Water Management in Citrus

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Water Management in Citrus
Yoseph Levy1 and Brian Boman2
Introduction
In most citrus producing regions of the world, irrigation and fertilization are important components of
commercial production. Irrigation is necessary to adequately replenish soil water lost through
evaporation and transpiration. Fertilizers replace nutrients removed during harvest and through leaching
and maintain tree growth and vigor. Irrigation and fertilization management are intimately linked
together. Managing one without regard for the other will result in less than optimum tree health, growth,
and yield. Irrigation and fertilization practices can also have significant impact on fruit quality and they
can affect the value of the fruit following storage and distribution.
Irrigation is one of the most costly cultural practices in arid and semi-arid areas, with long dry seasons.
It is also an important cultural factor in humid subtropical climates to maintain yields and fruit quality that
would otherwise be reduced during short-term dry periods. Studies have shown that money spent for
improved weed and pest control, pruning, and fertilization do not result in higher yields if irrigation
practices are inferior. It is only when such improvements are combined with good irrigation
management practices that the benefits can be fully realized.
Adequate soil moisture levels are critical for proper fruit-set and to support optimum fruit growth and
development through harvest. Drought stress can increase flower bud induction and be a useful
management tool, especially during warmer winters with inadequate cold induction or to induce out-of-
the-season flowering. Adequate soil moisture is important in the spring to ensure good fruit set
(Ginestar, 1996). Drought stress during spring fruit set can lead to the setting of a higher percentage of
late-bloom fruit of inferior quality.
During late spring and early summer, soil must be kept fairly moist to avoid stress, which may increase
“June drop” of young fruitlets. Proper water management is necessary throughout the year to establish a
dense, healthy root system and a vigorous tree.
1Agricultural Research Organization, Institute of Horticulture, Gilat Experiment Station, Mobile Post
Negev 85-280, Israel. email: vfjlev@agri.gov.il.
2University of Florida, Indian River Research and Education Center, 2199 S. Rock Road, Fort Pierce,
FL 34845-3138. email: bjbo@gnv.ifas.ufl.edu.

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Drought stress
Drought stress develops when tree water loss exceeds the rate of water uptake for a sustained period.
Stress can occur whenever the rate of water loss from the leaves by transpiration exceeds the rate of
water absorbed by the root system. Although citrus species are well adapted to conditions of moderate
drought stress, optimizing water management can provide significant benefit to the citrus grower.
Drought stress influences many components of citrus growth and development, with effects differing by
stage of growth and severity of stress. Severe moisture deficits result in familiar symptoms of wilting,
abscission of many leaves, poor fruit quality, and small fruit size. Although the effects are less dramatic,
milder drought stress in prolonged or repeated periods or at critical developmental stages may also
significantly reduce productivity and profitability of citrus.
Stress can occur in citrus trees before signs of water shortage are clearly evident. Reduced growth and
abnormal loss of leaves can result when citrus trees are allowed to go without irrigation until signs of
water shortage are evident. Stress can be caused by the following conditions:
• On hot, dry, windy days, water may be removed from the leaves faster than the capacity of the roots
to absorb water, and water shortage develops even though soil moisture is adequate. This is
especially found in soils with a limited root zone.
• Low soil-moisture may reduce the availability of water, so that the roots cannot extract water without
a very high hydraulic gradient existing between the root system and the leaves.
• A soil may be partially dry so that the hydraulic conductivity of the soil has been significantly reduced.
After the roots have absorbed the water with which they are directly in contact, the replacement of
this water by movement from points some distance away (these distances need not be more than a
few millimeters) is not rapid enough to meet needs of the tree.
• Under certain conditions, the root zone may be too wet and oxygen replenishment is difficult. The
rate of water adsorption by the tree is reduced when the oxygen level is low, which can cause stress
on a day in which there is high transpiration.
• Comparatively low root temperature can result in insufficient water uptake on warm, windy winter
days. Dry winds, especially during spring and autumn, can thus cause severe leaf drop and leaf
scorch (Reed and Bartholomew, 1930).
• Root injury such as that caused by cultivation can reduce water absorption rates and create stress.
Root hydraulic conductivity, essentially the ability of roots to transfer water from soil into the plant, is
quite low for citrus. This characteristic makes serious damage by drought stress less likely since soil
water is conserved; however, it can also result in some drought stress even when soil moisture is
plentiful. Citrus canopies release so much water into the air that water transport within the root system
and the rest of the tree cannot keep pace.

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As drought stress occurs, stomates close to reduce loss of water, and tree photosynthesis and potential
productivity is reduced. This property of citrus makes it impossible to prevent drought damage
completely. The challenge is to optimize efforts so that investments in water management provide good
returns. Frequently, this means that we manage water so that stomatal closure is delayed until later in
the day.
If there is adequate water in the plant, stomates remain open in the daylight. At night, stomates normally
close and evapotranspiration (ET) decreases to a very low level, but water absorption from the soil
continues throughout the night. By daybreak the next morning, drought stress in the tree is usually at a
minimum, and leaves have reached full turgor. This diurnal change in water loss and water uptake
results in small changes in trunk and fruit diameters, even with well-watered plants. These changes can
be monitored in order to assess the tree’s water relations.
As the soil dries out further, less water becomes available for uptake by the roots. Midday wilt can
occur, particularly on young, unhardened leaves, sometimes reoccurring several times during the day
(Levy and Kaufmann, 1976). Initially, wilted leaves recover turgor through water absorption overnight.
However, as soil drying continues, wilting of mature leaves starts earlier and lasts longer during the day.
Eventually leaves do not recover from their wilt by the next morning. At this point, the tree is said to be
in a permanent wilt and the soil reached the permanent wilting point.
Effects of Stress on Production
Drought stress anytime during the growth and development of citrus fruit can harm yield and fruit quality
compared to well-watered trees, losses may not be completely recovered through proper irrigation
during the rest of the season. Drought stress may result in smaller, lighter fruit with thicker peel and
reduced juice content. Excessive rainfall and/or irrigation immediately prior to harvest results in a
dilution of soluble solids whereas drought conditions concentrate soluble solids. Even though fruit from
drought stressed trees may have higher total soluble solids and acids per fruit, solids per land area may
be reduced because of lower total yields per orchard area unit.
Drought stress during flowering and fruit set directly reduces the number of fruit. During the fruit
enlargement phase (typically from June through harvest), the number of fruit remains almost constant,
and the main effect of water supply is on fruit size. Fruit size can be increased by irrigation, especially
during the last stages of fruit development, and considering this effect can give the grower some control
on the final size of fruit at harvest time. However, improved irrigation during fruit set may increase the
number of fruit enough to cause an overall decrease in fruit size. Drought increases peel thickness and
decreases the juice percent, and in lemons can cause endoxerosis (internal gumming and desiccation).
Moderate water deficits in the late summer and fall can be desirable to maintain high juice quality,
particularly in juice and early cultivars (Peng and Rabe, 1998); however, in early mandarin cultivars,
deficit irrigation may decrease fruit size and juice while it increases juice sugar.

