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IRRIGATION WATER MANAGEMENT NOTES |
Tensiometers: Irrigation Moisture Sensors
Tensiometer are best used when the soil moisture content will be maintained at about
50-70% of the available water.
Where irrigation water management is aimed at maintaining low-tension conditions which
are most favorable (optimal) for plant growth.
It covers the entire range of readily available soil moisture required for maximum
plant growth.
The useful limit of a tensiometer is at about 80 centibars of maximum tension.
Tensiometer do not indicate the osmotic tension (i.e., the effect of soluble salts) of
soil water.
For furrow or flood irrigation, the tensiometer station should be placed about 2/3 of
the way down the run (a station at each end of the field is highly recommended).
The use of two tensiometers per station is recommended, with the shallow tensiometer
placed in the upper root zone (e.g., about the 9 - 12" depth); the longer tensiometer
is placed at about 3/4 of the actual depth of the root zone. The shallow tensiometer
indicates the moisture status of the very active root zone area, while the bottom one will
detect any under or over-irrigations.
What can a Tensiometer do?
A tensiometer continuously measures soil moisture tension: the readily available soil
moisture in the crop’s root zone.
It can evaluate the soil moisture status in any soil type.
It continuously follows the changes in the soil matric potential (i.e., soil water that
is adsorbed on the soil particles and held between them by surface tension).
The tensiometer is like a root equipped with a gauge that continuously measures how
hard the roots are having to work to extract water: the higher the reading in centibars,
the more energy is being used by plant to maintain the transpiration process. e.g., a
reading of 40 centibars of tension indicates that the roots are extracting the same amount
of moisture whether they are in a sandy or clayey soil.
A tensiometer measures the existing soil moisture condition only at the depth at which
the porous ceramic tip of the instrument is placed, and cannot measure moisture conditions
above or below this point.
They can be helpful in determining to which depth the soil moisture has actually
penetrated and how quickly.
By recording and analyzing the tensiometer readings, optimum irrigation cycles can be
determined and scheduled.
Tensiometer Readings: What they Indicate
Tensiometer measures soil moisture tension; the reading is given in centibars and is
related to a given quantity of soil water that a soil can retain at a given tension, which
is unique for each soil type.
In sandy soils, the tensiometer can measure more than 80% of the total available soil
moisture. This is because of the relatively large pores spaces in these soils which
release water at lower tension.
In clayey soils, the tensiometer will read about 80 centibars when the soil moisture
content contains about 75%of the available water.
A zero reading indicates saturated soil conditions. Readings of around 10 centibars
correspond to field capacity for coarse-textured soils, while reading of around 30
centibars are field capacity for finer-textured soils. The upper working limit of 80
centibars corresponds to as much as 90% depletion of available water for the
coarse-textured soils (e.g., coarse sand), but is only about 30% depletion of the
available moisture for silt loam, clay loams and other fine-textured soils.
A reading of 0-10 centibars (i.e., saturated to wet soil), often occurs for a day or
two following a irrigation. Continued readings in this range indicate over-irrigation:
danger of water-logged soils (inadequate root aeration) or high water table.
Tensiometer readings in the range of 10-20 centibars indicate that there is ample
moisture and air in the soil for healthy plant growth in all types of soils. This range is
often referred to as the field capacity of a soil (i.e., the maximum amount of water that
a soil can hold, with any additional amount draining below the root zone).
When tensiometer readings are at 10-20 centibars, irrigations are discontinued to
prevent wasting water (deep percolation) and leaching of nutrients.
Low tension tensiometer are designed to operate in coarse sandy soils and in non-soil
planting mixes. They are used where soil moisture tensions above 30 centibars are rarely
expected and where finer resolutions near saturation is needed.
Tensiometer readings of about 30-80 centibars is the usual range for starting
irrigations, except in drip irrigations where soil moisture is generally kept at field
capacity; optimal soil moisture tension and root aeration is assured in this range.
Irrigations should start in the lower part of this range in hot dry climates and in
coarse-textured sandy soils with low water holding capacities. In cool, humid climates or
in soils with high water holding capacity, irrigations may start at the upper end of this
range.
Electrical resistance type sensor (e.g., watermark) provides accurate soil moisture
readings from about 10-200 centibars of soil moisture tension, which is generally
considered the entire soil moisture range required in irrigated agriculture (even in
heavier clay soils).
The electrical resistance soil moisture sensor is read with a hand-held meter which
gives a digital readout in centibars of soil water tension. It converts the electrical
resistance readings to calibrated centibar reading.
