Group Title: Circular
Title: Soil plant water relationships
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 Material Information
Title: Soil plant water relationships
Series Title: Circular
Physical Description: 5 p. : ill. ; 28 cm.
Language: English
Creator: Haman, D. Z ( Dorota Z )
Izuno, Forrest T
Publisher: Florida Cooperative Extension Service
Place of Publication: Gainesville ;
Publication Date: 1993
Subject: Plants -- Effect of soil moisture on -- Florida   ( lcsh )
Plant-water relationships -- Florida   ( lcsh )
Soils -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 5).
Statement of Responsibility: Dorota Z. Haman and Forrest T. Izuno.
General Note: Title from caption.
 Record Information
Bibliographic ID: UF00084356
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 28923253

Full Text


Soil Plant Water Relationships1

Dorota Z. Haman and Forrest T. Izuno2

Florida is classified as having a humid subtropical
climate. The average annual rainfall for most of
Florida is somewhere between 50 and 60 inches. This
is more than any crop uses during a growing season.
However, the typically erratic distribution of rain and
Florida's predominantly sandy soils make frequent
irrigation necessary in order to avoid plant stress
during drought conditions. To understand why
irrigation is necessary in Florida one must understand
soil-plant-water relations. A proper understanding of
these concepts is important to encourage wise use of
irrigation systems and promoting water conservation.


Water is essential in the plant environment for a
number of reasons. Water transports minerals
through the soil to the roots where they are absorbed
by the plant. Water is also the principal medium for
the chemical and biochemical processes that support
plant metabolism. Under pressure within plant cells,
water provides physical support for plants. It also
acts as a solvent for dissolved sugars and minerals
transported throughout the plant. In addition,
evaporation within intercellular spaces provides the
cooling mechanism that allows plants to maintain the
favorable temperatures necessary for metabolic

Water is transported throughout plants almost
continuously. There is a constant movement of water
from the soil to the roots, from the roots into the
various parts of the plant, then into the leaves where
it is released into the atmosphere as water vapor
through the stomata (small openings in the leaf

surfaces). This process is called transpiration.
Combined with evaporation from the soil and wet
plant surfaces the total water loss to the atmosphere
is called evapotranspiration.

One of the openings (stoma) is shown on the leaf
crossection in Figure 1. Guard cells which are found
on both sides of the stoma control its opening and
closing (Figure 2). Stomata can be found on one
(typically underside) or both sides of a leaf depending
on plant species.

Figure 1. Cross section of a leaf.

Well-watered plants maintain their shape due to
the internal pressure in plant cells (turgor pressure).
This pressure is also necessary for plant cell
expansion and consequently for plant growth. Loss of
this pressure due to insufficient water supply can be
noticed as plant wilting.

1. This document was published January 1993 as Circular 1085, Florida Cooperative Extension Service. For more information, contact your county
Cooperative Extension Service office.
2. Associate Professor, Irrigation Specialist, Agricultural Engineering Department and Associate Professor, Water Management Specialist,
Everglades Research and Education Center, Belle Glade, FL, Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville.

The Institute of Food and Agricultural Sciences is an Equal Opportunity/Affirmative Action Employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, or national origin.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean


Soil Plant Water Relationships

Figure 2. Stoma opening with guard cells.

The schematic effects of water stress on plant
growth are presented in Figure 3. The major
economic consequence of insufficient water in
agricultural crops is yield reduction. When too little
water is available in the root zone, the plant will
reduce the amount of water lost through transpiration
by partial or total stomatal closure. This results in
decreased photosynthesis since the CO2 required for
this process enters the plant through the stomata.
Decreased photosynthesis reduces biomass production
and results in decreased yields.



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Figure 3. The effects of water stress on plant growth.