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Physiological Responses
Physiological processes have different sensitivities to drought stress. Cell expansion is most sensitive,
and drought stress of only a few bars can slow down or stop this process. Cell wall and protein
synthesis are also quite sensitive to drought stress. As drought stress continues to develop, synthesis of
the hormone abscisic acid (ABA) increases, leading to earlier stomatal closure and decrease in
photosynthesis. With additional stress, respiration decreases and proline, an amino acid, increases.
Eventually stomates close completely and photosynthesis stops. This stomatal closure not only reduces
transpiration water-loss but also stops photosynthesis. As stress progresses even further, wilting and
leaf curling may occur, followed by leaf abscission. ABA is implicated in many components of plant
adjustment to drought stress, inducing characteristics like earlier stomatal closure and culminating in the
drastic step of dropping leaves to limit water loss. Prolonged severe drought stress can cause limb
dieback and eventual death of the tree.
Drought stress during the times of rapid vegetative growth should be avoided. In young trees,
vegetative growth determines their final canopy size and future yield potential. In mature trees, healthy
vegetative growth is required to maintain and replace fruit-bearing branches.
Citrus trees are evergreen, but unlike other evergreen fruit trees, such as avocado or mango, continuous
leaf replacement occurs as the tree grows. Leaves can remain on the tree up to two years. The leaves
have many xeromorphic characteristics, including an adaxial epidermis covered by a thick cuticle
without active stomata. Citrus roots can survive a prolonged drought and recover very quickly after a
complete drying of upper layers of soil. (Eissenstat et al., 1999).
Citrus stomata close under conditions of high evaporative demand (low humidity), or when soil water is
not readily available. Stomata may close when high vapor pressure deficits are present, even when
evaporative demand is fairly low (such as in winter months). Thus, transpiration rate and drought stress
of orange trees are similar on summer days in arid climates, such as Arizona and humid climates, such as
Florida, although the evaporative demands are much greater under Arizona desert conditions.
Irrigation can usually prevent severe drought stress in most commercial groves. Moderate drought
stress reduces shoot growth and leaf expansion, slows canopy development and cropping in young
trees, and reduces vegetative growth needed to support fruit production in mature trees. Twig and
branch dieback may result from either moderately insufficient or excessive water over prolonged
periods.
Canopy Development
The vegetative development of young trees depends closely on the irrigation and nutrition regime used.
During dry periods, irrigation of young trees at long intervals can delay canopy development although
drought stress symptoms may not be evident. During the first several years after planting, there is
generally a good relationship between canopy volume and yield. However, as trees reach full size,
excessive growth, induced by over-irrigation and fertilization can decrease yields, mainly because of

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crowding, shading and the need for hedging. At this stage, reduced irrigation and fertilization practices
can be used to limit canopy growth.
Measurement of growth of the trunk may be used to compare tree response to different irrigation
regimes, and recent developments enable accurate electronic dendrometric monitoring of small changes.
However, various rootstocks develop different trunk sizes and there may be a poor correlation between
trunk and canopy development across all rootstocks. For example, rough lemon develops a large
canopy with relatively small trunk as compared to Cleopatra mandarin, which has a large trunk but
develops a small canopy during the first years of orchard development.
Effects of Irrigation on Fruit Quality
Fruit quality is an important factor in determining the value and marketability of citrus, either for fresh
consumption or for processing. Market quality parameters can be divided into two main groups:
physical parameters of the fruit and chemical parameters of the juice. The physical factors include fruit
size, peel thickness, peel color, firmness, juice content, storability and shelf life. Chemical parameters
include acid, sugar, the sugar:acid ratio, and the amount of minor constituents, such as bitter or aromatic
compounds, which may influence fruit palatability.
The amount of sugar in the juice is expressed as total soluble solids (TSS, usually measured with a
refractometer) or as Brix (based on juice specific gravity and usually measured with a hydrometer).
Total soluble solids concentration in juice is an important parameter in determining the price of
processed fruit. Acid concentration and the ratio between sugar and acid are important parameters in
defining fruit quality and harvesting time. Acidity in grapefruit usually responds more than orange to
irrigation. In general, water shortage causes increased concentration of sugar in the juice, while
excessive rainfall or irrigation results in dilution of the sugars in the juice. Curtailing irrigation prior to
harvest may increase sugar concentration in the juice, but may reduce juice content.
Valencia and navel oranges that develop under luxurious moisture conditions have a more tender peel
and poor storability, compared with fruit from trees that are mildly drought stressed. Drought increases
peel thickness and the peel-to-pulp ratio, and this decreases the juice percentage.
The effect of irrigation on color break is difficult to assess since irrigation can affect the nitrogen nutrition
of the tree, and higher nitrogen will delay color break and enhance regreening. Usually color break is
delayed when the irrigation is increased. Increased irrigation of Ruby Red grapefruit (Huff et al., 1981)
has been shown to increase the chlorophyll and the red pigment (lycopene) content of the rind and
caused more re-greening, but did not increase the yellow pigment (beta carotene). The effects of
irrigation on citrus trees and fruit are shown in Table 1.

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Table 1. General effects of irrigation on citrus.
Parameters Increase Decrease No change
Tree size X
Fruit yield X
Fruit size X
Fruit weight X
Color X
Green fruit X
Peel thickness X
External
quality
Plugging X
Wind scar X
Russeting X
Creasing X
Plugging X
Scab X
Peel blemishes
Oleocellosis X
Juice content X X
Brix X
Acid X X
Sugar/acid ratio X
Color (red) X
Color (yellow) X
Solids/box X
Juice
quality
Solids/area X
Diseases and Physiological Disorders
Conditions of high leaf and fruit turgor greatly increases susceptibility to oleocellosis, which is caused by
the rupture of rind oil glands. Oleocellosis is avoided by delaying harvest when environmental conditions
favor high fruit turgor, e.g. shortly after irrigation or rainfall, early in the morning, or under weather
conditions which reduce water loss such as fog, dew, high humidity or clouds.

