SOIL-PLANT-WATER RELATIONSHIP

Soil Water Relationship

Soil is a heterogeneous mass consisting of a three phase system of solid, liquid and gas.
Mineral matter, consisting of sand, silt and clay and organic matter form the largest fraction of
soil and serves as a framework (matrix) with numerous pores of various proportions. There are
many variables in the physical characteristics of soil. These include soil texture, soil structure,
bulk density, and soil porosity. They all impact how soil, water, and air interact.
Soil composition: Soil is a mixture of mineral matter, organic matter, and pores. The mineral
matter makes up about one-half of the total soil volume. This mineral matter consists of small
mineral particles of either sand, silt, or clay. Organic matter is made up of decaying plant and
animal substances and is distributed in and among the mineral particles. Organic matter
accounts for about 1 to 5 percent of the overall soil makeup. The combination of mineral and
organic matter is referred to as the solids. The pores, spaces that occur around the mineral
particles, are important because they store air and water in the soil. Approximately 50 percent of
the soil makeup is pores. The overall composition of soil is 45 to 49 percent mineral particles, 1
to 5 percent organic matter, and 50 percent pores. Figure 1shows the approximate relationship
between the substances in the soil composition with the pore space shown split between air and
water. The amount of water and air present in the pore spaces varies over time in an inverse
relationship. This means that for more water to be contained in the soil, there has to be less air.
Soil texture: Soil texture is determined by the size of the particles that make up the soil. The
traditional method of determining soil particle size is done by separating the particles into three
convenient size ranges. These soil fractions or separates are sand, silt, and clay. Generally, only
particles smaller than 2 mm (1/12 inch) in size are categorized as soil particles. Particles larger
than this are categorized as gravel, stones, cobbles, or boulders. Sand particles range in size from
2 mm to 0.05 mm. There are subcategories assigned to this range that include coarse, medium,
and fine sand. Silt particles can range in size from 0.05 mm down to 0.002 mm. The physical
appearance of silt is much like sand, but the characteristics are more like clay. Clay particles are
less than 0.002 mm in size. Clay is an important soil fraction because it has the most influence on
such soil behavior as water-holding capacity. Clay and silt particles cannot be seen with the
naked eye.
Soil texture is determined by the mass ratios, or the percent by weight, of the three soil
fractions. The soil textural triangle, Figure 2, shows the different textural classes and the
percentage by weight of each soil fraction. For example, a soil containing 30 percent sand, 30
percent clay, and 40 percent silt by weight is classified as a clay loam.
Soil structure: Soil structure is the shape and arrangement of soil particles into aggregates. Soil
structure is an important characteristic used to classify soils and heavily influences agricultural
productivity and other uses. The principal forms of soil structure are platy, prismatic, columnar,
blocky, and granular.
These soil structure descriptions indicate how the individual particles arrange themselves
together into aggregates. Aggregated soils types are generally the most desirable for plant
growth. These terms also are used in conjunction with description words to indicate the class and
grade of soil. Class refers to the size of the aggregates while grade describes how strongly the
aggregates hold together. Structure less soils can be either single grained (individual unattached
particles—like a sand dune) or massive (individual particles adhered together without regular
cleavage—like clay pans or hardpans.) Soil structure is unstable and can change with climate,
biological activity and soil management practices.
Soil bulk density and porosity: Soil dry bulk density expresses the ratio of the weight of a soil
to its total volume. Wet bulk density is the ratio of the soil and water weight to the total volume.
The total volume includes both the solids and the pore spaces. The soil bulk density is important
because it is a measurement of the porosity of the soil. The porosity of a soil is defined as the
volume of pores in a soil. A compacted soil has low porosity and thus a higher bulk density. A
loose soil has a higher porosity and a lower bulk density. Like soil structure, a soil’s bulk density
