Soil water plant relationship

INTRODUCTION
• Soils are the natural media that support the growth and activities of many kinds of
plants, animals and micro-organisms which play a vital role for the existence of life
on earth.
Air
Water
Minerals
O.M
SOIL COMPONENTS

FUNCTION AND PROPERTIES OF WATER
• Water is a chemical compound of hydrogen and oxygen.
• Water together with dissolved nutrients forms the soil solution from which plants
get nutrients.
• Water helps to maintain turgor pressure of plant cells.
• Water is an integral component of photosynthetic relation.
• Water serves as a universal solvent.
• Water is very important for soil formation.
• Water is an important component of plant cell and constitutes about 80-90% of the
fresh weight of herbaceous plant parts and over 50% of the fresh weight of woody
plants.

ROLE OF WATER IN GROWTH AND PHYSIOLOGICAL
PROCESSES
• Almost every plant process is affected directly or indirectly by water supply.
Germination
Growth
Photosynthesis
Respiration
Transpiration
Stomata opening and closing
Flowering and Fruiting
Fruit ripening and Dormancy

ROLE OF WATER IN SOIL FUNCTIONING
Soil Formation
Soil Fertility
Regulating soil temperature

PROPERTIES OF WATER
• water is a chemical compound of hydrogen and oxygen. hydrogen and oxygen atom
are bonded together covalently (each hydrogen atom shares a single electron with
oxygen) and arranged in a V shaped structure at an angle of 104. 52A0 .

PROPERTIES OF WATER
1. Hydrogen bonding
2. Cohesion and adhesion
3. Surface tension
4. Capillarity
5. Polarity
6. Specific heat
7. Heat of vaporization
8. Heat of fusion
9. Heat conductor
10. Water as a Regent

CLASSIFICATION OF SOIL WATER
1. Gravitational water: It is of little use because it stays in soil for very short period of
time. It is present in soil at water potential greater than -0.1 bar and is always in acces of
field capacity.
2. Field capacity: . At field capacity water is held in soil at water potential -0.1to -0.3 bar.
3. Capillary water: it is present in soil at water potential -0.3 to -31 bar. However, plant can
use capillary water upto - 15 bar.
4. Hygroscopic water: Water that is held very tightly with soil solids at a water potential
lower than -31 bar.

Classification of water with respect to plants or biological
point of view
1. Available water: the available water is estimated as the difference between soil
water content at field capacity and permanent wilting point.
2. Permanent wilting point: It is generally the soil water held at less than 15 bar. It
is the lower limit of available water beyond which water is adsorbed so strongly
that plants cannot absorb it fast enough to meet their water requirements.
3. Unavailable water: Water held at water potential less than -15 bar and bound to
soil particles so tightly that is not available to plants.

SOIL WATER ENERGY CONCEPT
 Retention and movement of water in soil, its uptake and translocation in plants as
well as its loss to atmosphere all our energy related phenomenon. Soil water has two
types of energy.
1. Kinetic Energy: It is the form of energy due to movement of water molecules
within soil.
2. Potential .Energy: It is the form of energy due to position of water in soil.
 Free energy of water is always expressed relative to energy condition of water in a
standard reference state.

Reference State
 The reference state must have following characteristics;
1. It is pure.
2. It is free ,no adsorption at soil matrix.
3. It has pressure equal to atmospheric pressure.
4. It is at same temperature as that of soil water.
5. An arbitrary reference elevation must be set at a specific height.

Factors affecting free energy of water
Matric forces
Osmotic forces
Gravitational forces

Soil Water Potential
It is the difference between free energy of soil water and that of pure water in a
standard reference state is called soil water potential.
Characteristics of soil water potential;
1. Relative
2. Negative
3. Continuity
4. Driving force
5. Variability
6. Dynamic

Components of soil water potential
Matric potential
Osmotic or solute potential
Pressure potential or hydrostatic potential
Overburden potential or envelope potential:
Air pressure potential or pneumatic potential
Gravitational potential
• It is denoted by psy Ψ .
• Ψw= Ψm + Ψs+ Ψp.

MOVEMENT OF WATER THROUGH SOIL
1. Saturated Water Flow: It is the movement of water under saturated soil
condition and is mainly determined by two major forces;
• Hydraulic force
• Hydraulic conductivity
• Hydraulic force: It is the driving force that controls water movement through soil
under saturated condition. It originates from gravity. It is the ease with which soil
pores permit water movement.

