Institute of Foo
John T. Woe
ivid P. H. Tucker
erative Extension Service
d and Agricultural Sciences
of Florida, Gainesville
ste, Dean for Extension
(--G"17 (L-Ir ) I-arsa-arrmnu
v v v w w v v v v w v
CITRUS IRRIGATION MANAGEMENT
David P. H. Tucker'
Although Florida's average annual rainfall of 50-62 inches
exceeds the water requirements of citrus, irrigation is required
for maximum production. This is because of the seasonal nature
of the rainfall, the major proportion of which comes during the
June through September period, and the very low water-holding
capacity of the sandy soils on which most citrus is planted. That
period of the dry season extending from January through May
coincides with those times when adequate moisture is essential:
these are the critical stages of leaf expansion, bloom, fruit-set
and fruit enlargement. It is during this period that sound ir-
rigation practices should be emphasized. Irrigation during the
fall and winter months should be considered only when tree
stress is imminent. Excessive irrigation during this period can
result in lower internal fruit quality. Over 500,000 acres (about
3/4 of the total) of citrus in Florida are now under some type
of irrigation program.
As urban growth in Florida continues, demands on water
supplies by municipal and industrial users will limit the avail-
ability of good quality water for citrus production. Therefore,
growers must assess their water needs, determine the most ef-
ficient uses for the water allocated, and eliminate wasteful irriga-
tion practices. It is worth noting that in addition to being sea-
sonal in nature, Florida's annual rainfall has been quite variable.
Based on an average of 53 inches, during the 1950's Florida had
above average rainfall. Since 1970, however, the state has re-
ceived below the average. Such variability contributes to the
difficulties encountered in long-term water management plan-
Many water uses are incompatible and, therefore, there is a
conflict among water users. In effect, in certain areas of Florida
during the dry season water is a scarce resource in that not all
uses can be met without restrictions on some users. All waters
are subject to regulation under the Florida Water Resources Act
of 1972. The heart of the administrative approach to enforce-
ment is the permit system administered by 5 water manage-
ment districts. There are 2 types of permits--regulatory and
consumptive use. The regulatory permit provides control over
physical modifications and is required for well drilling and pump-
ing equipment, and for alteration or construction of all types of
1Extension Horticulturist, Fruit Crops Department, IFAS, University of
water control and impoundment structures. A consumptive use
permit must be obtained for removal of water from all sources
for all uses except domestic consumption by individuals.
Growers requiring assistance in determining irrigation needs
for filing consumptive use applications with various water dis-
tricts should study the IFAS Water Resources Council Fact Sheet
WRC-4 entitled "Water Requirements for Citrus," or consult
with their local County Cooperative Extension Service office.
Yield Response to Irrigation
A number of studies in Florida have shown that irrigation
can significantly increase citrus fruit production. Such yield in-
creases were obtained by maintaining adequate soil moisture in
the root zone in the spring when the fruit is small. Specifically,
soil moisture should be maintained above 65% of field capacity
(the amount of water held after excess water has drained away
and the rate of downward movement of water has materially
decreased) between fruit set (February-March) and such time
when young fruit have reached more than 1 inch in diameter
(June-July) when the summer rains start. Irrigation timing
during this time is complicated by the uncertain rainfall distri-
bution and rapidly changing seasonal temperatures.
It has also been shown that citrus varieties respond different-
ly to irrigation. Yield responses from irrigation over a 6-year
period were significant for Hamlin, Valencia, and Marsh grape-
fruit on rough lemon rootstock. Trees were irrigated at 1/3 de-
pletion of available soil moisture in the first 5 feet of Astatula
fine sand from January to June and at 2/3 depletion for the bal-
ance of the year. Yield increases were 35%, 29%, and 47% for
Hamlin and Valencia oranges and Marsh grapefruit, respectively.
Pineapple orange trees showed only a 10% increase. On this
soil type this variety should, therefore, be irrigated at % de-
pletion of available soil moisture in the first 5 feet of soil. Dancy
tangerines also responded favorably to irrigation with a 37%
yield increase reported following 9 irrigations. Bearss lemon
production was greatest when a high soil moisture content was
maintained throughout the irrigation season.
Irrigation Effects on Fruit Quality
Irrigation has been shown to have a significant influence on
citrus fruit size and quality. These effects are summarized in
Table 1. With Dancy tangerines, irrigation can advance the
maturity date in certain years. The average decrease in soluble
solids from irrigation over a 5-year period was about 10% as
compared to a 28% reduction in the acid content. This differ-
ence was reflected in the higher soluble solids-acid ratio. Irri-
gation also can be used favorably in heavy crop years when
many growers irrigate to increase fruit size. As mentioned
Table 1. Effects of irrigation on citrus.