and porosity can be affected by climate, biological activities, and soil management practices.
Soil water content: The variable amount of water content in unit mass of volume of soil and the
energy state of water in the soil are important factors affecting plant growth. The mass or volume
fraction of water in the soil can be characterized in terms of soil moisture content. The state of
soil water is characterized in terms of its free energy per unit mass, termed as potential. Soil
moisture content and potential are related to each other and graphical representation of this
relationship is termed as ‘Soil moisture Characteristic curve’.
Soil moisture constants: Soil moisture is always subjected to pressure gradient and vapour
pressure differences that causes it to move. Hence, soil moisture will not be constant at any
pressure. Certain moisture contents, given below are of particular importance in irrigated
agriculture and these are often called as soil moisture constants.
i. Saturation/ maximum water holding capacity/ maximum retentive capacity: This is
the moisture saturated stage of the soil, when all pores and capillaries are filled with water.
The cropped soil remaining at this stage for a longer period suffers from good aeration and
this condition is consequently harmful to most crop plants. The energy status of water at
saturation is zero. Water content of soil at saturation is approximately double that of the
field capacity.
ii. Field capacity: After application of water in the soil all the gravitational water has drained
away, then the wet soil is almost uniformly moist. The amount of water held by the soil at
this stage is known as the field capacity or normal moisture capacity of that soil. It is the
capacity of the soil to retain water against the downward pull of the force of gravity. In
 other words, when the excess or surplus water has been fully pulled out of any horizon, the
soil there in is said to be at field capacity. At this stage, only micro pores or capillary pore
are filled with water and plants absorb water for their use. Water at field capacity is readily
available to plants and microorganism. The energy status of water at field capacity is -0.1
to -0.33 bars. Field capacity is considered as the upper limit of water availability to plants.
Moisture equivalent is approximately equal to the amount of moisture held at field capacity
soil. The term moisture equivalent is defined as the percentage of water held by a one
centimeter thick moist layer of soil after subjected to a centrifugal force of 1000 times
gravity for half an hour.
iii. Hygroscopic coefficient: It may be defined as the percentage of moisture in the soil when
the soil is kept in an atmosphere having 100 percent relative humidity/saturation
atmosphere at 25oC. Dry soils kept in an open place absorb water vapour from the
atmosphere. This water absorbed in this way is called hygroscopic water. The soil moisture
tension at this point is equal to 31 bars (soil moisture potential -31 bars) and this water is
not available to plants, but available to certain microorganism
iv. Permanent wilting point: when the energy status of soil moisture reaches -15 bars, plants
can not absorb sufficient moisture. Plants show wilting symptoms even under high
humidity conditions. The soil moisture content at -15 bars is referred to as permanent
wilting point. At this moisture level plants wilt but do not die and are able to absorb small
quantity of water just sufficient for their survival and plants recover if water is supplied.
v. Ultimate wilting point: the soil moisture content at which plant die is known as ultimate
wilting point. Soil moisture at -60 bars is not available to plants and they die due to lack of
water or it may be defined as the percentage of moisture in the soil at which the soil can not
supply water to the growing plant at a sufficient rate to maintain their turgidity and plants
wilt permanently.
vi. Maximum available water wilting coefficient: The difference between the field capacity
and the wilting coefficient is the maximum available water wilting coefficient. It may be
calculated as
Wilting Coefficient = Hygroscopic coefficient/0.68
= Moisture Equivalent/ 1.84
vii. Available water: The quantity of water retained in the soil between the limits of field
capacity and permanent wilting point or between a tension of 15 and 0.33 atm is known as
available water for plant use. Sandy soil has the lowest amount of available water 8cm/m
depth while clay soil has the highest amount i.e. 23cm/m depth.