Synthetic electrical resistance blocks are much more stable and have a longer life than
gypsum blocks; these blocks are more responsive to soil water tensions less than 100
centibars and thus are adaptable to a wider rage of soil textures and irrigation regimes
than are gypsum blocks.
Essentially any device or method of evaluating, monitoring, and scheduling irrigations
can be a useful tool if recognized that it is a management aid only.
Reading the Instruments Consistently
By recording a periodic and consistent reading of the tensiometer during the entire
season, you will know the available soil moisture level and you will be able to know when
and approximately how much irrigation water is needed (this information is useful in
analyzing, monitoring, and scheduling irrigations based on actual soil moisture
conditions, and also in planning future irrigation requirements).
By reading your sensors 2-4 times between irrigations, you will begin to notice the
rate at which the soil is drying out (i.e., the tensiometer readings are getting higher as
the soil is depleted of its moisture). The rate of change is as critical as the actual
tensiometer reading in determining when to irrigate.
Irrigation Scheduling
As a general rule of thumb, irrigations should begin before the soil moisture level in
the upper two feet of the root zone profile falls below fifty % of the available soil
moisture.
A soil moisture depletion of about 50% of the available moisture in a loamy soil may
occur at about 80 centibars, but a 50% depletion of the available soil moisture in a
clayey soil may occur at about 200 centibars of tension.
The soil moisture of a sandy loam is almost all readily available to plants, i.e., it
is a held at tension of less than 100 centibars, and thus it is easy for the plant to
extract.
It is very difficult to infiltrate a minimum of less than 2.0"-3.0" on most
soils. This is due to irrigation system constraints and inherent inefficiencies.
Typical soil moisture deficits at irrigation can range between 1.0"-3.0" for
most crops in the top two feet of the root zone.
In fields where there are more than one soil type, soil moisture measurements are
almost always made in the sandier section, since stress will occur there first. Note: Some
plants need stress at specific growth stages (e.g., crops such as cotton may need to be
stressed at certain growth stages to maximize yields or crop quality).
Sandy soils, which have a very low Cation Exchange Capacity (CEC), do not store large
amounts of nutrients. Thus, necessity of applying frequent and small amounts of N
fertilizer along with the irrigation water.
Most agricultural plants begin to suffer some moisture stress when the soil moisture
tension in the top quarter of the root zone is in the range of 50-150 centibars of
tension.
Farmers tend to keep the soil moisture reservoir full (i.e., field capacity) when the
water supply is unreliable (i.e., they schedule their irrigations earlier than needed, in
order to avoid excessive drying of the soil moisture reservoir).
Because a shallow root zone has a small available water holding capacity, it is unwise
to deplete most of it, since a delay in the irrigation would quickly deplete the minimal
reserves and that would cause substantial crop stress.
The shallow tensiometer instrument tells you when to start irrigating, while the deep
instruments monitors deep percolation.
The success of irrigation monitoring and scheduling is based upon good communication
between the irrigators and farm managers.
Optimal Root Growing Environment
In general, a soil moisture content close to field capacity is the optimal moisture
level for vegetative growth. At this soil moisture level there is good balance between
soil moisture tension (i.e., water is easy for the plant root to extract) and aeration
(i.e., there is plenty of oxygen for the essential function of root respiration).
Soil temperature governs the types and rates of chemical reactions which take place in
the soil. Also, the soil temperature strongly influences biological processes, such as
seed germination, seedling emergence, growth, root development, and microbial activity
(i.e., the continual breakdown of fresh organic residues and stable soil humus).
Actively growing plants will reach field capacity within a few hours on a sand and
within one to three days on a clayey soil where there is no restriction to freely draining
water.
Roots will grow well in an environment that supports a warm, moist, well-aerated, and
proper nutrient balance.
Plants roots are generally confined to the larger pores (macro pores) between
aggregates.
By their demand for oxygen, microbes can affect soil aeration as whole, especially
where there is a readily decomposable supply of fresh organic residues (e.g., recent
incorporation of crop residues or application of animal manure).
It is very important to understand that plant roots need sufficient and continual
supplies of oxygen to function in normal conditions (i.e., plant roots must respire
constantly to maintain metabolic processes in their roots).
On clayey soils it is important to have a proper balance between water and oxygen
availability (i.e., due to high water holding capacities of clayey soils, they can be
easily water logged, thus inducing anaerobic environments). This is generally not a
problem with sandy soils. Again, the availability of oxygen to plants is extremely
important.
It is essential that water and fertilizers be applied uniformly, and be applied in the
correct amounts, in order to obtain maximum yields.