Increase Decrease No change
Tree size *
Fruit size *
Fruit color *
Juice content *
Sol. solids *
earlier, however, excessive fall irrigation can reduce the solids
content of oranges for processing. For Temple orange, adequate
spring irrigation was required to reduce the occurrence of late-
bloom fruit. With Bearss lemon supplemental irrigation affected
fruit size more so than for other citrus varieties and, therefore,
can be used to the grower's advantage. A high soil moisture
content maintained through the use of overhead sprinkler irri-
gation not only increased fruit production but also fruit size,
peel oil content and hastened fruit maturity. Since rainfall and
irrigation will create favorable conditions for the development
of scab infection, more emphasis should be placed on a scab
control program where lemons and other scab-susceptible varie-
ties are being grown for the fresh market.
Soil Moisture Measurements
Many growers have traditionally relied on visual symptoms
of tree wilting and soil condition as indices for irrigation. Such
practices have frequently resulted in irrigation being applied
too late to have any beneficial effect on fruit production.
Soil moisture measurements have not been widely used in
Florida for scheduling irrigation. In the past, the low cost of
irrigation and the ready availability of water have not created
the incentive to utilize precise moisture measurements in water
management. This is no longer the situation with the rapidly
rising costs of irrigation and the increasingly stringent regula-
tions in water usage. For research purposes and for the schedul-
ing of irrigations in areas such as California, soil moisture de-
terminations have been made using gravimetric, tensiometric
and neutron scattering methods. The tensiometric method is the
most widely used commercially.
The gravimetric method is the least technical, but most labori-
ous. Here, the moisture content of a soil sample is determined
by oven drying the sample at 1050C and calculating the moisture
loss. The soil moisture content is then transformed to a volume
percentage by multiplying the weight percentage by the average
bulk density value of the particular soil at the depth of sampling.
The tensiometric method utilizes an instrument called a soil
moisture tensiometer, the essential component of which includes
an unglazed ceramic cup, a connecting tube, and a vacuum gauge.
The water in the soil forms films over each soil particle. The pore
openings in the cup wall provide passageways between the water
in the cup and the films covering the soil particles. As the soil
dries out, the water films become thinner and more tightly bound
to the soil particles. The tension thus produced within these water
films causes water to be pulled from the cup, and this movement
produces tension or suction within the tensiometer, which is in-
dicated by a higher reading on the vacuum gauge. When water
from irrigation or rainfall reaches the vicinity of the cup, the
tension within the water films on the soil particles is reduced,
and water moves back into the tensiometer cup. The tension
within the instrument is thus reduced, and the gauge shows a
lower reading. This method has shown the greatest variability
in Florida soils, probably due to the very limited area which each
instrument measures. It is also not uncommon for the ceramic
cup to become root-bound, resulting in readings not representa-
tive of the surrounding soil. The neutron scattering method for
soil moisture determination utilizes an instrument consisting
of a fast neutron source, a slow neutron detector, and a counting
device. The theory behind it is based on the fact that hydrogen
ions in the soil, primarily in water, will react with fast neutrons
emanating from the source. The fast neutrons, upon colliding
with hydrogen ions, lose sufficient kinetic energy to become slow
neutrons. Some of these slow neutrons are deflected back to a
detector and are recorded on a counting device. The slow neutron
count over a fixed time is checked on a calibration curve to give
the soil moisture content.
Determination of Irrigation Requirements
The moisture requirement of mature citrus trees will vary
from 42-48 inches per year, depending on environmental consid-
erations such as temperature, day length, relative humidity and
wind. Although relatively high temperatures and moderate hu-
midity are favorable for tree growth, they are also conducive to
water loss through evapotranspiration (the amount of water
lost through evaporation from the soil plus that transpired by
plant surfaces) (ET). The mean monthly water use (ET) may
vary from a low of 0.08 inches per day in winter months upwards
to a peak of 0.17-0.20 inches per day in summer (Figure 1).
Citrus irrigation requirements are also influenced by soil
type. While some finer textured soils used for citrus store rela-
tively large amounts of water per unit volume, others with
coarser texture can store very little. Soils used for citrus in Flori-
da have water-holding capacities from 0.2-0.8 inches of water
per foot of soil. The variation in water-holding capacity between
2 particular soil types can be attributed to textural differences
which are largely dependent on the presence of organic matter,
J J A S
rainfall and daily
0 N D
clay, other water-retaining materials, and to depth of water
table. Generally, citrus on soils with low-water holding capacity
will have a greater need for irrigation than that on soils with a
The rooting depth of the citrus tree is a major factor in de-
termining the need for irrigation. It has been shown that trees
grown on well-drained sandy soils may have feeder roots extend-
ing 5 feet or more into the soil. On the other hand, groves on
flatwood soils rarely exceed 2 feet in rooting depth. This very
restricted rooting depth increases the importance and need of
irrigation. Table 2 shows rooting depths and usable stored water
of major soils planted to citrus.
Because of the factors involved (soil type, rooting depth,
planting density, time of year, rainfall, etc.) general recommen-
dations about the frequency of irrigation are difficult to make.
The question of when to irrigate must be answered by the indi-
vidual grower for each grove or part of grove. This is especially
true in areas where several soil types are present. Changes in
soil type, rooting depth and differences in rootstock or scion
variety within a given block require separate evaluation. Each
Table 2. Rooting depths and usable stored water of major
soils planted to citrus.