The role of soil in the soil-plant-atmosphere
continuum is unique. It has been demonstrated that
soil is not essential for plant growth and indeed plants
can be grown hydroponically (in a liquid culture).
However, usually plants are grown in the soil and soil
properties directly affect the availability of water and
nutrients to plants. Soil water affects plant growth
directly through its controlling effect on plant water
status and indirectly through its effect on aeration,
temperature, and nutrient transport, uptake and
transformation. The understanding of these
properties is helpful in good irrigation design and

The soil system is composed of three major
components: solid particles (minerals and organic
matter), water with various dissolved chemicals, and
air. The percentage of these components varies
greatly with soil texture and structure. An active root
system requires a delicate balance between the three
soil components; but the balance between the liquid
and gas phases is most critical, since it regulates root
activity and plant growth process.

The amount of soil water is usually measured in
terms of water content as percentage by volume or
mass, or as soil water potential. Water content does
not necessarily describe the availability of the water to
the plants, nor indicates, how the water moves within
the soil profile. The only information provided by
water content is the relative amount of water in the

Soil water potential, which is defined as the
energy required to remove water from the soil, does
not directly give the amount of water present in the
root zone either. Therefore, soil water content and
soil water potential should both be considered when
dealing with plant growth and irrigation. The soil
water content and soil water potential are related to
each other, and the soil water characteristic curve
provides a graphical representation of this
relationship (Figure 4).

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Soil Plant Water Relationships

The nature of the soil characteristic curve
depends on the physical properties of the soil namely,
texture and structure. Soil texture refers to the
distribution of the soil particle sizes. The mineral
particles of soil have a wide range of sizes classified
as sand, silt, and clay. The ranges of sizes for those
particles are presented in Table 1. The proportion of
each of these particles in the soil determines its

Figure 4. An example of the soil characteristic curve.

Table 1. Particles sizes for various textural groups.

Textural group Particle size Particle size
(mm) (in)

Gravel > 2.0 >0.08
Silt 0.002 0.05 0.00008 0.002
Clay < 0.002 < 0.00008
coarse sand 0.5 1.0 0.02 0.04
medium sand 0.25 0.5 0.01 0.02
fine sand 0.10 0.25 0.004 0.01
very fine sand 0.05 0.10 0.002 0.004

All mineral soils are classified depending on their
texture. Every soil can be placed in a particular soil
group using a soil textural triangle presented in
Figure 5. For example a soil with 60% sand and 10%

clay separates is classified as a Sandy loam (see point
A in Figure 5).




Figure 5. Soil textural triangle.

In addition almost all soils contain some organic
material, particularly in the top layer. This organic
material, together with the fine soil particles,
contributes to aggregate formation which results in
the improvement of the soil structure. Soil structure
refers to the arrangement of soil particles into certain
patterns. The structural pattern, the extent of
aggregation, and the amount and nature of the pore
space describe the structure of the particular soil. No
structure is usually present in the Florida's sandy
soils, however the presence of the organic matter can
improve the water holding capacity of the soil.

The size, shape, and arrangement of the soil
particles and the associated voids (pores) determine
the ability of a soil to retain water. It is important to
realize that large pores in the soil can conduct more
water more rapidly than fine pores. In addition,
removing water from large pores is easier and
requires less energy than removing water from
smaller pores.

Sandy soils consist mainly of large mineral
particles with very small percentages of clay, silt, and
organic matter. In sandy soils there are many more
large pores than in clayey soils. In addition the total
volume of pores in sandy soils is significantly smaller
than in clayey soils (30 to 40% for sandy soils as
compared to 40 to 60% for clayey soils). As a result,
much less water can be stored in sandy soil than in
the clayey soil. It is also important to realize that a

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Page 3

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Soil Plant Water Relationships

significant number of the pores in sandy soils are
large enough to drain within the first 24 hours due to
gravity and this portion of water is lost from the
system before plants can use it.