Nutrients move into plant roots via the soil water solution and this is optimal when
the soil moisture tension is close to field capacity.
A loam soil has the capacity to retain water and nutrients better than that of sand
while its drainage, aeration, and tillage properties are more favorable than those of
clayey soil.
Consequences of Poor Irrigation Efficiencies
Excessive irrigation (i.e., inefficient irrigations) can result in water logging,
nutrient leaching, salt accumulation, and a host of other detrimental soil problems.
Yields will decline from excessive applications of irrigation water. Declines are due
to disease, poor aeration, cooler soil temperatures, and fertilizing leaching.
An excessively wet soil will stifle root growth and function just as easily as an
excessively dry soil will desiccate them.
Poor aeration (i.e., water logged soils) can decrease the uptake of water and induce
early wilting.
The soil water content dictates the air content and gas exchange of the soil, thus
affecting the respiration of roots, the activity of microbes, and the chemical state of
the soil (i.e., whether the soil is in a reduced, anaerobic environment or an oxidizing,
aerobic environment).
At saturation, the soil is filled with water, with virtually no air pockets. The
saturated soil moisture tension is at 0.0 centibars.
Saturation conditions generally only occur at the very surface of the soil during an
irrigation, and then only for very high application rates and on heavy clayey soils with
poor soil structure.
The constraints of the irrigation system will be a critical component of the magnitude
of irrigation efficiencies obtained (e.g., non-lasered fields, excessive runoff, low flow
rate, poorly maintained irrigation systems, etc. will invariably lead to poor irrigation
efficiencies).
Maintaining Water Stable Aggregates at the Surface
The formation and maintenance of water stable aggregates is the essential feature of
soil tilth, i.e., that physical condition in which the soil is an optimally loose,
friable, and porous assemblage of aggregates permitting free movement of water and air,
easy cultivation and planting, and unobstructed germination and root growth.
Agriculturist are usually interested in having soil, at least in its surface zone, in a
loose and highly porous and permeable condition.
A well-aggregated soil is generally the most desirable condition for plant growth,
especially in the critical early stages of germination and seedling establishment.
The shapes, sizes, and densities of aggregates and consequent soil porosity will
generally vary within the profile.
Aggregate size distribution is a determinant of pore-size distribution, which directly
affects the water holding capacity of the soil and strongly influences drainage and
aeration.
Adjacent aggregates of variable size often adhere to each other, though of course not
as tenaciously as do the particles within each aggregate.
The integrity of aggregate stability is generally measured in relation to the
destructive action of water. The desired condition is for the individual soil aggregates,
especially at the soil surface, to be water stable (i.e., the aggregates don’t
disintegrate or dissolve in the presence or action of water).
A primary requirement for soil aggregation is that the clay particles be flocculated
(i.e., that the individual clay particles exist as packets or clusters and not existing in
a dispersed state). However, flocculation is a necessary but not a sufficient condition
for aggregation. Aggregation includes flocculation plus the binding effect of natural
organic polymers (e.g., polysacharrides) produced as by products of microbial metabolism.
Incorporated crop residues and animal manures plus root exudations and the continual
death of roots (particularly of root hairs) promote the microbial activity, specifically
at the root surface (called the rhizosphere) which results in the production of humic
substances or binding agents.
Typical respiration rates at given temperature tend to be greater during the spring
then during autumn. The cause seems to be the more vigorous population of microbes and the
greater availability of undecomposed organic residues in spring than in fall.
The composition of the soil micro fauna and micro flora depends on the thermal and
moisture regimes, on soil pH and oxidation-reduction potential, the nutrient status of the
soil substrate, and the type and quantity of the OM present.
Prominent among the many microbial products capable of binding soil aggregates are
polysaccharides, as well as numerous other natural polymers.
Some inorganic materials such as calcium carbonate or iron oxides can also serve as
cementing agents.
Microbially synthesized humic substances (i.e., waste end products) are binding
substances that are transitory, as they are susceptible to further microbial
decomposition. Thus, organic matter must be replenished and supplied continually if
aggregate stability is to be maintained in the long run.
In addition to increasing the strength and stability of intra-aggregate bonding,
organic products may further promote aggregate stability by reducing wettability and
swelling.
Solutions of active organic agents can penetrate into soil aggregates and then
precipitate more or less irreversibly as insoluble (though still biological decomposable)
cements.
The maintenance of water stable aggregates at the soil surface through organic matter
additions (e.g., animal manure), surface residue management, crop rotations, and tillage
practices (e.g., no-till or minimal till) can, at least, minimize surface compaction and
crusting.