Soil series rooting depth1 Usable stored water'
feet in. in root zone
St. Lucie, Paola, Lakewood 4-6 0.6-1.8
Astatula, Candler, Lakeland,
Blanton high 4-6 1.2-3.6
Apopka, Arredondo, Gainesville,
Ft. Meade, Lake, Orlando,
Eustis 2.5-4 1.2-3.2
Pomello, Tavares, Blanton low 1.5-4 0.9-3.6
Somewhat poorly to poorly drained:
Immokalee, Oldsmar, Wabasso,
Ona, Myakka, Wauchula, Leon 1.5-2 1.2-2.8
Charlotte, Felda, Placid,
Pompano, Pineda, Riviera,
Parkwod, Delray, Plummer,
Rutledge 1.5-2 2.0-4.2
Rockdale 1.0-1.7 0.7-1.4
1Rooting depth may vary significantly from these values due to specific
water management and soil. site characteristics.
2Seventy percent of the plant's available water capacity.
block should be analyzed carefully to determine the need for ir-
rigation for each area based on knowledge of existing conditions
and records of rainfall and irrigation applied.
For additional information on the water-retaining character-
istics and rooting depths of major soils planted to citrus, refer
to IFAS Water Resources Council fact sheet WRC-4 entitled
"Water Requirements for Citrus." In this publication soils are
classified as well drained, moderately well drained, and poorly
drained. Average rooting depths are given for each class as well
as average values for their water storage capacities, including
available and usable water in inches per foot of soil.
Those who make a careful analysis of the water available to
their trees (usable water per foot of soil x rooting depth) may
be able to efficiently utilize a bookkeeping system. Daily evapo-
transpiration losses are deducted and rainfall or irrigation are
added to the current moisture balance. This enables the grower
to keep a record which shows the current level of soil moisture.
Table 3 illustrates an example of the bookkeeping system in a
deep, well-drained soil. Without rainfall, 1/ depletion of available
soil moisture may occur within 10 to 13 days and irrigation
should be applied. Table 4 illustrates an example of the book-
keeping system on a poorly drained soil with a restricted rooting
Table 3. Irrigation schedule for Astatula fine sand.
Astatula fine sand with 3.5 inches of water per 5 foot of soil (0.7 inches
usable water per foot of soil X 5 foot rooting depth.)
Start accounting with maximum usable water.
Date Water use/day* Rainfall Balance
May 1 .14 3.5 3.50
2 .14 3.36
3 .14 3.22
4 .14 3.08
5 .14 2.94
6 .14 .3 (rain) 3.10
7 .14 2.96
8 .14 2.82
9 .14 2.68
10 .14 2.54
11 .14 2.40
12 .14 2.0 (irrigation) 3.50
Irrigation applied at /3 depletion of soil moisture.
(V3 X3.5= 1.17 inches. Irrigate when soil moisture gets down to 3.50-
*Average for May; this value may vary due to daily climatic changes.
Table 4. Irrigation schedule for Myakka fine sand.
Myakka fine sand with 2.8 inches usable water per 2 foot of soil (1.4
inches usable water per foot of soil X 2 foot rooting depth).
Start accounting with maximum usable water.
Date Water use/day* Rainfall Balance
May 1 .14 2.0 2.80
2 .14 2.66
3 .14 2.52
4 .14 2.38
5 .14 2.24
6 .14 2.10
7 .14 1.96
8 .14 1.0 (Irrigation) 2.80
9 .14 2.66
10 .14 1.0 (Rainfall) 2.80
Irrigation applied at / depletion of soil moisture.
( /3 X2.8=0.93 inches. Irrigate when soil moisture gets down to 2.80-
.93 = 1.87 inches.)
*Average for May; this value may vary due to daily climatic changes.
depth. Because of the low water-holding capacity, depletion of 1/3
of the available moisture may occur after only 6 days, necessi-
tating irrigation if maximum benefits from irrigation are to
Until more specific information is available on the response
of citrus to low-volume irrigation (particularly drip or trickle
irrigation) under Florida conditions, the rates and frequency of
application can best be estimated using currently available figures
Table 5. Daily tree evapotranspiration (gal/tree/day).
Month 15 year 24 year 20 year
January 26 30 21
February 25 30 21
March 34 36 27
April 39 44 36
May 49 54 46
June 59 67 61
July 63 71 68
August 64 71 70
September 57 65 43
October 44 49 41
November 32 35 28
December 27 30 19
on daily tree evapotranspiration, soil water-holding capacities,
and their respective wetting patterns. Table 5 shows values for
daily evapotranspiration by month for grapefruit and orange
trees of different ages.
Practically speaking, drip irrigation systems should not be
expected to provide the entire water requirements of the tree,
especially larger trees, as would an overhead sprinkler system
which usually covers the entire grove floor surface. The number
of emitters installed per tree should be such that their combined
wetting pattern area covers as much of the root zone as possible
or at least 50 to 60%. The poorer the soil type, the lower its
water-holding capacity, and the smaller the area of soil which
will be wetted under a drip emitter. With such soils it is pre-
ferable to use more emitters per tree, and a shorter irrigation
cycle with an increased application frequency. Conversely, on
heavier soils with greater water-holding capacities, fewer emit-
ters may be used, and systems should be operated for longer
periods at lower frequencies.