2. Unsaturated Flow: It is the movement of water in soil through capillary pores.
Unsaturated flow depends upon two factors;
Hydraulic conductivity
Driving force
• Driving force in case of unsaturated flow is generally the matric forces. Unsaturated
flow is inversely proportional to matric forces. Water flows from wet region (low
metric forces) to dry region (high matric forces).

3. Vapour Movement: Water vapours move from one point to another in response
to the difference in vapour pressure.
• Water vapors moves from moist soil where soil air is nearly saturated with water
vapours high vapour pressure to a dry soil where vapour pressure is somewhat
lower.
• Vapour movement in soil is very small and has limited practical significance to meet
crop water requirement.
• In a dry soil vapour movement may be considerable significance in supplying
moisture to drought resistant plants.

HOW PLANTS ARE SUPPLIED WITH WATER
• Two major phenomena are generally responsible for plant access to water.
1. Capillary movement of soil water to plant roots
2. Growth of plant roots into moist soil

Capillary movement
• When plant rootlet absorb water they reduce moisture content and thus result in
reduction in water potential in immediate surroundings of plant roots. In response to
this lower water potential, water tends to move towards plant roots.
• The rate of water movement depends on
Magnitude of potential gradient
Hydraulic conductivity of soil

Growth of plant roots to moist soil
• Capillary movement of water is complemented by rapid rate of root extension.
• The primary limitation of root extension is a small proportion of soil with which
roots are in contact.
• Roots soil contact commonly accounts for less than 1% of total soil surface area.
• Complemented operation of capillary and root extension is more effective to move
water from soil to plant roots

WATER ABSORPTION BY PLANT ROOTS
• The transport of water from soil to plant roots and to atmosphere takes place in
following three steps;
1. Water is first absorbed from soil by root hairs and other epidermal cells in or near
the root zone of young root tissue.
2. Lateral conductance across young root tissue epidermis pericycle into xylem duct
and vertical conductance within xylem vessel into leaves.
3. Passage of water across leaf tissue through the process of transpiration into
atmosphere.

MECHANISM OF WATER ABSORPTION
There are two mechanisms of water absorption
1. Passive absorption
2. Active absorption

Passive Absorption
• In passive absorption uptake of water by plants is generally controlled by
transpirational pull generated at leaves surface due to loss of water into atmosphere.
• During passive water absorption suction force originally generated at leaf surface
moving down through a continuous column of water is transmitted to root system
and is responsible for the absorption of water from soil.

Active Absorption
• Absorption of water by plant roots due to activity of living roots and usually
involving the expenditure of energy.
• When there is high concentration of salts in soil, due to adsorption of water
molecules to salt ions water potential of soil solution becomes low as compared to
root cells.
• Under these conditions plant roots have to synthesize and accumulate different kind
of osmolytes (Proline, glycine batane, sucrose and mannitol) into root cell sap.
• Due to accumulation of these osmolytes in the root cell sap water potential within
cell sap decreases and water starts to move from soil to plants along water potential
gradient.

FACTORS AFFECTING WATER ABSORPTION
1. Absorption efficiency of roots
2. Availability of water and soil type
3. Concentration of soil solution
4. Transpiration
5. Soil Temperature
6. Soil Aeration

Nutrient Movement From Soil To Plant Roots
• Before taken up by plants the nutrients must reach from soil to root surface. It takes
place by three processes.
1. Mass Flow
2. Diffusion
3. Root interception

Mass Flow
• It is the transport of soil solution containing nutrients to plant roots caused by water
potential gradient developed due to loss of water through transpiration or
evaporation.
• Contribution of mass flow to carry nutrients to plant roots varies with following
factors:
Nutrient in consideration and its concentration in soil solution
Water consumption or requirement of plants.
Plant species and plant age
Time of the day and season
Soil moisture content

Diffusion
• It is the transport of nutrients to root surface along the concentration gradient. P, K,
S, Fe and Zn move through diffusion.
• The transport of nutrients to plant roots through diffusion is high when plant uptake
exceeds nutrients supply through Mass flow.
• Depletion zones develop along the root surface depending upon following factors
• Uptake by roots
• Replenishment of soil
• Mobility of ion by diffusion

Root Interception
• It refers to the exchange of Ions between root surface and soil minerals surface
through the physical contact between root and soil.
• The quantity of nutrients that can come in direct contact with plant roots is the
amount in the volume of soil equal to volume of root.
• Roots can contact 1-3% of available soil nutrients but normally 1%.
• Root interception can be increased by mycorrhizal infection.

