifugal force (CF).
The corresponding CF to field capacity is called centrifugal
moisture equivalent (cme).
It also called the upper limit of soil moisture storage.
The FC ranges from 8% for coarse sand, to 40% for fine clay.
Soil Water

Filed capacity is affected by various factors:
Soil texture, soil layer, organic matter, depth of
wetting, and evapotranspiration. However, it is
assumed as constant over the growing period.
It is an idealized concept due to soils do not drain to a
given water content and cease to drain further.
The concept is not valid for the presence of
impermeable layer or water table.
The FC concept may not be application to those soils
with swelling and shrinking problems.
Soil Water

Permanent wilting point (WC or θwp):
The amount of water left in the soil
when plants are unable to extract any
more water to meet its demand.
The lower limit of soil water available
to plants.
Still some water in the soil but not
enough to be use to plants.
Water held by adsorptive force.
Hygroscopic water.
Traditionally defined as the water
content corresponding to-15 bars of
SWP.
Soil Water

Available water (AW):
Water held between field capacity
and wilting point in the soil.
Available for plant use.
 
100
wpfcrzD
AW
 

Where;
AW = available water (cm or mm)
Drz = root zone depth
θfc = field capacity (% volume basis)
θwp = permanent wilting point (% volume basis)
Soil Water

Water holding capacity of soils (Effect of Soil Texture
Soil Water

Readily available water (RAW):
Relatively small declines in actual transpiration associated with
soil water content reduction between FC & PWP indicate that the
water is more readily available and that higher crop yield should
be expected.
Irrigation are normally scheduled above PWP.
Soil Water

Maximum Allowable Deficiency (MAD):
Used to estimate the amount of water that can be used without
adversely affect the plant
For most of the crop it is 0.65
Deficit irrigation: Sacrificing crop revenues to achieve reduction
in water usage and energy cost that exceeds the sacrifice crop
revenue.
AW
RAW
MAD 
Soil Water

 Feel and Appearance:
Take field samples and feel them by hand.
Advantage: free of cost, Multiple locations.
Limitation: experience required, very inaccurate method.
Soil Water Measurement

 Gravimetric method:
Measures mass of water content (thetha dm)
Take field sample, weigh it, oven dry soil sample and weigh it.

 Gravimetric method:
 Advantages:
Simple, direct, precise and extensively method to measure soil
water content.
Used to calibrate other indirect methods.
Multiple locations.
 Limitations:
 Laborious and time consuming (at least 24 hours)
 Destructive method.
 Bulk density required to estimate volumetric moisture content
 Error may encounter for soils rich in clay or organic matter.

 Neutron Scattering:
 Measures volumetric moisture content.
Components:
1. Probe: a source of fast neutron either a
mixture of americium & beryllium or radium
& beryllium.
2. Scalar (detector for slow neutron): time
control, a counter, a display and functions for
processing and calibration of the probe.

 Neutron Scattering:
Working principle:
1. Fast neutron emitted from the source to surrounding soil
colloid with hydrogen molecules, gradually lose their kinetic
energy, and results in cloud of slow or thermalized neutrons.
2. Sphere of influence is spherical in shape and detector create a
small electric pulses which are amplified and then counted by
scalar over specified time interval.
3. Number of slow neutrons counted in a specified time interval
is linearly related to volumetric moisture content
qV
= a +b(CR)

 Neutron scattering method:
 Advantages:
Samples is a relatively large soil sphere.
Repeatedly sampling at same site and several depths
Accurate method
 Limitations:
Costly instrument.
Radioactive licensing and safety.
Not reliable for shallow measurement near the surface of soil.
Not for the farmers, only for research work.

Dielectric constant:
 A soils dielectric constant is depends on soil
moisture.
 Time domain reflectrometry (TDR)
 Frequency domain reflectometry (FDR)
 Primarily used for research and academic purpose.

 TDR:
 Measures volumetric moisture content
based the dielectric constant of soil.
 Dielectric constant:
 2 and 5 for dry soils (soil solids)
 80 at frequencies between 30 MHz and
1GHz
 Relationship between dielectric constant and volumetric
moisture content is only weekly dependent on salt content, soil
temperature, soil type and density.
 Hence, method can be used without calibration in many soil
with an accuracy of +/- 0.02 m3/m3

 TDR:
 Advantages:
Provides accurate volumetric moisture content at desired depths
including surface layer advantage over neutron probe.
Does not require calibration for most of the soils.
TDR can be connected with data loggers to collect, store, and
retrieve data automatically over a long period of time.
Used to estimate salinity from measurements of attenuation of
signal which are independent of time measurement used to
estimate soil water.
 Limitations:
Expensive due to high cost.
Installation of probes in stony and heavily compacted soil is
difficult.