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Citrus foot rot, a fungal disease caused by Phytophthora spp in susceptible rootstocks. is closely
related to conditions of high soil water content. Among the most susceptible to foot rot we find rough
lemon, sweet orange, sweet lime, Rangpur lime, and self rooted limes. The best preventive measure is
good soil drainage. Care should be taken not to plant susceptible trees too deep, one should keep the
soil around the trunk dry and avoid irrigation regimes that wet the trunk for long time periods. Dripper
lines should be kept at a safe distance from the trunks.
Water flooding causes asphyxiation of the roots and can cause severe damage or death to citrus trees
planted in heavy, poorly-drained soils. The susceptibility of citrus rootstocks to flooding is different and
almost opposite from the susceptibility to Phytophthora infection. Some rootstocks, such as sour
orange and Carrizo citrange, may be very sensitive to this problem, while rough lemon and sweet lime
have more tolerance. Citrus is more tolerant of flooding at the lower temperatures experienced in winter
(Ford, 1964), mainly because of reduced metabolites that accumulate by bacterial action at high
temperatures.. Flooding increases root resistance to water movement and reduces water absorption.
Therefore, high ET rates during warm summer days combined with reduced water uptake from flooding
can promote damage sooner in warm weather.
Lime-induced chlorosis in calcareous soils is a disorder closely related to irrigation and soil aeration and
can be aggravated by high soil water content. Great differences exits between rootstocks, with
Trifoliate orange (Poncirus trifoliata), Swingle Citrumello, Troyer and Carrizo citranges among the
most susceptible, and rough lemon and volkameriana comparatively tolerant. Frequent irrigation caused
chlorosis in Arizona (Hilgeman and Sharples, 1957), and in California "over-moist" soil conditions were
regarded as one of the most common causes of chlorosis (Chapman, 1968). In Israel, chlorosis was
increased by shortening the interval between sprinkler irrigations; however, chlorosis was reduced when
using drip irrigation (Levy, 1984). Even though the irrigation was frequent, and the irrigated soil volume
remained wet all the time. Practicing deficit irrigation in order to reduce lime-induced-chlorosis can be
dangerous, since these rootstocks are also susceptible to salinity that may be aggravated by deficit
irrigation, which means that all the salts that are supplied with the irrigation water remain in the root
zone, and are not leached. The application of iron chelates to correct lime-induced-chlorosis, instead of
limiting irrigation, should be considered.
Root Systems
The root systems of citrus trees are relatively shallow compared to deciduous fruit trees such as walnuts.
The typical maximum rooting depth in deep, well-drained soils is 1.2 to 1.5m, with the main root system
reaching only a depth of 0.6-1.0 m. Roots can develop much deeper in well-drained sandy soils. In a
study on the deep sandy soils of central Florida, roots of rough lemon rootstock reached a depth of 3.6
m, while roots of sour were found at a depth of 2.7 m (Castle and Kresdorn, 1977). Water depletion
studies confirmed that the roots were active at this depth but most of the water extraction came from the
top 0.6 m. In arid climates, roots will not penetrate deep soil layers, which may have higher salinities.
Citrus can also survive in very shallow soils, especially when the limiting factor is a high ground water
table. Citrus trees grown in the coastal flatwoods areas of Florida typically have rooting depths of only
0.4 to 0.6 m (Ford, 1954). Citrus roots exposed to extended periods of dry soil apparently maintain or

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very quickly recover P and water uptake capacity. This behavior is consistent with an overall rooting
strategy where essentially no surface roots are shed following prolonged exposure to dry soil (Eissenstat
et al., 1999).
Drought stress decreases the hydraulic (water) conductance of roots. In winter citrus trees may suffer
from drought stress when the soil is cold but climatic conditions bring about high evaporative demand
due to low air humidity. Typically, low-humidity winds can cause severe leaf drop or leaf scorch when
soil temperature is low, especially in heavy soils.
Reduced summer irrigation decreases root development in citrus trees. Trees that are repeatedly
subjected to water deficit develop thicker feeder roots. Under favorable soil characteristics (good
aeration, low salinity), these roots may extend to greater soil depths than those under trees maintained
with more abundant soil-moisture conditions. Excess water in the root zone and/or low aeration exerts a
greater adverse effect on the root system than water deficiency. The decline of citrus tree health
resulting from excess water and poor soil aeration is nearly always due to decay of fibrous or larger
roots. In Israel, citrus growing in clay soils tends to produce lower yields than orchards in sandy soils.
This is especially true when the upper soil layers dry out between irrigations at long intervals. Long
periods of drought or long intervals between irrigation force the trees to increase water uptake from
deep soil layers, if these layers are wet, aerated and non-saline. Soil-water status is a major component
of the root environment, affecting the growth and health of roots. Other components of the root
environment affected by irrigation are soil aeration, temperature, and salinity. The ideal environment for
citrus roots is a porous, medium-textured, well-drained soil, where water is easily available but not in
excess. When such a soil is irrigated, water distributes itself throughout the soil profile in the root zone
or drains away leaving no excess.
The growth and production of citrus may be impaired either by an excess of water in the root zone or
by a lack of easily available water. A shortage of water in the root zone has detrimental effects on root
growth. As the soil dries, root growth is reduced and eventually ceases. If some of the roots of a tree
remain in soil containing easily available water, the roots in dry soil are not damaged. Roots do not
grow through dry soil to reach moist soil, but remain healthy and ready to become active whenever the
soil in which they exist receives moisture. In groves that suffer from chronic conditions of excess water,
the most healthy roots are those existing in portions of soil that remain well-drained during the rainy
season.
Citrus roots need good soil aeration if they are to remain healthy. It is therefore important to manage
the drainage system to provide a well-aerated environment for tree roots. A common approach is to
plant citrus on ridges, which drain rainwater away from the trunks, and increase soil aeration. Another
benefit from the construction of ridges is that soil compaction by machine travel is reduced in the tree
row, which can be especially important, in young orchards. Low soil oxygen is most damaging to plants
during hot weather. Increased temperature also reduces the solubility of oxygen in water. Therefore, it
is especially important to avoid water logging during hot weather.
The roots of citrus and the trees themselves respond to differences in soil temperature. One study
showed that citrus roots grow best at soil temperatures of 26oC (Castle, 1978). As the temperature

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rose to 30oC, the growth rate was reduced by 25%. At 34oC, the growth rate was reduced 90%.
When the temperature dropped lower than 26oC, the growth rate was again reduced. Little root growth
was found to occur below 20oC.
The water-supplying ability of citrus roots is also affected by soil temperature. In a study that measured
the transpiration rate of rooted lemon cuttings grown in various root-temperature environments with
similar aerial environment, the transpiration rate increased as root temperatures varied from 5oC to 35oC
(Ramos and Kaufmann, 1979).
Flooding Damage
Research in Florida has shown that in the hot summer season, there is potential for damage to citrus
roots by water excess (Fig. 1) if water saturates the root zone for 4 days or more during extended
summer rains (Ford, 1964). However, during the cooler winter months, citrus trees can tolerate
flooded conditions for much longer periods.
In soils that have a hardpan or other barrier to downward movement of water, water must move
laterally, to be drained from saturated soils above these layers. Planting on ridges can thus help drain
excess water away from the trees. The rate at which water moves through soil expressed in units of
distance/time (mm/hr or cm/day) is called hydraulic conductivity. Sands typically have saturated
hydraulic conductivity of 500 mm/hr or more, while the saturated hydraulic conductivity of many soils
with significant clay content is in the range of 5-10 mm/hr. Hydraulic conductivity also varies
tremendously with soil moisture content. For example, the saturated hydraulic conductivity of a fine
sand soil is about 500 mm/hr, but when soil tension is increased to -85 kPa, the hydraulic conductivity
drops to less than 2 mm/hr.
Porosity is the pore space (air space between soil particles) divided by the total volume of soil. The
quantity of water that drains out of the soil as the water table is lowered is directly related to the soil
porosity. Gravity is the force that moves water in saturated soils. Therefore, water in saturated soils
moves from a higher elevation to a lower elevation. The difference in elevation between two free water
surfaces divided by the distance between points is called the hydraulic gradient. The steeper the
gradient, the faster water will drain from the higher elevation to the lower elevation. Excessive rainfall or
irrigation will cause a perched water table to develop above hardpans in the soil. Water infiltrates the
soil and moves downward to the free water surface (where the soil is saturated), and then must move
laterally towards the water furrow for drainage to occur. Consequently, a "mound" in the water table
develops between drain tiles or ditches.
Problems occur when elevated water tables continue for several days. Once the soil is near saturation,
a small amount of rainfall or irrigation will fill the available pore spaces in the soil, thus saturating the root
zone. When the air is excluded (pore spaces are filled with water), root decay can occur (and may
result in infections by disease organisms).
Water table observation wells (Fig. 2) are good tools for monitoring soil-water dynamics. They are a
reliable method for evaluating water-saturated zones in sites subject to chronic flooding injury. These