Now let us consider an application of 1 inch of water through
an under-tree microsprinkler system as we might in conven-
tional irrigation. Assume that the particular sprinkler delivers
water at the rate of 10 gal/hour (GPH) and wets an area of 10
feet in diameter over the tree root zone (covering in excess of
the 50 to 60%).
The area wet by sprinkler = X 52= 78 sq. ft. approx.
78 sq. ft. is 0.0018 acre
( 78 )
There are 27,154 gallons in 1 acre inch. Therefore, 27,154X
Forty-eight gallons applied to a 10-foot diameter area would
be equivalent to a 1-inch application. With a sprinkler applica-
tion rate of 10 GPH, the system should be run 5 hours to apply
50 gallons or slightly less time for 48 gallons.
Selection of an Irrigation System
For an investment in an irrigation system to be reasonable,
a grower must have assurance that water will be available. The
grower must take into consideration a number of factors when
selecting an irrigation system for his particular conditions.
1. Should an irrigation system be regarded simply for in-
surance of tree survival or should it be utilized to increase
yields to a ppint where the net returns from the crop will
pay for the system and allow for profit?
2. The availability of water, its source and quality. A low-
volume system would be an appropriate choice in areas
of the state where critical water shortages occur season-
ally. The source of water, be it well or surface, will de-
termine the type of consumptive usage regulations in-
volved. Water quality will also influence the system chosen.
Sources high in salt content will not be suitable for over-
head sprinkler irrigation, while those high in iron, sul-
fides or algae content will be less suitable for low-volume
3. Energy requirements of systems are now a major consid-
eration due to the increasing costs of fuel and electricity.
Systems with lower power requirements should certainly
be considered favorably.
4. Fixed or initial costs of systems will certainly be an im-
portant budgetary consideration.
5. Labor requirements in terms of availability and cost
should be examined. Systems requiring high labor inputs
would be undesirable under present conditions.
6. Systems with a history of high maintenance requirements
may be undesirable.
While all systems presently in use have one or more of the
above undesirable features, individual growers will have to make
a decision based on their particular situation.
Methods of Irrigation
The various methods of irrigation used in Florida citrus will
be discussed briefly with respect to their advantages and disad-
vantages. Cost information including fixed costs, variable costs
and costs per acre inch of application are not presented in this
publication as these values are constantly changing. For current
information, growers should contact their local County Coopera-
tive Extension Service Office.
Portable Perforated Pipe. These systems are no longer popu-
lar due to their high labor requirement.
Portable and Self-propelled Volume Guns. Portable guns, un-
like self-propelled units, have a high fuel cost. Self-propelled
units are highly flexible in their usage and have a low labor re-
quirement. Self-propelled and portable units are often inefficient
under windy conditions. Such high-volume systems are highly
visible to the public and are frequently a source of criticism with
Fig. 2. Portable perforated pipe.
respect to water wastage. They are not considered as effective
under bedded grove conditions as much of the water runs off
the beds into the furrows and ditches.
Invariably, large grove operations on poorer soils utilizing
volume gun systems do not have enough units to adequately ir-
rigate the acreage at the required frequency. This results in tree
stress during the critical periods of bloom, fruit set and enlarge-
Permanent Overhead Sprinkler Systems. The popularity of
these systems is declining due to the rapidly rising fixed costs
Fig. 3. Portable volume gun.
involving their purchase and installation. Their once-low oper-
ating costs are now rising due to the increased costs of fuel and
electricity. Growers who use these systems inefficiently in terms
of over-irrigating should curb such practices. Such systems are
Fig. 4. Permanent overhead sprinklers.
also inefficient when operating under windy conditions. Like
other overhead systems, they are highly visible and, therefore,
do nothing to curb public criticism of excessive water usage.
They should be installed so that they do not interfere with the
application of herbicides under tree canopies.
Permanent sprinkler systems installed on short risers under
tree canopies are used on a limited acreage. For satisfactory
water distribution, such systems should be installed at a closer
spacing and in groves with high skirts. Low tree skirts will in-
terfere with the water distribution.
The so-called "pop-up" type sprinkler systems installed in
the middles between tree rows, while having many advantages,
are susceptible to damage by grove equipment and to impair-
ment of riser movement by sand.