The structure of most coarse-grained (e.g., sandy soils) soils is single grained, as
there is little tendency for the individual grains of sand to adhere to each other and to
form aggregates. Also, these soils do not have the surface area or activity of clay
particles, which can form organic-clay complexes which protects the organic matter (humus)
from further decomposition. Consequently, under similar soil management practices, sandy
soils will have lower organic matter contents than clayey soils.
The most common causes of poor soil structure which are due to general agricultural
management practices include: equipment compaction due to heavy tractors or an excess
amount of equipment traffic through the field, and the depletion of organic matter in the
soil (e.g., low yielding organic residue crops and excessive cultivation)..
Irrigation factors which degrade soil surface structure include 1) irrigation with
water having a high % of Na, 2) irrigation with water having a high ratio of Mg/Ca, 3)
irrigation with a very pure water, and 4) sprinkler at low pressure with large droplets.
If there is a disproportionate amount of sodium of bicarbonate or carbonate in the
water the soil surface will seal up, causing infiltration problem. Very pure water causes
similar effects.
The resulting combination of high ESP and low salt concentration induces colloidal
dispersion (i.e., the separation of individual clay particles from one another), this
contributes to the formation of a dense crust.
Soil dispersion (i.e., disintegration of soil aggregates) generally occurs with
monovalent and highly hydrated cations such as sodium. Conversely, flocculation occurs at
high solute (e.g., irrigation water with an elevated level of soluble salts) concentration
and/or in the presence of divalent cations such as calcium.
The aggregates exposed at the soil surface are most vulnerable to the destructive
forces of irrigation water, cultivation practices, and heavy machinery traffic. The
surface aggregates which collapse and slake down during wetting may form a slick layer,
called a surface seal, of dispersed soil particles which may be several centimeters thick.
This process clogs the surface Macro pores and thus tends to inhibit the infiltration of
water into the soil and the exchange of gases between the soil and the atmosphere. Upon
drying, this layer shrinks to become a dense, hard crust which inhibits seedling emergence
by its hardness.
Arid-zone soil typically contain low organic matter contents and consequently can have
unstable soil aggregates which are particularly vulnerable to compaction, crusting, and
erosion.
Annual cropping systems, as compared to a pasture, will hasten the decomposition of
humus and the destruction of soil aggregates.
The influence of a cropping system on soil aggregation is seen to be a function of root
activity (density and depth of rooting and the rate of root proliferation), density and
continuity of surface cover, and the mode and frequency of cultivation and traffic.
Soil wetness at the time of cultivation has a great bearing on whether aggregated
structure is maintained or destroyed. Whereas cultivation of excessively dry soil is
likely to result in grinding or pulverizing the soil into dust.
Soil Matrix (Sand, Silt, and Clay)
The solid matrix of the soil includes particles which very in chemical and
mineralogical composition as well as in size, shape, and orientation. It also contains
amorphous substances, particularly organic matter which is attached to the mineral grains
and often binds them together to form aggregates.
The material of which the soil solid phase is composed includes discrete mineral
particles of various sizes, as well as amorphous compounds, with the latter generally
attached to, and sometimes coating, the particles. The solid phase consists by and large
of distinct particles, the largest among which are visible to the naked eye and the
smallest of which are colloidal and can only be observed be means of an electronic
microscope.
The sand fraction is often further subdivided into subfractions such as coarse, medium,
and fine sand. Sand grains usually consist of quartz, but may contain other primary
minerals (feldspar, mica).
Mineralogically and physically, silt particles generally resemble sand particles (which
are predominantly made up of quartz), but since they are smaller and have a greater
surface area per unit mass and are often coated with strongly adherent clay, they may
exhibit, to a limited degree, some properties of clay (e.g., cation exchange capacity,
stickiness).
The clay fraction, with particles ranging from 2 micro meters downwards. Clays
generally belong to a group of secondary minerals called the alumino-silicates.
Clays are secondary minerals, formed in the soil itself in the coarse of its evolution
from the primary minerals contained in the original rock.
The relative inert sand and silt fractions can be called the soil skeleton, while the
clay, by analogy, can be thought of as the flesh of the soil. Together, all three
fractions of the solid phase, as they are combined in various configurations, constitute
the matrix of the soil.
The organization of the solid components of the soil determines the geometric
characteristics of the pore spaces in which water and air are transmitted and retained.
The arrangement and orientation of the particles in the soil is called soil structure.