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wells can also be used to measure the rate of water table drawdown, which is the real key to how long
roots can tolerate flooding. Observation wells fitted with measuring rods allow water tables to be
visually observed while driving by the well site.
Short-term estimates of flooding stress can be obtained by digging into the soil and smelling soil and root
samples. Sour odors indicate anaerobic conditions, an oxygen deficient environment. The presence of
hydrogen sulfide (H2S, a rotten egg odor) is an indication of anaerobic reducing condition and that
feeder roots are dying. Anaerobic bacteria (which live in the absence of oxygen) develop rapidly in
flooded soils and produce toxic substances that contribute to the destruction of citrus roots. In a field
survey of poorly drained groves, toxic sulfides, nitrites and organic acids that are toxic to roots were
found in these flooded soils.
Good drainage allows air to move into the soil and prevents oxygen-deprived conditions. Citrus tree
stress caused by flooding is usually reduced when soil water is moving than when it is stagnant. A higher
subsoil pH may help to delay, for a few days at least, the death of citrus roots under flooded conditions.
With experience, flooding injury can be diagnosed during periods when groundwater levels are high.
Even before visible tree symptoms develop, augering and digging in the root zone may give an estimate
of future tree response. Indications of problems include high water tables with saturated soils in the root
zones, sloughing roots, and sour odors in the soil. When the water table recedes, visible damage to the
trees may become more obvious. New feeder roots appear and grow rapidly on trees that have
survived and received adequate irrigation.
Damage symptoms may develop over a period of time depending on the severity of root damage.
Symptoms usually appear after the water table drops and the soil dries out. Symptoms of root damage
include leaf yellowing, wilting, fruit drop, leaf drop, and twig dieback. Because the root system was
reduced by the flooding, the full extent of damage may not be known for several months or until drought
conditions occur.
Young trees are often more sensitive to flooding and may develop symptoms resembling winter
chlorosis. More subtle symptoms include reduced growth and sparse foliage. This can occur at shallow
depressions only a few cm lower in elevation that the surrounding area. Harvesting operations in a
grove after recent flooding may also further damage surface roots that have been injured by the flooding.
Hot, dry conditions following flooding, will hasten the onset of stress and symptom expression. The
reduced root system resulting from flooding is incapable of supporting the existing tree canopy. When
this occurs, irrigation management becomes critical. Irrigation must provide moisture to a depleted
(shallow) root system. Excessive water could compound existing problems. If the damage to the root
system is extensive and tree canopy condition continues to deteriorate with permanent wilt and foliage
dieback, some degree of canopy pruning may be necessary to reestablish a satisfactory shoot/root
balance.
Light, frequent irrigations will be required until the root zone has become reestablished. Subsurface
moisture should be maintained to promote root growth into the deeper root zone. If root damage is

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severe, frequent irrigation may be required throughout the winter months, especially if dry winds persist.
If irrigation water is high in salts, frequent irrigations (of short duration and low volumes) are essential to
prevent salt buildup that may compound the flooding problem.
Soils
Soil consists of various-sized solid particles and the pores (spaces) between them. In most citrus soils,
pore volume is 40-55% of the total soil volume. These pores hold water and air that are necessary to
citrus roots. Water is held in the soil pores by attraction between the water molecules and the surfaces
of the solid particles. At a low water content, water is spread as a thin film over the surfaces of all the
soil particles. The thinness of the film is dependent upon the amount of water present and the total
surface area of all the solid particles. Fine-textured (heavy, clay) soils have more particle surface area
than coarse-textured (light sandy) soils. Therefore, water molecules are held tightly very close to the
particle surfaces. The amount of energy needed to extract a unit of water from the soil is measured in
units of negative pressure (bars or atmospheres). This force is called soil matric suction or soil-moisture
tension and can be easily measured in the field with a tensiometer. Tensiometers were used extensively
to monitor soil moisture in citrus orchards (Creighton et al., 1990; Kaufmann and Elfving, 1972; Van
Nort, 1967). Tensiometers will function best in orchards that are irrigated frequently (such as by
microirrigation) and will not function properly once the soil dries, as is common for longer frequency
sprinkler or surface irrigation methods.
During irrigation or rainfall, water infiltrates into the soil and is re-distributed within the soil by gravity
and soil capillary forces (attraction for water). When all of the pore space is filled with water, a soil is
saturated. In this condition, gravitational forces dominate. Water drains downward through the soil
since the energy required to remove a unit of water from saturated soil is low. Drainage is initially rapid
since water is first drained from the largest pores, where it is held least securely. As the water drains,
air space is created in these pores. The remaining water is somewhat closer to the surfaces of soil
particles and is held firmly enough to prevent rapid drainage by gravity. This water condition, when
measured in the field, is called field capacity (FC). At this point, the soil water in the root zone will be
depleted primarily by plant transpiration or evaporation from the soil surface.
When water is used by plants, the medium-sized pores lose water first. As the soil dries, eventually the
smaller soil pores lose water. As the water extraction progresses, water films on the soil particles
gradually become thinner, and a greater energy is required to remove each subsequent increment of
water. At first, the change in energy requirement is slight and has little effect on the plant. Since citrus
roots do not come in contact with every particle of soil in the root zone, water must move through the
soil to reach root surfaces. The distance traveled may be less than a mm or it may be several cm,
depending on soil moisture status. Water moves more rapidly in thick films than in thin films. As water
films become thinner, the rate of flow decreases and eventually becomes too slow to meet the needs of
the tree.
The physical availability of soil water to a citrus tree is a continuously variable characteristic largely
depending on the water film thickness, and the soil tension can be measured in pressure units with a