Seepage and Crown Flood Irrigation. When discussing these
methods of irrigation the term 'water management' must be con-
sidered. On the flatwood soils of the east coast and southwest
Florida, poor drainage and wet conditions in the summer con-
trast to very drought spring and fall conditions. Drainage and
irrigation must, therefore, be considered inseparable. While the
variable costs are low, the large expensive pumping equipment
installed for drainage water removal and for moving water into
groves for irrigation contribute to fixed costs. A disadvantage
of such systems would be the very large volumes of water re-
quired. However, consumptive use requests for the large acre-
feet volumes of water have been justified using the reasoning
that much of this water is only temporarily removed and is
available for re-use when returned to the shallow aquifer follow-
ing irrigation. Critical factors in the design and layout of such
systems are topography, bed width, distance between ditches,
depth to hardpan layers, and soil type characteristics including
its potential for lateral movement and upward movement by
capillary action. An adequate pumping capacity is also very im-
portant to ensure that water can be moved into and out of the
grove for adequate irrigation and to minimize prolonged flooding
conditions which would damage tree root systems. A good man-
agement system would keep water tables at depths suitable for
maximum tree rooting depth instead of having high water table
conditions continually trim root systems. Shallow root systems
make trees poor foragers for nutrients and moisture making fre-
quent light irrigations necessary to keep soil moisture adequate
in the shallow soil layers. The crown flood system, in which the
water level is brought to the top of the bed, has been shown to
rig. a. riooa rnrgaTion.
supply more water with a better distribution throughout the soil
rooting volume than seepage irrigation.
Low-volume Irrigation Systems. These include the drip or
trickle and low-volume microsprinkler-type irrigation systems.
Drip ir igationl may he defined as a low pressure irrigation sys-
rig. o. microspinklers
ter which supplies water to a portion of the root zone of a plant
maintaining this moisture at somewhere between saturation and
field capacity during the growing season. It places moisture in
an area around the plant where the root system can obtain it
with little difficulty and thus, water losses from the soil are
Advantages claimed for such systems are reduced water
usage, lower energy requirements, and lower fixed and variable
costs. The latter 2 claims are not necessarily valid. In an effort
to install such systems at low cost, adequate design has fre-
quently been sacrified and poor materials used. Such systems,
in order to be given a fair chance to perform satisfactorily,
should be installed properly with the best equipment, with less
emphasis on cutting costs. Low-volume systems are also not
maintenance-free, as lines and emitters or sprinklers have to
be checked at regular intervals to ensure their satisfactory per-
formance. Emitter and sprinkler head blockages may occur with
poor quality water.
Numbers of emitters per tree will depend on the water re-
quirements of the tree, soil type, and tree spacing. The closer
trees are set the more likely are adjacent root systems to feed
:. *.m an emitter situated between them in the tree row. Drip
Fig. 7. Trickle.
irrigation systems left to run continuously will result in water
wastage, thus negating one of the major advantages of low-
volume systems, namely, water conservation. Also on heavier
soil types damage has occurred to root systems under emitters
run continuously due to the creation of water-logged conditions.
Cultural practices must be modified when low-volume sys-
tems are installed with above-ground lines, and mechanical till-
age operations should not be used, especially close to the trees.
Herbicides should be used for strip weed control in tree rows.
Weed-free conditions enable easier inspection of emitters to ob-
serve whether they are operating correctly. In the case of micro-
sprinklers, weed growth will intercept the pattern of water dis-
tribution. Emitters and microsprinklers are subject to damage
by grove workers, by equipment moving across rows and by
harvesting personnel and their equipment. Emitters installed
under tree canopies and closer to the trunks are always better
protected. Continuous wetting of tree trunks should be avoided
by using sprinklers that are designed to avoid this and by not
placing drippers too near the tree. Lines are subject to twisting
with expansion and contraction. This is not as much a problem
with drip emitters, but microsprinkler spray pattern efficiency
may be considerably reduced as the sprinkler head is turned from
the vertical. To reduce this problem, lines have been pegged to
the ground or partially buried.
Young Tree Irrigation. In Florida it has been the custom
to plant young trees in the center of a water ring constructed
by pulling the soil up on a circular bank. Following planting and
sometimes for several months thereafter, trees are watered by
a water wagon which supplies 5 to 10 gallons into the ring
around each tree. This method of watering is becoming increas-
ingly costly, especially when such vehicles have to travel consid-
erable distances through groves where resets are widely scat-
tered. Although the need for such a watering procedure is elimi-
nated immediately after the planting procedure where a per-
manent irrigation system is installed, the satisfactory schedul-
ing of irrigations is still difficult due to the great difference in
water requirement between small and large trees.
A knowledge of water quality is of fundamental importance
to the grower who is deciding on the installation of an irrigation
system or indeed is involved in irrigation management on a daily
basis. For overhead sprinkler systems the salt content of the
water source is important while for low-volume irrigation such
factors as sulfides, iron, and algae contents need to be examined
along with other water quality measurements. Typical salt burn
symptoms on citrus tree foliage have been produced by low-rate,
long-duration overhead sprinkler irrigation with water contain-
ing 800-1000 ppm total dissolved solids (TDS) or more. A water
source containing more than 800 ppm should be considered un-
suitable. Salt injury may be reduced by irrigating at high rather
than low rates, and by irrigating when conditions are least con-
ducive to evaporation, as at night rather than during the day.