Soil structure is strongly affected by changes in climate, biological activity, and
soil management practices, and it is vulnerable to destructive forces of a mechanical and
physicochemical nature.
Soil structure is important as it determines the total porosity as well as the shapes
of individual pores and their size distribution. It affects the retention and transmission
of fluids in the soil, including infiltration and aeration. It may affect such phenomenon
as germination, root growth, tillage, and erosion.
Air capacity depends upon soil texture and soil moisture.
The total porosity reveals nothing about the pore size distribution, which is a very
important property.
Coarse-textured soils tend to be less porous than fine-textured soils, though the
average size of individual pores is greater in the former than in the latter.
In a sandy soil, most of the pores are relatively large, and once these large pores are
emptied at a given tension, only a small amount of water remains.
In clayey soils, the porosity is highly variable as the soil alternately swells,
shrinks, aggregates, disperses, compacts and cracks.
The bulk density of the soil is affected by the structure of the soil, i.e., its
looseness or degree of compaction, as well as by its swelling and shrinkage
characteristics, which are dependant upon clay content and wetness.
The texture of a soil can only be changed by massive plowing to mix the textures of
various soil depths.
The measurement and study of clay particles can help provide a basis for evaluating and
predicting soil behavior.
Clay is the decisive fraction which has the most influence on soil behavior. Clay
particles adsorb water and hydrate, there by causing the soil to swell upon wetting and
then shrink upon drying.The clay fraction is very important in understanding the various
soil properties of the soil.
The clay fraction, i.e., its content and mineral composition, largely determines the
specific surface of a soil.
The fraction which influences the physical behavior of the soil most decisively is the
colloidal clay, since it exhibits the greatest specific surface area and is therefore most
active in physicochemical processes.
It is the soil particle size and mineral composition, which largely determine the
behavior of the soil (e.g., its interactions with fluids and solutes, as well as its
strength, and thermal regime).
The cation-exchange phenomenon is of great importance in soil physics as well as soil
chemistry, since it affects the retention and release of nutrients ans salts, and the
flocculation and dispersion processes of soil colloids.
Clay mineral differs somewhat in surface charge density ( the number of exchange sites
per unit area of a particle surface), and differ greatly in specific surface area. Hence,
they differ also in their total cation exchange capacity.
Sand and silt have relatively small specific surface areas and consequently exhibit
comparatively little physicochemical activity.
The clay fraction differs mineralogically, as well as in particle sizes, from sand and
silt, which are composed mainly of quartz and other primary mineral particles.
The measurement of soil texture gives us an idea of the quantity of clay in the soil,
but it reveals very little of the specific character and activity of the clay.
The various clay minerals (e.g., montmorillonite, kaolinite, illite, sesquioxides)
differ greatly in properties and prevalence.
The most prevalent clay minerals are the layered aluminosilicates (e.g.,
montmorillonite).
Colloidal clay sizes may remain in suspension indefinitely while in their dispersed
state.
Water Storage in the Soil
Water is stored in the soil as a film around each soil particle and in the pore spaces
between the soil particle.
In an unsaturated soil, the water is constrained by capillary and adsorptive forces,
hence its energy potential is generally negative.
The micro pores are intra aggregate capillaries responsible for the retention of water
and solutes.
The finer the soil particles (as in clayey soils), the more water the soil can hold,
but the harder the plant has to work to draw moisture out of the soil.
The coarser the particles (as in sandy soil), the less moisture the soil will hold, but
more of the moisture is available for plant use.
Field capacity: is a measure of the water held by the soil against the influence of
gravity. If a soil is saturated by rainfall or irrigation and then allowed to drain freely
for 24 hours, the soil is usually at field capacity.
Available water holding capacity: the amount of water held between field capacity and
permanent wilting point
(the moisture content in the soil at which plants begin to irreversibly wilt; it is not
important to precisely know if the correct value is 20 bars or 10 bars because there is
generally very little change in soil moisture between those two values.). It is usually
expressed as inches/foot of soil or as total inches in the rootzone.
Readily available moisture is approximately 75% of the total available water.
Increased pore space permits more water to be held at field capacity and enhanced humus
content enables plants to extract more water before the wilting point is reached.
Because soil textures and structures vary with depth, samples must be taken at least
every foot of depth to determine available water holding capacity of the root zone (note:
nearly all stratified soils hold more water than if the profile were of a single uniform
texture, as much as 50-60% more available water than held in a similar depths of uniform
soil).
Each soil has different moisture holding characteristics. Such characteristics include
soil depth, available water holding capacity per foot of soil.