Measure of energy status of the soil water.
Important due to it reflects how hard plants must work to extract
water.
Units of measurements are normally bar or atm.
Soil water potentials are negative pressures (tension or suction).
Water flows from a higher (less negative) potential to a lower
(more negative) potential.
Soil Water Potential

 Total Soil Water Potential:
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.
omgt  

 Gravitational potential:
Component of total potential which is due to position of a point
relative to some reference or specified elevation.
Gravitation potential
At reference point is zero.
Above reference point is +ve
Below reference point is –ve.

 Gravitational potential:
Example: Find out the gravitational potential of the points A & B
located a distance of 125 mm above and 75 mm below an arbitrary
reference line. Also determine change in gravitation potential
between A & B.
Solution:
ΨgA = -125 mm,
ΨgB = -75 mm,
Ψg = ΨgA – ΨgB = 125-(-75)=200 mm

 Matric potential:
Application of pressure or suction to the
soil water causes change in water potential.
This change in water potential is called the
pressure potential.
Pressure potential may be +ve or –ve
Depends on increase or decrease in potential
energy with respect to free water (atmosphere).
Under unsaturated conditions:
Soil water pressure is negative.
-ve pressure potential is also known as capillary
pressure, or metric potential, suction or tension.
Water table is the locus of atmospheric
pressure in the soil water system.
Below water table soil water pressure is
positive.

 Matric potential:
Representing total energy in terms of head (energy per unit of
weight) with the assumption that the osmotic head is
everywhere same or negligible,
H= hydraulic head (m),
h = pressure head (m),
z = elevation head (m).
zhH 

Depth
(mm)
h
(mm)
z
(mm)
H
(mm)
0 -600 600 0
100 -500 500 0
200 -400 400 0
300 -300 300 0
400 -200 200 0
500 -100 100 0
600 0 0 0
700 100 -100 0
800 200 -200 0
900 300 -300 0
1000 400 -400 0
 Example:
In a 1000 mm soil profile, soil
water is in equilibrium with at
water table at 600 mm. Estimate
pressure, gravitational & hydraulic
(total) heads through the profile at
an interval of 100 mm assuming
that solute concentration is
negligible.
Solution:
Considering water table as
reference, value of pressure,
gravitational and total heads are
determined and given below.

Tensiometer.
 Resistance block.
 Thermal dissipation blocks.
 Pressure plate apparatus.
Measurement of soil water potential

 Tensiometers:
Measure soil water potential (tension)
Indirect method because soil water related with soil water
pressure potential.
Practical operating range is about 0 to 0.75 bar of tension (this can
be a limitation on medium-and fine-textured soils)

 Tensiometers:

 Tensiometers:
 Tensiometers generally are effective only at less than 85 centibar
of tension.
 Because the gauge will malfunction when air enters the
ceramic tip or the water in the tube separates.
 The usable range from 0 to 85 centibars
 Most important range for irrigation management.
 Tensiometers do not directly give readings of soil water content
 To obtain soil water content, a moisture release curve(water
content versus soil tension) is needed

 Electrical resistance blocks:
Measure soil water potential (tension)
Tend to work better at higher tensions
 (lower water contents).

 Electrical resistance blocks:
Meter resistance readings change as moisture in the block
changes.
 The manufacturer usually provides calibration to convert
meter reading to soil tension.
The blocks ten to deteriorate over time, and it may be best to use
them for only one season.
Problems may occur with highly acid or highly saline soils.

Curve of matric potential (tension) vs water content.
 Less water then more tension
At a given tension
 Finer-textured soils retain more water (larger number of
small pores).
Soil water release curve

 The tension or suction created by small capillary tubes (small soil
pores) is greater than that created by large tubes (large soil
pores).
 At any given matric potential coarse soils hold less water than fine
–textured soils..
Matric potential and soil texture

An important class of flow events is related to water entry through
the soil surface- in a process known as infiltration.
The rate of infiltration relative to the rate of water supply on the
surface (rain, irrigation) determines how much water enters the soil,
how much, if any, will be ponded on surface or create surface runoff.
Movement of water into soils

Movement of water into soils

Infiltration
Infiltration on land surface

Infiltration
Definition: The process of entering rainwater in to the
soil strata of earth is called Infiltration.

Infiltration Process:
When water is applied at the
soil surface, then water flows
through following zones.
Saturation Zone
Transition Zone
Transmission Zone
Wetting Zone
Wetting Front
Infiltration Process

Infiltration Process
Infiltration Model

Infiltration Capacity
Definition: The maximum rate at which a given soil at a
given time can absorb water is defined as Infiltration
capacity(fp).
pff  when pfi 
if  when pfi 
The Actual rate of Infiltration (f )
can be:
Where, i = intensity of Storm
For:
Dry soils f is more
Moist soils f is less

Factors affecting Infiltration Capacity
Slope of the land
Degree of saturation
Porosity
Packing of soil grains
Compaction
Surface cover condition
Land use
Temperature
Other factors

Slope of the land: The steeper the slope(gradient), the
less is the infiltration or seepage.

Degree of saturation: The infiltration capacity is less for
saturated soils and more for dry soils.

Porosity: The greater the porosity, the greater is the
amount of infiltration.