Water widl somewhat highle salt content (1200 ppm) has blvi
used without injury especially at night when irrigating it-
high-volume, self-propelled, and portable guns. Water with TI)D
in excess of 2000 ppm should not be used even for flood iiriga-
Drip irrigation systems with their smaller emitter opening
and more intricate labyrinth-type internal structures are more
conducive to clogging problems. The incidence of clogging has
been shown to be less in those types of emitters through which
water moves at the higher velocities. The ideal water should bt
free of particulate matter and the agents which enhance the de
velopment of biological slimes. Bacterial slimes are the fund
mental cause of most of these clogging situations and may be
divided into 3 groups: sulfur slime, iron slime, and non specific
filamentous and non-filamentous slices. There are no types oJ
low-volume irrigation system absolutely free from clogging prob-
lems. Chlorine combined with suitable filtration procedures will
control particulate clogging, sulfur bacteria, iron bacteria, and
certain non-filamentous aerobic slimes in waters low in organic
matter. It should be used as a maintenance or preventive rather
than a corrective treatment, as it is very difficult to clean out
systems once they are clogged. Chlorine precipitates ferrous iron
and is less effective at high pH. Its activity is also considerable
reduced by exposure to sunlight and high temperatures. Chlorine
is not a compound to be just "dumped" into irrigation systems:
it must be accurately nmi,:ere in according to the need. One or
the most important factors that must be measured accurately is
the free residual le el rather than the total chlorine level. Fo0
example, 9 ppm of sodium hypochlorite (NaOCI) for each 1 ppn.
of total sulfides yields a free residual chlorine level of 0.5-1,0 ppn,
--the amount needed to kill bacteria. Water containing iiol
levels of up to 5 ppm is suitable, providing an intermitten,
chlorine injection treatment is used which results in an 0.5 ppni
free residual level of chlorine at the end of the system.
Surface waters, under certain circumstances, may be suitable
for low-volume irrigation providing chlorine is injected at the
pump and a sand filter used to trap algae and particulate matter
prior to entering the lines and emitters.
Prior to investing in a low-volume irrigation system, th.
grower should have the appropriate water quality tests run on
samples from his water sourcess, be they surface or well water.
Tests should be run for pH, iron, sulfides, total dissolved solid.
(TDS), and surface water should also be observed for color and
particulate matter in suspension. If the latter two characteristics
are found, further tests may be required. Levels of sulfides and
iron will change with time as the well is pumped. Although this
variation cannot be entirely eliminated, wells should be pumped
for a few minutes prior to sampling. Also as certain chemical
constituents will change with time, the appropriate tests, especi-
ally for iron and hydrogen sulfide, should be conducted immedi-
ately on water samples at the grove site. Samples which have
to be transported to a laboratory for analysis should be chemi-
cally stabilized at sampling time.
Irrigation Application Etficiencies
Water application efficiency or irrigation elti.nerny may be
defined as the ratio of the quantity of water effectively placed
into the crop root zone and utilized by the crop to the quantity
actually delivered to the field, the efficiency being expressed as
There are several factors involved in determining field ap-
plication efficiencies. The losses occurring during irrigation may
be due to any one or a combination of the following factors:
(1) Evaporation losses from the surface of flowing water or
evaporation in the air from sprinkler nozzle spray, (2) losses
to deep percolation below the root zone, (3) evaporation from the
soil during irrigation, or (4) run off from the field.
Water application losses from properly designed sprinkler
irrigation systems are limited to: (1) Evaporation losses in the
air from the sprinkler spray, (2) evaporation from intercepted
leaf surfaces, considered negligible in Florida citrus, (3) deep
percolation losses, and (4) evaporation from soil.
Water distribution and evaporational loss of permanent over-
head and volume gun irrigation systems have been measured
under field conditions. Higher coefficient uniformity values.were
found for night irrigation than day irrigation for both systems
because of less air movement. Water distribution was also af-
fected by the spacing of sprinklers in permanent overhead sys-
tems with closer spacings providing more uniform water distri-
bution. Evaporational loss of water was higher during day irri-
gation than night irrigation. This loss ranged from 11-19% for
permanent overhead sprinkler systems and 0-7% for volume
gun systems. Night irrigation is preferable over day irrigation
both from the standpoint of lower evaporation losses and more
uniform water distribution. This is especially true for permanent
overhead irrigation systems. While the evaporation losses from
the volume gun are less than half that of the permanent over-
head in day operation, wind drift is an important factor and
operating during very windy conditions should be avoided.
The efficiency of overhead sprinkler systems is also reduced
when such systems are operated in recently planted widely
spaced groves in which young tree root systems only occupy a
small percentage of the total soil volume. Consequently, such
trees only have access to a small proportion of the water dis-
tributed over the entire grove floor area.
The overall efficiency of flood irrigation decreases with ap-
plications in excess of tree needs and the water-holding capaci-
ties of soils. Such excess water percolates beyond the tree root
zones, and while it may be returned to the shallow aquifer, the
efficiency is still reduced as it is not utilized by the trees. In the
case of low-volume irrigation systems, deep percolation is likely
to make the greatest contribution to reduced efficiency. This oc-
curs when such systems are run continuously or for long periods,
resulting in water movement beyond tree root zones, especially
in poorer soils with lower water-holding capacities.