Energy Concept of Soil Water

The free energy of water to do work is called by the scientists its water potential. The
absolute value of water potential of pure water is not measured. Instead it is arbitrarily assigned
by definition a value of Zero bar (percent of water at STP). When either a solute or water
adsorbing colloidal is added to water, its free energy is reduced in proportion to the concentration
and kind of the solute or colloid added. The lowered water potential is obviously below zero bars
and it, therefore, carries a negative sign. A completely wet soil may have its water potential near
zero bars while a dry one near -15 bars or less, more negative is the value of water potential in a
system, lower will be the energy of its water available to do work.
The forces that keep the soil and water together are based on the attraction between water
and soil molecules (adhesion) and amount water molecules themselves (cohesion). The surface
tension is most important force in the wet range, while adhesion is the main factor in dry range.
Thus higher the moisture content, smaller is the attraction of soil for water. When water is
present in fine capillaries, the energy with which it is attached is a function of the surface tension
and capillary size. But, when it is present in bigger pores, it is bound loosely to the soil and can
be acted upon by the gravity. Dissolved salts in water decrease the free energy of water. By
virtue of dissolved salts, soil water has lower free energy than pure water. Further, soil water
bound to solid particles as hygroscopic water is tightly held by the surface of contact and has a
low free energy by virtue of binding forces. Thus, there are two types of interactions that
decrease the free energy of water due to solubility of salts and due to interaction of water and
solid surfaces.
Soil water potential
Soil water potential or soil moisture tension is a measure of the tenacity with water is
retained in the soil and shows the forces per unit area that must be exerted to remove from the
soil. It is measured in terms of the potential energy of water in the soil measured, usually with
respect to free water. It is usually expressed in atmosphere (atm), the average air pressure at the
sea level. Other pressure units like cm or mm of water or mercury are also used.
Component of soil water potential: There are three main sources causing reduction in water
potential in soils to bellow zero bars viz. solute present in the soil, Colloidal soil matrix and
Force of Gravity. The water potential reduced due to presence of solutes is called its osmotic
potential, while that reduced due to the presence of soil colloids, its matric potential. Reduction
in water potential due to gravity forms its gravitational potential.
A. Osmotic potential: When pure water is separated from say a sucrose solution by a
differentially permeable membrane pure water gushes through the membrane into the
sucrose solution to raise its free energy level. The process is called osmosis and the water
potential on the solution side, its osmotic potential. Like wise when two solutions varying
in their osmotic potentials are separated by a differentially permeable membrane, again
osmosis will occur from the solution at higher(less negative) osmotic potential towards the
lower osmotic potential (more negative) solution till equilibrium is reached. At this stage
the osmotic potential gradient (earlier called the diffusion pressure deficit) becomes zero.
In all the normal, non saline, agricultural soils the osmotically active solute concentration
 are not high enough to offer any significant amount of the osmotic potentials. Therefore, in
such soils osmotic potential of water can be neglected. But in salt affected soils the osmotic
potential may become a very significant component of soil water potentials, which may be
lowered to the extent that the plant roots may fail to absorb water from it, even when it was
sufficiently moist. Some times, in extreme cases, water may even move out from plants
roots into the soil by the process of reverse osmosis. In all normal agricultural soils the
osmotic potential as a component of water potential is ineffective and can, therefore be
ignored.
B. Matric potential: water held in soil capillaries by adhesion and surface tension greatly
contribute to the matric potential. Water molecules, with their dipole characteristics are
rapidly attracted by soil matrix. The adhesion of water molecules to soil matrix lowers the
water potential in proportion to the intensity of the adhesion. Since this time the water
potential is reduced due to adhesion to soil matrix, it is called its matric potential. In
saturated soils, where there is almost no adhesion of water molecules to the soil, colloids,
the matric potential is about equal to that of pure water i.e. near zero bars. But, since all
crop production is conducted in unsaturated soils except rice, the matric potential forms a
very important component of soil water potential and it determines the movement of water
in soil as well as from, soil into the plant roots.
C. Gravitational potential: the third component of water potential in soils arises from the
gravitational pull of water. This is called gravitational potential. It is of importance only in
saturated soil where free water loses its energy and moves in response to gravity as
drainage water. In unsaturated soils, where water is held against gravity by force of
adhesion and cohesion gravitational potential of water can be easily overlooked.
Reviewing the three components of water potential in soils it is clear that in normal
agricultural soils the osmotic and gravitational potential do not influence the soil water and plant
relations. Here matric potential of water alone determines the availability of moisture to the plant
roots.

Movement of water in the soil

Movement of water in the soil comprises of three phases:
i. Infiltration
ii. Redistribution
iii. Withdrawal.
Immediately after application of irrigation or receipt of rain water enters the soil. It
gradually redistribute into different layers. Water moves from higher potential to lower potential.
Depending upon differences in water potential in different parts of the soil, the movement of
water may be lateral, upwards or downwards. Water movement in the soil occurs in three form i)
Saturated flow ii) Unsaturated flow iii) Vapour movement.
 A. Saturated flow: saturated flow of water occurs when all pores are filled with water either
due to rain or irrigation or under water logged condition.
B. Unsaturated flow: The unsaturated movement is in micropores. Unsaturated flow is
more important from crop production point of view.
C. Vapour movement: When soil becomes dry water from micropores is also emptied and
water is mainly present in the form of water vapour. Water vapours moves from one zone
to another due to vapour pressure gradient.

Particulars Saturated flow Unsaturated flow Vapour movement

Major force Gravitational Metric Vapour pressure
Water form Liquid Liquid Vapour
Major direction of Downward Lateral All directions
flow
Pore space All pores filled Micropores filled All pores are empty
with water with water
Volume of water Large quantity Small Negligible
movement (375000 kg/ha in (100000 kg/ha in 15 (15kg/ha in 15cm
15 cm depth) cm depth of soil) depth of soil)

Plant Water Relationship

The total quantity of water required for the essential physiological functions of the plant
is usually less than 5% of all the water absorbed. Most of the water entering the plant is lost in
transpiration. However, failure to replace the water lost by transpiration results in the loss of
turgidity, cessation of growth, and eventual death of the plant. The main areas of plant-water
relationship are water absorption, water conduction and translocation, and water loss or
transpiration. These processes are responsible for uptake of plant nutrient, creation of energy
gradient and ultimately all the metabolic and physiological activities in the plant.
Energy concept of plant water relations
Energy status of water in plant cells is determined by three major factors i.e, turgor
pressure, imbibational pressure and solute pressure. Pressures arising both from gravitational
forces and inter-cellular pressure can be included in the turgor pressure terms.