Compaction: The infiltration capacity is very less for
compact surface.
The clay surface soils are compacted even by impact of
raindrops which reduces infiltration.
Infiltration capacity can be improved by tillage.

Vegetation: Grass, trees & other plant types capture
falling precipitation on leaves and branches keep that
water from being absorbed into the earth & take more
time to reach ground.
More the vegetation slower is the infiltration but
increase its capaity.

Land Use: Roads, parks and buildings create surfaces
that are no longer permeable, thus infiltration is less.

Temperature: At high temperature viscosity decreases
& infiltration increases.
 during summer infiltration increases
 during winter infiltration decreases

Other Factors:
 Entrapped air in pores: entrapped air can greatly affect
the hydraulic conductivity at or near saturation.
Quality of water: Turbidity by colloidal water.
Freezing: Freezing in winter may lock pores
Annual & seasonal changes: According to change in the
land use pattern, except for massive deforestation &
agriculture.

Infiltration characteristics of soil
Important Characteristics:
Type of soil
Texture
Structure
Permeability
& underdrainage.

Measurement of Infiltration
Infiltration characteristics of soil at a given location can
be estimated by:
Using flooding type infiltrometer
Measurement of subsidence of free water in a large
basin or pond
Rainfall simulator
Hydrograph analysis

Flooding Type Infiltrometer
(a) Simple(Tube-type) infiltromenter (b) Double ring infiltrometer

(c) Recording Type Double ring infiltrometer
Flooding Type Infiltrometer

Infiltration by Rainfall Simulator:
 plot of land (2m X 4m)
The specifically designed nozzles produce raindrops
falling from height of 2m.
Under controlled conditions with various combinations
of intensities & durations, the surface runoff rates &
volumes are measured in each case.
Rainfall Simulator

Modeling Infiltration Capacity
Cf
Ct
)( CP tF
tvstfP )(
Cumulative
infiltration capacity
FP(t)
The graph shows a typical variation of infiltration
capacity fp with time
Curves of infiltration Capacity & Cummulative Infiltarion

Cumulative Infiltration Capacity FP(t):
Defined as the accumulation of infiltration volume over a
time period since the start of the process & is given by
Thus the curve FP(t) vs time is the mass curve of
infiltration. It may be noted that from the following
equation.
dttfF
t
PP .)(
0

dt
tdF
tf P
P
)(
)( 

Different infiltration Models to describe FP & fP:
Horton’s Equation (1933)
Philip’s Equation (1957)
Kostiakov Equation (1932)
Green-Ampt Equation (1911)

HORTON’S EQUATION (1933):
Equation expressed the decay of infiltration capacity with
time as an exponential decay given by
for
Where,
fp = infiltration capacity at any time t from the start of the rainfall
f0 = initial infiltration capacity at t=0
fc = final steady state infiltration capacity occurring at t=tc. (Also
called constant rate or ultimate infiltration capacity).
Kh = Horton’s decay coefficient which depends upon soil
characteristics & vegetation cover.
  tK
CCP
h
effff 
 0
Ctt 0

PHILIP’S EQUATION (1957):
Philip’s two term model relates FP(t) as
Where,
s= a function of soil suction potential & called as sorptivity.
K = Darcy’s hydraulic conductivity
Then, infiltration capacity could be expressed as
KtstFP  2
1
KstfP 

2
1
2
1

KOSTIAKOV EQUATION (1932):
Expresses cumulative infiltration capacity as
Where, a & b are local parameter with a>0 & 0<b<1.
The infiltration capacity would now be expressed by
b
P atF 
)1(
)( 
 b
P atabf

GREEN-AMPT EQUATION (1911):
A model for infiltration capacity based on Darcy’s law as
Where,
η = porosity of the soil
Sc = Capillary suction at the wetting front
K = Darcy’s hydraulic conductivity
Infiltration capacity could be
where m & n are Green-Ampt parameters of infiltration model.







P
c
P
F
S
Kf

1
P
P
F
n
mf 

Numerical Examples
Numerical Exmples related to following models:
Horton’s Equation (1933)
Philip’s Equation (1957)
Kostiakov Equation (1932)
Green-Ampt Equation (1911)

Classification of Infiltration Capacities
Infiltration
Class
Infiltration
Capacity
mm/h
Remarks
Very Low < 2.5 Highly clay soils
Low 2.5 to 12.5
Shallow soil, Clay soils, Soils low
in Organic matter
Medium 12.5 to 25.0 Sandy Loam Silt
High >25.0
Deep sands, well drained
aggregated soils
Based on Hydrological Soil group used in SCS-CN method:

Irrigation water quality

Salinity Hazard

Sodicity Hazard

Sodium absorption ratio

Sodium to calcium activity ratio (SCAR)

Alkalinity Hazard

Residual Sodium Carbonate (RSC)

Magnesium

Chlorides

Classification of Infiltration Capacities

Sulphate

Fluorine

Management practices for using poor
Quality water

Soil, Water and Plant Relationship (Day-11, 18 July 2020)

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