For laboratory determinations with disturbed soil samples, the soil moisture content at
30 centibars is called the soil moisture content at field capacity. In the field, the soil
moisture tension at field capacity is closer to 10-20 centibars.
The water content of a saturation paste is about twice the field capacity for some
soils.
Soil Water Adsorption and Capillarity: Matric Potential
It is the matric potential which characterizes the tenacity with which soil water is
held by the soil matrix.
Capillary moisture is the water in pore spaces held by the surface tension between the
water and the soil particles.
This potential of soil water results from the capillary and adsorptive forces due to
the soil matrix. These forces attract and bind water in the soil and lower its potential
energy below that of bulk water.
A negative pressure potential has often been termed capillary potential, and more
recently, matric potential, This potential of soil water results from the capillary and
adsorptive forces due to the soil matrix.
In non-salty well-drained soil, the matric potential is almost equal to the water
potential.
Matric potential is the dominant portion of the total water potential in most
situations where the soil salinity level is minimal.
Water in an unsaturated soil is subject to capillarity and adsorption, which combine to
produce a matric tension.
In sandy soils, adsorption of water is relatively unimportant and the capillary effect
predominates.
In the very high suction range, the predominant mechanism of water retention is
adsorptive rather than capillary.
Clays hold water at greater tensions than sandy soils.
Because of the finer pore spaces in clayey soils, they hold water at higher tensions.
A matrix potential of 15 bars (or 1,500 centibars), the difficulty to extract soil
moisture is so extreme that plants permanently wilt
Under normal conditions in the field, the soil is generally unsaturated and the
soil-water potential is negative.
To measure matric potential in the field, and instrument known as the tensiometer is
used.
Soil moisture tension defines matrix stress at single point and depth in the field. It
indicates when to irrigate, but not how much to apply.
Soil Water Salinity: Soluble Salts
Soluble salts in the soil solution exert additional soil moisture tension, which can
exceed the crops salt tolerance (salinity threshold), thus causing potential yield
reduction. The Electrical Conductivity (ECe) of the soil saturation extract is used in
evaluating the potential problem.
Estimates of the effects of the EC of the soil water (ECsw) on crop growth are made
from the ECe.
The ECe of a sample is always less than the ECsw, since the soil water is diluted with
distilled water during the process.
Since the porous cup walls of the tensiometer are permeable to both water and solutes,
the water inside the tensiometer tends to assume the same solute composition and
concentration as soil water.
A tensiometer soil solution access tubes can be used for easy extraction of soil
solution samples for testing as to salinity or plant nutrients in the soil water at actual
field conditions.
Total salinity is often referred to as TDS (Total Dissolved Solids).
Soil particles in the water are referred to as TSS, Total Suspended Solids.
Osmotic potential effects in the soil solution tend to reduce the range of available
moisture in such soils by increasing the amount of water left in the soil at the time the
plants wilt permanently.
Osmotic forces due to salt in the soil water create the same effect as a dry soil in
making it more difficult for water to move into the plant roots,
The potential energy of soil water is also reduced by the presence of solutes, i.e., by
the osmotic effect.
The presence of solutes in soil water affect its thermodynamic properties and lowers
its potential energy.
Water quality (salinity) can influence the availability of soil water to plants.
The relationship between EC and total part per million (ppm) of salt for irrigation
waters in the western US is: 1 mmhos/cm = 700ppm.
A soil water concentration of one mmho/cm (ds/,) is equivalent to approximately 36
centibars of soil moisture tension.
The ECsw at field capacity is about twice that at saturation.
As the soil dries from saturation to field capacity, obviously the soil solution would
concentrate, EC would increase, ant the osmotic potential would increase. Offsetting this
would be the tendency to precipitate solid salt phases (mainly carbonate and gypsum).
Assumption on salt concentration from saturation to field capacity, is that there is no
precipitation of solid phases.
If a sample is taken right after an irrigation, and then right before the next
irrigation, there is the same amount of salt (lb/ac) in the soil. However, the ECsw will
be very different.
In arid regions, salts such as gypsum and lime, dissolved from the upper part of the
soil, may precipitate at some depths to form a cemented pan.
Leaching Requirement
Leaching Requirement values are typically (2-8% of that applied), but may be as high as
30-40% with a high ECsw, a sensitive crop, ans high temperatures.
A good irrigation water has a moderately low ECiw, i.e., the EC of the irrigation water
(0.3-0.8 mmhos/cm) and a low adjusted Sodium Adsorption Ratio, SAR (Less than 3).
Leaching is the percolating of water through and out of the root zone, and thereby
reducing the total amount of soluble salt in the root zone.