To study soil-water-plant relationships it is
convenient to subdivide soil water into water available
to the plant and water unavailable to the plant. After
the soil has been saturated with water one can
observe a vertical, downward movement of water due
to gravity. In Florida soils, this drainage process
happens quickly. Usually 24 hours is sufficient to
remove most of the gravitational water in sandy soils.
The exact time depends on the soil type; the drainage
of the gravitational water generally takes a little
longer for clayey soils. Most gravitational water
moves out of the root zone too rapidly to be used by
the plants. The remaining water is stored under
tension in the various size pores. The smaller the
pore the greater the tension and the more energy
required to remove its water. As a result plants have
the ability to remove water only from the certain size
pores. The removal of water from very small pores
requires too much energy and consequently, this
water is not available to the plant. There is also
some water which is very closely bound to soil
particles. This water is called hygroscopic water. It
is also very difficult to remove, and is not available to
the plants.

The range of water available to plants is between
field capacity (FC) and the permanent wilting point
(PWP). The soil is at field capacity when all the
gravitational water has been drained and a vertical
movement of water due to gravity is negligible.
Further water removal for most of the soils will
require at least 7 kPa (7 cbars) tension. The
permanent wilting point is defined as the point where
there is no more water available to the plant. The
permanent wilting point depends on plant variety, but
is usually around 1500 kPa (15 bars). This means that
in order for plants to remove water from the soil, it
must exert a tension of more than 1500 kPa (15 bars).
This is the limit for most plants and beyond this they
experience permanent wilting. It is easy to see that
soils which hold significant amounts of water at
tension in the range plants are able to exert (up to
1500 kPa (15 bars) of tension) will provide better
water supply for plant growth (Figure 6).

Figure 6. The amount
different soils.





of available water in two

Unfortunately, Florida has very sandy soils which
do not provide good water storage. The pores in
sandy soils are generally large and a significant
percentage drain under the force of gravity in the first
few hours after a rain. This water is lost from the
root zone to deep percolation. What remains is used
very quickly and the state of PWP can be reached in
only a few days.

Ranges of available water for various soils are
presented in Table 2. For a typical sandy soil found
in Florida this value is approximately .75 in/ft. This
means that only .75 in of water available to plants can
be stored in one foot of root zone. The amount of
water available to the plant depends on the depth of
the root zone. Naturally, a plant with a deeper root
zone will have more water available than a seedling
with roots only 2 to 3 inches deep. That is why root
depth is so critical in irrigation scheduling and young
plants with a shallow root zone require more
frequent, light application of water. To learn more
about irrigation scheduling the reader is referred to
the IFAS Bulletin 249 "Basic Irrigation Scheduling".








Page 4

Soil Plant Water Relationships

Page 5

Table 2. Available water for various soil types.

Available water
Type of Soil
range average
(in/ft) (in/ft)
Sands and fine sands 0.4 1.00 0.75
Moderately coarse- 1.00 1.50 1.25
textured sandy loams
and fine
Medium texture very 1.25 1.75 1.50
fine sandy loams to
silty clay loam sandy
Fine and very fine 1.50 2.50 2.00
texture silty clay to clay
Peats and mucks 2.00 3.00 2.50


Bouma J., R.B. Brown, and P.S.C. Rao. 1982. Basics
of Soil-Water Relationships -Part I Soil as a
Porous Medium. Soil Science Fact Sheet SL-37.
Florida Cooperative Extension Service. IFAS.
Gainesville, FL.

Bouma J., R.B. Brown, and P.S.C. Rao. 1982. Basics
of Soil-Water Relationships -Part II. Retention of
Water. Soil Science Fact Sheet SL-38. Florida
Cooperative Extension Service. IFAS.
Gainesville, FL.

Bouma J., P.S.C. Rao, and R.B. Brown. 1982. Basics
of Soil-Water Relationships -Part III. Movement of
Water. Soil Science Fact Sheet SL-39. Florida
Cooperative Extension Service. IFAS.
Gainesville, FL.

Merva G.E. Physioengineering principles. 1975. The
AVI Publishing Company, Inc. Westport, CT.

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