Moisture Stress Relationships
Several things happen in plant responses to water stress.
When plants are subjected to a fairly severe level of stress for
short periods, they quickly adapt to those stress conditions. The
size of plant tops and transpiring surfaces is minimized relative
to the size of the root system which is the absorbing surface.
This is done automatically in the plant environment by loss of
leaves during drought conditions and by reduction of new top
Citrus trees have a rather tight canopy, with considerable
self-shading of leaves. In fact, half of its leaves are shaded all of
the time, and that is not true for many other plants. Citrus trees
have a root system that is reasonably well-developed, even though
it doesn't usually reach down to the water table in well-drained
soils. If the soil profile is fairly deep and expansive, the root
system tends to match it. Another side effect of drought is that
roots become less permeable to water absorption and movement.
Less water is taken in and transported more slowly, not because
of dieback, but because of physiological changes in the root cells.
Stomatal closure and a reduction in photosynthesis are also in-
Both water stress and excess available moisture have been
implicated in fruit quality problems (both internal and external)
of citrus. A rapid change from dry to wet conditions can be par-
ticularly detrimental. It has long been known that fruit volume
growth responds to irrigation. Diurnal shinkage of fruit occurs
with fruit recovering its original size plus some gain in size each
night until soil moisture is depleted. An irrigation at this time
allows the fruit to resume growth in the diurnal pattern. It has
been found that leaves draw water from fruit during transpira-
tion and this moisture loss from the fruit comes from the peel.
The fruit itself transpires moisture also and both processes con-
tribute to 'drying of the fruit peel. The following fruit quality
problems have been shown to be related in some degree to water
stress: Valencia segment dehydration, Pineapple pitting, stem
end rind breakdown (SER), and Valencia aging. Problems as-
sociated with water excess are: diluted total soluble solids and
acid, fruit splitting, water spot of Navel oranges, oleocellosis,
and stylar end breakdown of limes.
Antitranspirants (film-forming materials) form discontinu-
ous layers on leaves and fruit which block or partially block some
of the stomata and coat the interstomatal areas. Such materials
are a means of alleviating plant moisture stress and fruit dis-
orders induced by such stress have been reduced by sprays. Anti-
transpirants have also been used in nurseries on trees after top
pruning, but prior to digging. Such treatment reduces leaf mois-
ture loss and reduces tree stress during the moving and planting
operation, but does not substitute for adequate irrigation prac-
tices after planting.
Interaction of Irrigation with Other Horticultural Practices
Irrigation practices will be influenced by rootstocks which
have been shown to influence the ability of trees to glean water
from greater soil depths due to greater rooting depths in deep
sandy soils. For instance, on the deeper soils rough lemon has a
deeper root system than Carrizo citrange and Cleopatra man-
darin, which in turn have deeper roots than sweet orange.
Proper irrigation enables the tree to utilize fertilizers applied
during the dry periods by moving nutrients into the soil. On the
other hand, excessive irrigation will leach nutrients beyond the
root systems of trees and result in monetary loss and possibly
contribute to pollution of nearby ground water sources.
In the absence of rainfall, irrigation must be used for the
activation of soil-sterilant type herbicides, especially during the
spring and fall. Weed control is influenced by irrigation in that
weed growth is stimulated during dry periods and herbicides
may be leached beyond the zone of germinating weed seed.
Soil moisture should always be adequate at the time of appli-
cation of oil sprays. Irrigation may therefore be required prior
to such sprays if tree stress is suspected.
The higher the density of tree plantings, the greater the de-
mand for soil moisture per unit volume of soil because of the
greater root concentration. Irrigation practices should therefore
be modified accordingly to meet the increased demands of the
Pruning operations such as hedging and topping should not
be done under conditions of moisture stress. During dry periods,
irrigation should be applied prior to such operations to ensure
adequate soil moisture conditions. These precautions are es-
pecially important where trees are severely pruned.
It is a well-established fact that groves under good manage-
ment are less susceptible to cold damage than those under poor
management. Proper irrigation practices are an integral part
of good grove management.
Irrigation can be an invaluable tool for cold protection under
winter conditions. If the soil is dry and a cold spell is anticipated,
irrigation several days before the expected freezing temperatures
may be advantageous. The moist soil tends to store more heat
during the day for release at night. It is important that this be
done in advance, because if it is done as the front arrives there
can be evaporation with colder conditions resulting. When con-
templating the use of overhead sprinkler irrigation for cold
protection, it is most important to realize that each freeze night
calls for different heat requirements. Sometimes when small
amounts of heat are needed under specific environmental condi-
tions, excellent results can be obtained. The important thing is
to understand what comprises these conditions before a system
is turned on during a particular cold night or disastrous results
can result with respect to tree damage. Growers unfamiliar with
this method of cold protection are strongly advised to obtain
the appropriate information from their local County Cooperative
Extension Service Office. There are reports of varying degrees
of cold protection from low-volume under-tree sprinkler systems;
however, documented research data is not available. Under cer-
tain environmental conditions small temperature increases may
be expected from these systems, such as in a situation where
wind speed is low and relative humidity high. However, under
windy conditions and with the dew point expected to drop to 20
or 15 degrees, the relative humidity will usually be quite low and
the air very dry. In such a situation temperatures could well drop
to the danger zone with evaporative cooling if low-volume under-
tree sprinklers are used. The potential for cold protection under
flood irrigation conditions is well known due to the heat stored in
such large masses of water in the grove.