Component of water potential in plants

Water is present in all living plant tissues. Vacuoles of the cell are the chief sites of its
accumulation, although in lesser quantities, it is also present as adhered to protoplasmic and cell
wall constituents. The water potential in plants comprises of three components viz. solute
potential, matric potential and pressure potential.
 A. Solute potential: In plants, water is present in the form of solution of variable solutes of
different concentration called cell sap. The quantum of water potential lowered by the
presence of osmotically active solutes in plants tissues is called by plant physiologist as
its solute potential (osmotic potential in soils) Solute potential in plants may also vary in
different hours of the day and night due to variable water deficit prevailing in plants cells.
Thus, solute potential is the most active component of water potential in plants.
B. Matric potential: In all normally growing herbaceous plants, matric potential is
considered negligible. It is so because in these plants there is very meager water present
as adsorbed to the cell, the matric potential is considered as an insignificant component of
the cell water potential and it is frequently omitted.
C. Pressure potential: In living plants growing under adequate water supply the tissue cells
are usually turgid especially at night. Since, cell wall possess an elastic stretch, an inward
cell wall pressure is created in turgid cells. This, in turn develops an outward pressure on
cell wall, called Turgor pressure and cell wall pressure are of equal dimension in fully
turgid cells and these operate in opposite directions. The Turgor pressure being + ve,
tends to increase the water potential in plants tissue. Therefore, it is called pressure
potential of water. Solute potential carries a negative sign and pressure potential carries a
positive sign. Thus, in a fully turgid cell solute and pressure potential being numerically
equal, the cell water potential is nearly zero. But in a flaccid cell the pressure potential of
water is zero.
Soil Water-Plant-Atmosphere Continuum
Availability of soil moisture, its uptake by plants, its translocation through the plants, and
its evaporation into the atmosphere are various steps in the transfer of water through the soil
plant atmosphere system. Soil supplies water to plants and weather parameters controls the
demand of water for various agro-physiological processes in the plants. Thus, soil-plant-water
relationships are mainly related to the properties of soil and plants that affect the movement,
retention and use of water. The rate of entry of water into the soil and its retention, movement
and availability to plant roots are affected by the amount and size distribution of the soil pores.
The amount of water present in soil is not very useful unless the soil water potential is known as
far as the water availability to plants is concerned. Under field conditions water content of soil is
always changing constantly with time and also with depth of soil and is not static or constant.
Water contents in soils under certain standard conditions are referred to as soil moisture
constants. The concept of soil moisture constants greatly facilitates in taking decision in
irrigation management.
Water absorption by plants
Water is absorbed mostly through the roots of plants, though an insignificant absorption
is also done through the leaves. Plants normally have a higher concentration of roots close to the
soil surface and the density decreases with depth. In a normal soil with good aeration, a greater
portion of the roots of most plants remain within 45cm to 60cm of surface soil layers and most of
the water needs of plants are met from this zone. As the available water from this zone decreases,
plants extract more water from lower depths. When the water content of the upper soil layers
reach wilting point, all the water needs of plants are met from lower layers. Since, there exists
few roots in lower layers, the water extract from lower layers may not be adequate to prevent
wilting, although sufficient water may be available there.
When the top layers of the root zone are kept moist by frequent application of water
through irrigation, plants extract most of the water (about 40 percent) from the upper quarter of
their root zone. In the lower quarter of root zone the water extracted by the plant meets about 30
percent of its water needs. Further below, the third quarter of the root zone extracts about 20
percent and the lowermost quarter of root zone extracts the remaining about 10 percent of the
plants water. It may be noted that the water extracted from the soil by the roots of a plant moves
upwards and essentially is lost to the atmosphere as water vapours mainly through the leaves.
This process, called transpiration, results in losing almost 95percent of water sucked up. Only
about 5percent of water pumped up by the root system is used by the plant for metabolic purpose
and increasing the plant body weight.
Rate of water absorption is controlled by the rate of transpiration but is regulated by the
size and distribution of roots and several soil factors such as soil temperature, soil water
potential, soil water content, concentration of soil solution, aeration etc. Soil water content and
soil water potential are important determinants of water uptake. When water potential in leaves,
roots and soil become equal water absorption become negligible. Soil temperature is known to