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tensiometer. At some point (normally at about -15 bars of soil tension), the soil-water films become so
thin that the roots cannot absorb water and the tree wilts. When the tree remains wilted overnight, the
soil-water status has reached a level termed permanent wilting point (WP). Citrus will generally suffer
daytime wilting repeatedly before this condition is reached and yield is typically reduced long before WP
is reached.
The difference between FC and WP is called the available water (AW). Table 2 lists typical values of
AW for various soil types. Local soil surveys and irrigation guides can provide information on specific
soil types.
Once AW is known, the total available water (TAW) can be determined. TAW represents the
maximum amount of water in the root zone that can be used by the tree. It is obtained by multiplying
AW by the depth of the tree’s root zone. For layered soils, TAW is calculated by adding the multiples
of AW and depths of all soil layers contained in the root zone.
The effective root depths can range from 0.3 m in some poorly-drained soils to about 0.6-1.2 m in
many well-drained soils. The best way to determine effective root zone depths is by digging and
observing where most of the roots are located. The effective root zone is where most of the roots
actively involved in water uptake are located. In humid areas, irrigations should be concentrated in this
upper portion of the root zone where the great majority of the tree’s roots are located.
Allowable Soil Water Depletion
The allowable soil water depletion is the fraction of the available soil water that will be used to meet ET
demands. As ET occurs, the soil water begins to be depleted. As the soil dries, the remaining water is
bound more tightly to the soil, making it more difficult for trees to extract it. For this reason, ET will
start to decrease long before the WP is reached. Lower ET generally results in smaller fruit, lower
overall total soluble solids (TSS) production, and lower fruit yields. Therefore, irrigations should
commence before the root zone water content reaches a level that restricts ET.
The critical soil water depletion level depends on several factors: crop factors (rooting density and tree
age/size), soil factors (AW and effective root depth), and atmospheric factors (current ET rate).
Therefore, no single level can be recommended for all situations. Allowable depletions of to 50% of the
available soil water are commonly used in scheduling irrigations during the non-sensitive periods. Lower
depletion levels should be used during critical stages such as at bloom and fruit set. As a rule of thumb,
soils should be allowed to deplete no more than 33% from February through the “June drop” and at
50% depletion of AW during other times of the year.

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 13 of 41
Table 2. Typical available water (AW) for various soil types
Available Water (AW)
Type of Soil Range
(cm/30 cm)
Average
(cm/30 cm)
Sands and fine sands 1.0 to 2.5 1.9
Moderately coarse-textured: loams and fine
sandy loams 2.5 to 3.8 3.1
Medium texture: very fine sandy loams to silty
clay loam 3.1 to 4.4 3.8
Fine and very fine texture: silty clay to clay 3.8 to 6.3 5.0
Peats and mucks 5.0 to 7.5 6.2
Irrigation Water Requirements
Citrus trees require water for transpiration throughout the year. Citrus evapotranspiration is largely
determined by climatic factors. Water requirements vary with soil type, climate, ground cover,
cultivation practices, weed control, tree size and age, scion, rootstock, and tree health. Typically, trees
only require 3-4 L/day during the first couple of years after planting. Water requirements for mature
trees generally range from 800-1300 mm per year.
Citrus evapotranspiration is largely controlled by climatic factors. Under similar climatic conditions,
citrus trees are known to have lower transpiration rates than many crop plants. For example, the
midsummer daily evapotranspiration in Israel was found to be 7-8 mm/day for many field crops, 8.5
mm/day for apple orchards, but only 4.5 mm/day for citrus orchards (Shalhevet et al., 1981). The
reduced transpiration rate is due to low citrus canopy conductance. Canopy conductance calculated
from soil water depletion data and potential evapotranspiration are typically 0.22-0.29 cm/second
compared with 2.0-3.3 cm/second for a number of field crops (Van Bavel et al., 1967).
In Arizona, the daily consumptive use of Marsh grapefruit ranged from 1.5 mm/day in January to 5.6
mm/day in July, and from.0 to 4.4 mm/day for Navel oranges (Eric et al., 1965). The total annual
evapotranspiration for the two varieties was 1217 and 990 mm, respectively. In a report for Yuma,
Arizona, daily evapotranspiration values for Navel oranges were 0.8 mm/day in December and 7.5
mm/day in August (Hoffman et al., 1984). The high summer evapotranspiration values reflect the very
dry and hot climate of southern Arizona.

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Water use from individual trees is highly dependent on tree size. In Florida, large trees at low planting
densities (130-150 trees/ha) may use 75-115 L/day during the winter months and 230-260 L/day in
July and August (Tucker, 1985). Average annual evapotranspiration (ET) was reported to be 1200
mm for a developing grove with Bahia grass cover over a 10-year period in Florida (Rogers et al.,
1983). Daily water use by these trees peaked at about 150 L/day during the June-July period.
Large, vigorous, high yielding trees require more water than young or non-productive trees. Large trees
at low planting densities may use twice as much water per tree as trees planted at higher densities that
result in trees that are hedged and topped to control tree size. The water use per unit land area is
generally assumed to be constant, regardless of tree planting density, once the trees mature and form
near-continuous coverage of the ground in the tree rows. At planting densities of about 375 trees/ha,
typical trees may consume water in the order of 40-80 L/day during the winter months and peak at
150-200 L/day during the summer.
Studies have shown that irrigating when 1/2 of the total available water in the soil is depleted during the
period from February through June, and at 1/3 depletion the remainder of the year, can provide efficient
use of water while maintaining high yields (Koo et al., 1978). In another Florida study (Boman, 1997),
grapefruit trees irrigated at soil tensions of 10-15 cbar (1/2 depletion) yielded 12% more and produced
9% more TSS than those irrigated at 35-45 cbar (1/3 depletion). In addition, irrigation at 1/2 depletion
yielded more larger-sized fruit.
Young Tree Water Use
It takes a number of years (4 to 10, depending on planting density) to achieve maximum land coverage
and thus maximal evapotranspiration from trees in a newly planted grove. The amount of water applied
to a young grove should take into account the tree canopy size and percent of the land are that is
covered by tree canopy. Newly planted trees require only a fraction of the water that is needed by
mature trees. Average water use of about 4 L/day has been reported in Florida for newly-planted
Midsweet orange trees during the first year (Boman,1994). However, irrigation systems that apply
water to grassed areas in row middles and between trees can raise the water requirements considerably.
One study found nearly double the water requirement for 2-year-old citrus trees with grass cover as
compared to trees with bare soil (Smajstrla, 1985).
In Cuba, summer water requirements for young Valencia orange trees were reported as 2 mm per day
for newly planted trees and 3 mm per day for 4-year-old trees (Toledo et al., 1981). In a Florida study,
water use for 5-6 year-old Valencia orange trees peaked at about 60 L/day in May, before summer
rains started. During November through January, ET rates averaged only about half the May rate
(Boman, 1993). Water use was found to increase at a rate of about 20% each year from years 4
through 6.
With modern microirrigation methods, it is possible to irrigate individual trees and increase the size of the
wetted area or the number of emitters as the tree develops. With drip irrigation, it is possible to plug
some of the emitters during the first years, however with close plantings, roots will may fill the irrigated
portion in one year. The recommendation for midsummer applications in Israel is 10, 15, 25, 45, and