Leaching Requirement (LR) computations: the extra percentage of water which must
infiltrate into the soil beyond the amount needed for meeting the evapotranspiration
needs, non-uniformity, and non-infiltrated losses such as spray evaporation, uncollected
runoff, is called the leaching requirement.
Leaching Fraction: the actual depth of water which infiltrates at the point of the
field having the least deep percolation.
In many cases, the water used for LR only represents a small percentage of the total
deep percolation.
The controlled salt salinity level should be less than that which would harm crops.
The leaching requirement is part of a maintenance program once the soil is no longer
saline.
High salt levels inhibit plant germination and emergence.
For farmers the most important aspect of salinity management occurs during germination.
Simply put, if a crop does not germinate and emerge, there is no need to worry about later
crop damage.
Reclamation Leaching: A rule of thumb for reclamation leaching of high salt levels is
that the soil salinity can be reduced by 90% if an equivalent root zone depth of water is
applied.
Water Flow Through Soil
Macro pores provide aeration and drainage.
A good structure will have many large pore spaces throughout the root zone, allowing
water and air to move.
The Macro pores are mostly the interaggregate cavities which serve as the principal
avenues for the infiltration and drainage of water and for the aeration. The separation
between the Macro pores and the micro pores is often arbitrary.
Wider pores conduct water more rapidly, and the liquid in the center of each pore moves
faster than the liquid in close proximity to the particles.
As tension develops, the first pores to empty are the largest ones, which are the most
conductive, thus leaving water to flow only in the smaller ones.
At some distance from the particle surfaces, water is held so weakly that the pull of
gravity causes some of it to drain.
When soils are at field capacity, any additional water that is added drains out of the
root zone within a day or two, before it can be used by the growing plant.
The soil water content at which gravitational movements stops is known as field
capacity. Field capacity is the relatively stable point where water has moved out of large
soil pores and is replaced with air.
For these reasons, the transition from saturation to unsaturation generally entails a
steep drop in hydraulic conductivity.
The water holding capacity of the soil refers to the amount of water that can be held
by the soil with only negligible drainage occurring.
The soil will lose very little water after if has drained to field capacity if there
are no plants growing in it.
Even portions of gravitational water may flow slowly from one layer to another of
contrasting texture (stratified).
The presence of impeding layer in the profile, such as layers of clay, sand, or gravel,
can inhibit redistribution. Thus, in the long run, it is the layer of lesser conductivity
which controls the process.
The differences in potential energy of water between one point and another give rise to
the tendency of water to flow within the soil (e.g., water moves from relatively high
energy to one of less energy).
Water tends to be drawn from a zone where the hydration envelopes surrounding the
particles are thicker to where they are thinner.
In the soil, water moves constantly in the direction of decreasing potential energy. In
other words, water moves spontaneously from where matric suction is lower to where it is
higher.
Water flow from a fine-textured layer (such as clay loam) into a coarse-textured one
(sand) is slow except at water potential near ) (free-flowing water).
A heavy clay soil exerts strong capillary forces and resists downward water movement by
gravity.
When water is applied during an irrigation, each upper increment of depth of soil must
reach field capacity before water will move down to the next depth.
Flow into a dry sand layer can take place only after the pressure head has built up
sufficiently for water to move into and fill the large pores of the sand.
Sandy soils are characterized by large voids between individual soil particles. These
large voids exert relatively weak capillary forces, but offer little resistance to
gravitational flow, with the result that lateral and upward water movements is limited,
while downward water movement is rapid.
During a typical irrigation, the soil profile does not reach saturation except perhaps
at the very surface. Free drainage : this drainage may be complete in few hours in a sandy
soil, but can take several days or weeks in a heavier clay soil.
Irrigation management typically affects the top of the soil surface, which in turn
affects infiltration rates.
A very thin layer of impermeable soil can reduce infiltration rates very effectively.
In general, the conductivity decreases with decreasing concentration of electrolytic
solutes, due to swelling and dispersion phenomena.
The infiltration rate is apt to be relatively high at first, then to decrease, and
eventually to approach a constant rate that is characteristic for the soil profile.
For soils with zero slopes, irrigation water management is much more simple than for
most sloping furrow systems because all of the applied water infiltrates.
Plant Uptake of Soil Water
The water stored in the pore spaces is held by surface tension and is the easiest for
the plant to extract.
Capillary moisture is the primary source of water for plants (while micro pores provide
water holding for plant use).