Irrigation Systems as Carriers for Nutrients and Pesticides
Citrus can be satisfactorily fertilized with nitrogen, phos-
phate and potash through overhead sprinkler irrigation systems
providing systems are run long enough after injection to ensure
the rinsing of all fertilizer from the foliage. Most of the nutrients
applied in this manner eventually reach the soil and are avail-
able for absorbtion by 'the tree root systems. Although nitrogen
and potassium have been applied as a liquid (a commonly used
analysis being 8-0-8) through low-volume drip irrigation and
under-tree microsprinklers, no documented research data is
available to provide guidelines for this procedure. Past research
on fertilizer placement would indicate that for optimum response
as much of the tree root system as possible should have access
to the applied fertilizer. The application of nutrients through
drip systems has in some cases resulted in algae growth, especial-
ly in the presence of sunlight. To prevent clogging of emitters,
a chlorine treatment may have to be initiated.
Under current stringent pesticide labeling procedures, users
should be cautioned that the application of pesticides by methods
other than those specified on the product label is a violation of
federal law. This consideration would, of course, apply to irri-
gation as an application method.
Certain highly insoluble herbicides as liquid formulations
have been applied at appropriate rates in irrigation water to
water rings around young trees with good results. They are
suitable for this method of application due to their very low
solubility properties and hence their retention in the surface
soil layers. More soluble herbicides would not be suitable for
this method of application as they would be quickly leached into
the young tree root zone with consequent tree damage. Highly
volatile materials can cause bark damage and tree death. Contact
and soil sterilant herbicides have been applied through low-
volume under-tree microsprinkler systems in South Africa and
Israel with reported success; however, recommendations for
this method of application are not available in Florida. Of im-
portance would be good distribution through systems, accurate
calibration in terms of rate of application per treated area, ap-
plication without excessive leaching into the tree root zone, and
avoiding wetting green tree trunks and foliage with contact
Influence of Irrigation on Mite Populations
and Disease Incidence
No published information is available in Florida on the ef-
fects of overhead sprinkler irrigation on insects, mites, and fungi
in citrus. However, in deciduous fruit-growing areas a number
of studies have shown that sprinkler irrigation contributed to
the suppression of certain mites on apples and pears. There have
been observations in Florida that populations of spider mites
are suppressed by irrigation during the dry spring and particu-
larly fall months. Irrigation also tends to increase the presence
of certain parasitic fungi that reduce mite populations. Over-
head sprinkler irrigation is known to increase the incidence of
scab on varieties susceptible to this fungus disease by creating
conditions favorable for infection. Indiscriminate use of irriga-
tion on Dancy tangerines in the spring has been observed to in-
crease the incidence of the fungus disease Alternaria on the
leaves and fruit of this variety. A thorough irrigation prior to
leaf flush expansion instead of frequent irrigation during this
period will probably result in less favorable conditions for dis-
It has been shown that trees infested with burrowing nema-
todes, which would normally steadily decline and lose production,
can be maintained in a state of economic productivity through
the liberal use of irrigation during dry periods when trees are
susceptible to the greatest stress. This is an example of irriga-
tion management as an integral part of a pest management
Irrigation with Waste Water
As demands on water supplies intensify, recycling of munici-
pal and industrial waste water for irrigation becomes more
feasible. However, such sources have not been evaluated for
citrus. In Florida, sprinkler irrigation is used for some disposal
of waste waters from citrus processing plants. Experimental
work indicates that it is feasible to irrigate citrus trees with
waste water from such sources. No ill effects from waste water
were observed on trees grown in the greenhouse for 2 years
and in the field for up to 4 years. From the standpoint of trees
and soil, it appears that the quantity is more critical than the
quality of the water applied. Further work is needed before firm
conclusions can be drawn.
Conversion: U. S. and Metric Measures
1 inch (in) =2.5400 centimeters (cm)
1 foot (ft) =0.3048 meters (m)
1 acre =0.4047 hectares (ha)
1 gallon (gal) =3.7853 liters (I)
This publication was printed at a cost of $1097.00 or 11.0 cents per copy
to inform commercial growers about citrus irrigation management.
Single copies are free to residents of Florida and may be obtained
from the County Extension Office. Bulk rates are available upon
request. Please submit details of the request to C.M. Hinton, Publi-
cation Distribution Center, IFAS Building 664, University of
Florida, Gainesville, Florida 32611.
COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
Cooperative Extension Service, IFAS, University of Florida
and United States Department of Agriculture, Cooperating
K. R. Tefertiller, Director