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 15 of 41
65 L/day from the first to the sixth year, respectively. From the sixth year on, the recommended
irrigation requirement is 3-5 mm/day or 100 L/day/tree.
Irrigation Scheduling
The total seasonal amount of irrigation water, needed by a fully grown orchard for optimum yield,
depends on the daily course of evapotranspiration, the rainfall distribution, the citrus cultivar grown and
anticipated yields. In a Mediterranean-type climate, rainfall is concentrated in the winter months with
little or no summer precipitation. During the winter period there is normally no need for irrigation except
under drought conditions (Ali and Lovatt, 1996).
An important consideration for winter irrigation in arid climates is salt leaching (discussed later). It
should be noticed that 40mm of rainfall is much less affective than an irrigation of 40mm (400 m3/ha),
since the irrigation-wetted area of the orchard may occupy less than 25% of the total orchard land area,
and the canopy itself may act as an umbrella and divert some of the rainfall away from the irrigation-
wetted area. In summer rain climates, there may be a considerable amount of rainfall during the summer
growing season. However, oftentimes in these areas, the timing of rains results in periods when rainfall
is insufficient to satisfy crop water requirements and supplemental irrigation is beneficial. Most of the
rain in Florida falls from June to September but during the critical period of January to June, rainfall is
low. In Florida, irrigation was found beneficial in 8 of 9 years and the average yield increase was 22%
when an annual application of 280 mm of water was added to the 1100 mm of annual rainfall (Koo,
1989), similar results were reported for Brazil (Tubelis et al., 1999) and even Japan (Suzuki et al.,
1967).
The required timing and amount of applied water is governed by the prevailing climatic conditions, stage
of growth, soil moisture holding capacity, and root development. In one study, microsprinkler irrigation
of young Valencia orange trees in Florida was most effective when irrigations were initiated at tensions
of -20 cbar (Koo and Smajstrla, 1985).
In a 7-year Texas irrigation experiment using Marrs and Valencia oranges, and Ruby Red grapefruit,
irrigation supplemented the 730 mm of annual rainfall with annual water application of 220 to 680 mm
(Wiegand and Swanson, 1982). Water was applied via surface methods at an available water depletion
of 30 to 40%. The daily water consumption for summer was 4 mm/day and 1.3 mm/day in the winter.
The optimum interval between irrigations depends on the design of the irrigation system, ET, time of
year, and soil characteristics as well as tree size and condition. Evapotranspiration rates of trees are
highly dependant on climatic conditions. In many modern irrigation systems in arid climates, the same
irrigation frequency is maintained during the whole irrigation season, and the amount of water per
irrigation is changed according to climatic conditions and fruit development.
Field Water Balance
Delivering water to a crop in the field results in losses which increase the amount of water which must be
delivered to supply the crop water requirement. Losses may occur because of inefficiencies in the

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conveyance system, evaporation and wind drift (if water is sprayed through the air), surface run-off, or
percolation below the root zone. These losses can be minimized through good management practices,
but they are impossible to completely eliminate, and they must be considered when determining the total
(or gross) irrigation water requirement. The total irrigation water requirement is the total amount of
irrigation water which is required for crop production (including ET), plus all losses incurred in delivering
water to the crop, and other needs such as leaching of salts, and freeze protection. In humid areas, a
large part of the crop water requirement can be provided by rainfall. Effective rainfall, that is stored in
the root zone and available for crop use, proportionally reduces the amount of water which must be
irrigated. Runoff losses are normally minimal on sandy soils, but may be significant on heavy soils.
Application losses, including evaporation and wind drift, can occur during irrigation, especially from
sprinkler systems on hot, dry, windy days. These losses are, however, relatively small during periods of
low radiation, low wind velocities, and high humidity. Also, water which evaporates during application,
or which is intercepted and later evaporates from soil, plant, or other surfaces is not entirely lost.
Rather, some evaporation during application promotes cooling and this compensates for ET by reducing
temperatures and thus ET that would have occurred if the intercepted water had not been evaporated.
Evaporation and wind drift losses can be minimized by irrigating at night, early mornings, and late
afternoons when climatic conditions are not severe. However, cultural aspects such as disease must be
considered when wetting the foliage may promote pest or disease development.
Advantages of drip irrigation include minimizing evaporation, wind drift, and wetting of leaves, and the
ability to utilize low quality water, both saline and reclaimed. Deep percolation losses from irrigation
systems can be minimized by good irrigation management. If water is applied uniformly and the water
holding capacity of a soil is not exceeded, water losses to deep percolation will be minimized.
However, when saline water is used for irrigation, it may be necessary to leach excess salts from the
root zone by applying water in excess of the soil water holding capacity, this can be done continuously
in every irrigation or periodically. If the losses are kept to a minimum, most of the irrigation water
applied will evaporate or transpire in proportion to the climatic demand. Unfortunately, rainfall is
relatively unpredictable and its occurrence immediately following an irrigation reduces rainfall
effectiveness. Irrigation can be reduced by anticipating rainfall and providing soil storage capacity (that
is, irrigating to less than field capacity to leave room for rainfall storage) to increase rainfall effectiveness.
However, even in summer-rain areas, care should be taken not to drought-stress the trees in the spring;
such stress then can reduce normal fruit set, and cause out-of-season bloom.
Upflux
The movement of water upward within the soil profile from the water table is called "upflux". As water
is removed from the soil by the tree roots and by evaporation at the ground surface, water content of
the soil decreases. By capillary action, water moves from the water table into the drier soil above.
Water tends to adhere to soil particles due to surface tension between adjacent particles. Smaller soil
particles have smaller inter-particle voids. The smaller particles provide greater surface areas upon
which water can adhere. In addition, the smaller voids allow water to be retained at higher surface
tensions. As a consequence, soils with smaller particles have the ability to move water greater distances

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 17 of 41
by capillary action than coarser soils. As a result, the upflux process can move water into the root zone
from a much deeper water table in clay soils than it can in sandy soils.
Excess water drains by gravity into the shallow water table after a saturating rain or irrigation cycle. The
removal of soil water by evaporation and transpiration results in water movement upwards (upflux) by
capillary action to replace some of the water in the root zone. The deeper the water table is, the farther
the water has to travel upwards into the root zone. Therefore, the effectiveness of the water table for
providing moisture to the roots decreases as the water table level drops. If the level is allowed to drop
too low, capillarity is broken and upflux action ceases until another saturating rain or irrigation cycle
refills the soil profile.
Under saline conditions, upflux of water from a saline water table to the surface should be avoided,
since it may cause salinization and destruction of the soil
Water Budgeting
Two questions must be answered in order to schedule irrigations: (a) when to irrigate? (b) how much
should be applied. A water budget (Figs. 3 and 4) accounts for irrigation and rainfall plus water use by
the trees. The water budget procedure can be used to answer both questions.
The crop root zone can be visualized as a reservoir where water is temporarily stored for use by the
crop. Inputs to that reservoir occur from both rainfall and irrigation. If the capacity of the soil-water
reservoir (the volume of water stored in the crop root zone) and the daily rates of ET extraction from
that reservoir are known, the date of the next irrigation and the amount of water to be applied can be
determined. Thus, ET and soil-water storage in the plant root zone are the basic information needed to
use the water budget method for irrigation scheduling.
The most significant crop factors that affect ET from a well watered citrus grove are the size of tree,
overall tree health, the time of year, and the leaf area with respect to the ground surface on which
radiation is incident. Methods of expressing plant size and leaf area include the degree of ground cover
or percent canopy coverage. ET rates are greatest when the entire soil surface is covered by the crop
canopy. Exceptionally low relative humidity and high winds will increase ET rates above normal. Hot
dry winds may raise the ET rates of isolated groves by 25% or more above the normal, although such
periods are usually brief. Citrus orchards do not totally shade the ground, especially during their early
stages of growth, and evaporation from the dry soil surface between plants is low. This is especially
true for sandy soils which act as a mulch to greatly reduce evaporation when the surface dries; weed
cover can change this. When the tree canopy is not complete, the ET rate is strongly influenced by the
area of leaf surface that is intercepting sunlight, that is, the percent of soil surface shaded by the trees.
For this reason, ET during early years is considerably less than the ET that would occur from a
complete canopy. As the orchard canopy develops, ET reaches its maximum at nearly complete
ground cover. ET measurements indicate that when the percent of ground covered by the canopy is
above 60-70%, full ground cover and full ET rates can be assumed. Immediately after an irrigation,
evaporation from the wet soil occurs at approximately the same rate as full cover ET, but as the soil
dries, rates of evaporation are quickly reduced. Thus, frequency of irrigation plays an important role in