The smaller soil pores remain filled with water held in place be attractive forces,
Capillary water held at field capacity provides the moisture that a growing crop will take
up and use. I additional water is not added, a crop will eventually deplete the capillary
water and the crop will reach its wilting coefficient. This is the point where
transpiration rate exceeds the absorption rate.
As a soil dries out, the remaining water is held in smaller and smaller pores.
As tension is further increased, more water is drawn out of the soil and more of the
relatively large pores, which cannot retain water against the suction applied, will empty
out.
Total soil-moisture potential is often thought of as the sum of matric and osmotic
potentials and is a useful index for characterizing the energy status of soil water with
respect to plant water uptake.
The soil water potential is a combination of the effects of the surface area of soil
particles and small soil pores that adsorb water (matric potential), the effects of
dissolved substances (solute or osmotic potential).
Water from the soil is pulled or sucked into the plant root due to a higher
concentration of salts in the plant root.
Approximately 90 % of the moisture requirements of the plant are obtained from the
upper 3/4 of the root zone.
About 70% of the plant’s moisture requirements are obtained from the upper half of
the root zone.
In many cases the extraction pattern can be approximated by the 40-30-20-10 rule. Water
sued in the top 25% of the root zone represents 40%of the total water used.
Plants differ in their abilities to withdraw water from soils.
Plants differ in their water use rate.
Rooting depths will be greater on sandy soils than on clay soils.
Plants can use moisture if the soil moisture content is above field capacity (e.g.,
plants use water even while they are being irrigated).
Evapo-Transpiration (ET)
Evapo-transpiration (the amount of water given off through the leaves of the plant and
soil surface) determines the amount of water needed.
The four elements that affect evapotranspiration are 1) sun, 2) humidity, 3) wind, and
4) temperature. The tensiometer evaluates all of these factors.
Water moves from the soil into roots, up the plants, and into the air in response to
tension gradients (differences of water energy between two points).
Only about 2% of the transpired water in most field plants actually stays in the plant.
The rest simply passes through the plant.
In most irrigated situations, transpiration is the main component of beneficially used
water, but it may not be the main destination of applied water.
Plant variety in a given location has a genetic potential to transpire a certain
quantity of water during the growing season.
The ET values are generally estimate and then field verified with soil moisture
depletion measurements for average irrigation conditions,
ET of specific plants and the reference ET, depending upon plant type, stage of growth,
and root zone depth.
Evaporation from a soil surface during a regular irrigation season does not usually
affect the soil moisture content beyond 3-4 inches under the soil surface during the
growing season.
Wet soil and plant surfaces have more evaporation than dry soil surfaces.
The drier the soil, the slower the transpiration rate, because the water is held
tighter in the soil (more negative matric and osmotic potential) as it dries out.
The drier the soil, the greater will be the resistance to water extraction.
An increase in soil-water suctions is associated with a decreasing thickness of the
hydration envelopes covering the soil-particles surfaces.
Approximately 70-80 % of the available water in a coarse textured soil may be depleted
before the plant begins to reduce transpiration; transpiration reduction may begin when
only 30% of the available water is depleted from a clay. Nevertheless, the 30% of water
from the clay will generally be more inches of water than the 70% from the sand.
When a plant is stressed for water, the guard cells at the entrances of the leaf
stomata close as a protective measure to reduce water loss. Vegetative growth also
declines at that point because the flow of CO2 gas into the stomata is reduced, and this
gas is necessary for plant photosynthesis.
For some plants it may be desirable to stress the plant and reduce transpiration, e.g.,
stressing processing tomatoes prior to harvest to increase the percentage of soluble
solids contents.
As plant water status changes in the direction of water stress, ET tends to decrease.
As soil water potential decreases (soil dries), transpiration will start to decrease at
some critical point. Evaporation will decrease as the soil surface dries.
The crop yield per unit of irrigation water transpired is higher under good management
than under poor management.
High transpiration rates are not bad if they improve yield; a main purpose of
irrigation is to increase transpiration and yields.
A wet soil root zone will provide less matrix and salinity stress to a plant, and
therefore it will transpire more water than under large depletion scenarios.
More frequent irrigations would result in higher ET rates. Crop varieties also have
different season lengths.An efficient irrigation system may result in higher plant
transpiration rates than an inefficient irrigation system because there will be less dry
spots on the field.
Technical Questions about HIT may possibly be quickly answered by
contacting:
Rudy Garcia
Natural Resources Conservation Service
Soil Conservationist & Water Quality Specialist
e-mail: rgarcia@nm.nrcs.usda.gov
or call: 1-505-522-8775, extension 116
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