influence water absorption and ultimately transpiration to considerable extent. Water absorption
below a soil temperature of 10°C is reduced sharply and increases linearly upto 25°C and slows
down later. Most plants are not able to absorb water when soil aeration is poor as in waterlogged
conditions. Root system is the most important plant factor influencing uptake of water.
Transpiration plays a significant role in soil water absorption. This process determines the
gradient between the atmosphere and the plant through leaves.
The effectiveness of roots in absorption of water depends on the extent of the roots
system and on the efficiency of individual root. Continuous absorption of water is essential for
the growth and even survival of most plants. Absorption and transpiration are linked by the
continuous water column in the xylem of the plant. The absorption of water occurs when there is
a decreasing gradient in water potential from the soil or solution surrounding the root to the root
xylem.
Translocation
Water movement in soil plant atmosphere is proportional to the driving force between
evaporating surfaces and inversely proportional to the various resistance in the pathway. Water
moves from sites of high potential to those of low potential. Plants can extract water from the
soil only when their water potential is lower than that in the soil. This potential difference or
water potential gradient between the evaporating tissues of the plant and the soil water in the root
zone increases with the evaporating demand and liquid flow resistance in the pathway. The rate
of water movement (flux) between any two points is described by the relationship:
Flux = Ψ1 – Ψ2 / Σr
Where Ψ1 and Ψ2 are the water potentials at the two points and Σr is the sum of
resistances to water movement between those two points. Thus, in soil plant atmosphere system,
Flux = ΨSoil – Ψroot surface / rsoil
= Ψroot surface – Ψxylem / rroot
= Ψxylem – Ψleaf / rxylem + r leaf
= Ψleaf – Ψair / rleaf + r air
Resistance to water movement in the soil is determined by its hydraulic conductivity,
water content and path length. Water movement in the plant can be considered in two phases i.e.
the liquid phase from the root surface to the mesophyll cell and the vapour phase from the
surface of the leaf mesophyll cell to the stomatal pores. The overall resistance to flow is
relatively low in the liquid phase, being highest in the roots, intermediate in the leaves and
lowest in the stems, where the movement is largely in vascular system. The transformation of
water from the liquid to the gaseous phase requires energy, which comes from the solar radiation
falling upon the leaf. There are several resistance to water movement from with in the leaf to the
external atmosphere but the most important are stomatal resistance and the resistance to diffusion
through boundary layer of still around the leaf.
Plant structure: Morphologically a plant consists of roots, stem and leaves. The leaves are born
throughout the stem in all the plants and are mainly responsible for the loss of water. Similar
variation in plant water relations among different species and between surfaces also exist. The
internal surface of leaf has small pores surrounded by two cells. These pores are called stoma
and the cells surrounding them are called guard cells.
The stomata regulate the loss of water as vapours and exchange of carbon dioxide in the
leaf and other organs. The leaves maintain their continuity of structure with the stem which has
conducting tissues called xylem, the main channels of water transport, and phloem. The stem
maintains its continuity with the root, which eventually is in contact with the soil. The outermost
cell of the root usually gets elongated into a long hair which has its continuity with the remaining
cells. Other epidermal cells are also capable of absorbing water but the root hair provides the
advantage of exploring more area because of its enlarged surface. Thus, a large number of root
hairs draw moisture from their vicinity and supply water to the cortex.
Rooting characteristics and moisture use: The amount of soil moisture that is available to a
plant is determined by the moisture characteristics of the soil, the depth to which the plant roots
extend and the proliferation of the roots. Little can be done to alter soil moisture availability.
Greater possibilities lie in changing the plant characteristics, enabling it to extend its rooting
system deeper into the soil, thereby enlarging its reservoir of water. Plants vary genetically in
their rooting characteristics. Vegetable crops, such as onions and potatoes, have a sparse rooting
system and are unable to use all the soil water within the root zone. Forage grasses, sorghum,
maize and such other crops have very fibrous, dense roots. Lucerne has a deep root system.
Perennial plant has already established root depth, and needs only to extend its small roots and
root hairs to utilize the entire amount of available soil water.
Moisture extraction pattern within root zone: The moisture extraction pattern is the relative
amounts of moisture extracted from different depths within the root zone. About 40% of the total
moisture used is extracted from the 1st, 30% 2nd, 20% 3rd and only 10% from the last quarter of
the root zone (Fig. 6.1). So to have a fair estimate of the soil moisture status, it should be
measured at different depths within the root zone.