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 18 of 41
determining evaporation losses from the soil, especially when the entire soil surface is wetted.
Evaporation rates from sandy soils are quickly reduced when the soil surfaces dry. However, because
of their low water holding capacities, the surfaces must be wetted more frequently than those of heavier-
textured soils because more frequent irrigations are required.
Calculating the Water Budget
The water budget procedure is also called a water balance or bookkeeping procedure. It is similar to
keeping a bank account balance. If the balance on a starting date and the dates and amounts of
deposits and withdrawals are known, the balance can be calculated at any time. Most importantly, the
time when all funds (water) would be withdrawn can be determined so that an overdraft is avoided (or
an irrigation can be scheduled).
The water budget equation for irrigation scheduling on a daily basis can be written as follows:
S= R+ I -ET -(D + RO) Eq. 1
where
S = change in available soil water (mm)
R = rainfall measured at the field site (mm)
I = irrigation applied (mm)
ET = evapotranspiration estimated from pan evaporation or other method (mm)
D + RO = drainage (D) and runoff (RO): calculated as rainfall in excess of that which can be stored
in the soil profile to field capacity (mm)
The soil water content on any day (I) can be calculated in terms of the water storage on the previous
day (I-1), plus the rain and irrigation, and minus the ET, drainage, and runoff that occurred since the
previous day as:
Si = Si-1 + R + I - ET - (D + RO) Eq. 2
where
Si = soil water content on a particular day
Si-1 = soil water content on the previous day
The above assumptions need corrections for irrigation that wet only part of the soil volume, such as drip
or microsprinklers.

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The starting point for irrigation scheduling is often after a thorough wetting of the soil by irrigation or
rainfall. This brings the soil reservoir to full capacity and S(I) to TAW. If this does not occur, the initial
available soil water must be determined by direct observation (measurement or estimation).
Daily measurements or estimates of ET are subtracted from the available soil water until the soil water
has been reduced to the allowable depletion level. At that point, an irrigation should be applied with a
net amount equivalent to the accumulated ET losses since the last irrigation. Therefore, soil reservoir is
recharged to full capacity, and the depletion cycle begins again.
The water budget procedure also accounts for rainfall since it refills the soil profile and raises the soil
water content. If large rainfalls occur, only that portion required to restore the soil water content to field
capacity will be effective. Greater amounts of rain will either run off the soil surface or drain below the
plant root zone.
The management decision concerning the level of allowable water depletion (AD) is one that will need
to be made by each irrigation manager. It will vary depending upon soil, tree size, and climatic factors.
Commonly it will vary during the growing season. For example, AD may be set at 50% during July-
January, but it should be decreased to 33% during critical growth stages during bloom through the June
drop period. Decreasing AD increases the frequency of irrigation (but decreases the amount per
irrigation) to provide a more favorable crop root environment to reduce drought stress during critical
growth stages. Decreasing AD will generally result in greater irrigation requirements because the soil
will be maintained wetter and thus rainfall will be less effective. More frequent irrigations will also
promote increased evaporation from the soil surface.
The soil depth to be managed for irrigation must be refined by field experience. In shallow soils, the
managed root zone may only be only 0.3-0.6 m. Even on deep soils, the managed citrus root zone
should be much less than the deep depths where roots exist. Rather, the irrigated zone should be the
upper 0.6-1.2 m of the root zone where the majority of the roots are located. This practice also has the
advantage of allowing some soil capacity for rainfall when it occurs.
Daily ET values for specific water use periods should be estimated from pan evaporation or ET
equations. If current daily ET estimates are not available, the use of soil moisture measurement
instrumentation or the installation of evaporation pans should be considered. In many areas (especially
where there is sandy or shallow soil), the use of long-term average ET data can result in stress because
day-to-day ET rates are highly variable. Long-term average ET data can be used as a guide for daily
ET estimates, but they will need to be modified for climatic variability. For instance, they will need to be
increased during long-term hot, dry periods, and decreased during mild weather periods.
Irrigation Methods
Irrigation methods differ in the way water is distributed over the field. Gravity irrigation, where water is
conveyed to the point of consumption directly on the land surface, is the traditional method of irrigation
of citrus in many areas. However, surface irrigation methods are gradually being replaced by

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pressurized irrigation systems in many areas. Pressurized systems include both sprinkler (overhead and
under-tree) and microirrigation methods (drip, minisprinklers, bubblers, and microsprayers).
Gravity Irrigation
Water may be distributed in furrows (Fig. 5), border strips along the tree row, or in flood basins around
each tree. Water distribution by this method is inherently less uniform than by alternative methods and
usually larger quantities of water need to be applied in order to obtain reasonable coverage. In Arizona,
drip irrigation used a third less water (with 20% leaching fraction) than flood irrigated plots while
obtaining similar yields (Hoffman, et al., 1984). In a 5-year experiment in a Valencia orange orchard
near Yuma, Arizona (Rodney et al., 1977), flood irrigation yields were lower (112 kg per tree) and
water applications were considerably higher (1700 mm per year) than for sprinkler (390 mm, 146 kg
fruit per tree), basin irrigation (360 mm, 232 kg fruit per tree) or drip irrigation (430 mm, 194 kg fruit
per tree).
Sprinkler Irrigation
Sprinkler irrigation may be applied by overhead solid-set sprinklers (Fig. 6), traveling gun (Fig. 7),
undertree low angle solid-set or portable systems. Although sprinkler systems were once a common
irrigation method for citrus, less-costly and more flexible microirrigation systems have taken their place
in many areas. When the irrigation water contains even a low concentration of salts, foliar damage may
occur with overhead irrigation. For this reason overhead sprinklers (overhead solid set, traveling gun,
overhead portable systems) have been abandoned in many citrus producing areas. Compared to
microirrigation, evaporation is usually greater from above canopy systems that keep the whole canopy
area wet for a longer period.
Microirrigation
Microirrigation has become very common in citrus orchards. The methods include microsprinklers and
drip irrigation. The advantages of microirrigation include high application efficiency, low pressure
requirements, ease of operation, fertigation and chemigation, ability to automate, and good control of
soil aeration. Conversion of existing mature orchards from flood and sprinkler to microsprinkler and
drip irrigation must be done with consideration to changes in the wetted surface area, preferably during
the rainy season The optimum length of time to operate a microirrigation system at each irrigation
depends upon the individual system owner's management procedures as well as the irrigation system
design, the soil hydraulic properties, and the crop root zone. Therefore, the optimum time may be
unique for each system manager because of his management constraints, even with identical soils, crops,
and irrigation systems. There are, however, constraints on the maximum and minimum periods of time
which the systems should operate. The average direct energy use for drip and furrow are comparable at
about 915 Kwh/ha compared with 2270 Kwh/ha for sprinkler irrigation. Total energy consumed was
least for drip irrigation (Aljibury, 1981)