Factors affecting nutrient movement to plant roots
• Factors affecting nutrient movement to plant true fruits can be grouped into three
categories.
• 1. Nutrient related factors
• 2. Plant related factors
• 3. Soil related factors

Nutrient Related Factors
Nutrient under consideration
Concentration of nutrient in soil solution
Form of nutrient in soil

Plant Related Factors
Plant species and varieties within species
Plant age
Root type
Presence or absence of root hairs
Root length
Root induced changes in rhizosphere

Soil Related Factors
Soil texture
soil structure
Soil aeration
soil temperature
pH
Eh
Organic matter
Microbial activities
Soil moisture

ION OR SOLUTE UPTAKE BY PLANTS
Absorption of salts by plants takes place through an immediate contact of plant
roots with the soil solution.
 Absorption of salts takes place in following steps.
1. Movement of Ions from soil to plant roots
2. Accumulation of ions in the root surface
3. Radial movement of Ions from root surface to xylem
4. Translocation of Ions from Roots to shoot

Factors Affecting Ions/ Salts/ Solute Absorption
Plant Species
Extent of root system &Metabolic activities in root tissues
Internal concentration of salts & sugars
Hydrogen ion concentration
Respiration
Light & Aeration & Temperature
Soil Moistures Content
Concentration and composition of external solution
Interaction between ions

ION UPTAKE MECHANISM
• Ion uptake is the process in which mineral nutrients enter the cellular material
following the same pathway as that for water. Nutrients may be taken up by plants
by two mechanisms.
Passive uptake
Active uptake

Passive Uptake
• It is the absorption of mineral nutrients by plants along the concentration gradient
without direct expenditure of metabolic energy. Passive absorption is not affected by
temperature and metabolic inhibitors.
• There are 4 theories lying under passive absorption/uptake.
1. Mass flow theory
2. Contact exchange theory
3. Carbonic acid exchange theory
4. Donnan equilibrium theory

Mass Flow Theory
 According to this theory ions are absorbed by the roots along with mass flow of
water under the effect of transpiration.
 An increase in transpirational pull increases the uptake of ions by roots .
Mass flow of ions through root tissue occur due to transpirational pull in the
absence of metabolic energy.

Contact Exchange Theory
According to this theory ions adsorbed on colloidal particles get absorbed to the
roots in exchange for hydrogen ions previously adsorbed on the roots.
 Ions adsorbed on the colloidal particles oscillate within a small space, when two
particles(root and soil) are close enough the oscillation space of an ion adsorbed to
another particle overlaps, thus exchange of ions may take place between soil
particles and plant roots.

Carbonic Acid Exchange Theory
The soil solution provides a medium for the exchange of Ions between the root and
colloidal particles.
 Carbon dioxide released during respiration forms carbonic acid by reacting with
water of soil solution.
 This carbonic acid is then dissociated in soil solution to form hydrogen ion and
bicarbonate ions.
 Hydrogen ions adsorbed to colloidal particles exchanges for cations such as
potassium ion, which are released into soil solution from there they may diffuse to
plant roots and are taken up by plants.

Donnan Equilibrium Theory
Cell membrane is composed of macromolecules of proteins and lipids that have
different functional groups like carboxylic group and phosphate group from which
positively charged particles like proton dissociates, leaving the macromolecules
with net negative charge.
The negative charge so produced are not diffusible because they are within the
membrane structure.
These negatively charged membrane are called donnan phase.
As a result of net negative charge on the membrane structure the cations like
potassium will tend to diffuse through the membrane because of electric potential
difference.

Active Ion uptake
The active transport of Ions from the outer space of cell to the inner space generally
occurs against the concentration gradient and hence requires metabolic energy.
 During active and transport the carrier proteins, picks up an ion from one side of
the membrane and discharges it on the other side. This picking up and discharge of
an ion by carrier protein require energy. This energy is obtained from the hydrolysis
of ATP.
The carrier protein may carry one ion inward and exchange it with another ion at
the inner surface of the membrane, so that other ion is carried by the same protein
carrier outward.

REDISTRIBUTION OF IONS IN PLANT BODY
The continued movement of salt ions into leaves along transpirational pull results in
an increase of salt content during growing season.
Not all of salt which enter a given leaf will retain in that leaf, some of the salt
ions are exported/transported back into stem.
Redistribution of salts with in plant body seems to be controlled largely by
metabolic activities of various parts of the plant.
The redistribution of salt ions mainly occur through xylem and phloem.
Salt ions tend to be moved from older leaves to younger leaves, reproductive
structure and metabollically active regions before the fall of older leaves.

ADAPTATION OF PLANTS TO DROUGHT STRESS (Take Notes)
SOIL PLANT ATOMSPHERE CONTINUM SPAC (Take Notes)

Soil water plant relationship

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