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 21 of 41
Microsprinklers
The diameter of wetted soil surface under many minisprinklers (Fig. 8) ranges from 3-6 m, a distance
similar to or slightly greater than normal tree spacing. Microsprinklers can be tailor-designed to the size
of the tree, with the discharge and the wetted diameter increased as the tree grows. Microsprinklers
have an important advantage over drip irrigation in the improved frost protection that this system can
provide, especially to young orchards. Most of the microsprinklers are prone to mechanical damage
due to harvesting operations and orchard maintenance.
Drip irrigation
Drip irrigation (Figs. 9 and 10) is common in dry-summer countries: Mediterranean, Australia and
South Africa, under such conditions most of the root system concentrates in the irrigated soil volume.
Drip irrigation is particularly suitable for high salinity situations in arid climates, since they maintain high
water content and move accumulated salts out to the periphery of the wetted zone. Higher levels of
salinity in the irrigation water can be tolerated with this system than with other methods of irrigation
(Rhoades and Loveday, 1990), when care is taken to prevent leaching of salts from the surface into the
root system by rain.
Drip irrigation necessitates careful attention to water filtration. Often water chlorination is needed to
prevent bacterial blockage of emitters. Advances in filtration techniques make it possible to use surface
water and even sewage water. When drip systems are used, the grower can decide to block some of
the emitters and start with a few drippers near the young tree, and adding more emitters as the roots
expand. Currently, the usual practice for drip irrigation is one or two dripper lines per row, with built-in
pressure-compensating emitters spaced at distances of 0.5-1 m. Advances in the manufacture of
pressure-compensated inline drippers, make possible longer rows and uneven topography than were
used in the past.
Drip irrigation systems are cheaper and require less maintenance than sprinkler systems. Attention
should be given to periodic flushing of the dripper lines, which can be done automatically or manually.
The use of hard water may necessitate periodic flushing with acid, as recommended by the
manufacturer, to dissolve lime deposits.
Irrigation frequency can be more than once a day in sand dunes, but is typically twice a week in medium
soils in Israel, less frequently in very heavy soils. However, many orchards are irrigated daily in Spain
with great success.
Early underground drip systems in citrus orchards failed, mainly because of root penetration into the
emitters. New and better emitters, coupled with chemical control of the penetration of roots, make
underground drip feasible. Underground drip is beneficial in cases where pests, such as rodents or
woodpeckers, damage aboveground plastic lines. Buried drip systems have the potential for reduced
water requirements since little water is lost to evaporation, and contact with raw sewage water is
eliminated.

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The minimum operation time for a microirrigation system should allow the water to move into that
portion of the crop root zone where the majority of the roots actively involved in water uptake are
located. Because the soil surface in the irrigated area will be wetted, nonproductive evaporation losses
will occur. Those losses will be greatest immediately following each irrigation while the soil surface is
wettest. Irrigation applications should be of sufficient duration to allow the applied water to penetrate
into the soil to a depth where the bulk of the roots are located.
Since a microirrigation system is only capable of irrigating a small portion of the total root zone, it may
be necessary to operate the microirrigation systems more frequently than other systems. The frequent
water applications allow maintenance of nearly constant low soil water potentials (moist soil conditions)
which minimize drought stress in the trees.
The maximum operating time for a microirrigation system depends on rooting depth, soil texture, and
antecedent conditions. Generally, irrigations should be scheduled to thoroughly wet the soil to the depth
of the effective (managed) root zone. Unless irrigating with high salinity water, care should be taken not
to apply water volumes that will wet to depths below the root zone. Over irrigation increases the
chances of nutrient leaching below the root zone.
An important consideration with drip irrigation is fertigation, due to the constant movement of water;
fertigation should be applied continuously during most of the irrigation season. Advances in irrigation
and fertigation control make it feasible to irrigate with a constant concentration of fertilizers during the
irrigation cycle.
Irrigation and Salinity
All natural waters and soil solutions contain soluble salts. The quantity of salts is commonly reported in
units of total dissolved solids (TDS) or electrical conductivity (EC). TDS are measured by evaporating
a sample of water and weighing the residue. The results are reported in parts per million (ppm) or
mg/L, ppm is equal to mg/L.
Salts exist in solution as ions that conduct electrical current and the EC increases in proportion to the
concentration of dissolved salts. EC measurements are taken with platinum electrodes and presented in
units of conductance. The SI unit of measurement is deci-Siemens per meter (dS/m) which is equal in
magnitude to the commonly used conductance term of millimho/cm (mmho). Both of these terms are
generally in the range of 0 to 5 for waters used for irrigation. The conversion from electrical
conductance to TDS depends on the particular salts present in the solution. Conversion values of 630
to 700 x EC (in dS/m) are used, depending on the area.
Chloride ion is toxic to citrus, so the concentration of chloride (mg chloride per liter), is an important
parameter, in deciding the suitability of water for citrus irrigation. Another toxic element of great
concern is Boron (B), which occurs in some soils, some natural water sources, and may increase in
reclaimed water, and in desalinized sea water.

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Since the concentration of salts in soil depends on soil water content, soil salinity is often related to the
electrical conductivity of standard saturated extract (ECe). ECe standardizes the amount of salts in the
soil to conditions when the soil is saturated. Depending on soil moisture content, the actual salinity level
in the vicinity of the tree roots may be several times greater than the ECe. In sandy soils, where salts are
easily leached, management decisions based solely on ECe measurements are not advised. The ECe on
these soils is only an indication of soil salinity at the time of measurement and can change rapidly
following irrigation or rainfall. Without proper water and nutrient management, citrus irrigated with high
salinity water can suffer the reduced growth and production which accompanies salt stress.
Dissolved salts exert an osmotic effect that reduces the availability of free (unbound) water through
physical processes. Roots are therefore not able to extract as much water from a solution that is high in
salts than from one low in salts. In effect, the trees have to work harder to move water through their
system. This can result in an immediate reduction in root growth followed by reductions in shoot growth
and yield.
Citrus is a salt-sensitive crop and sensitivity is mainly related to the specific toxic effect of accumulation
of chloride and sodium in the leaves. In humid areas, the injury symptoms on citrus trees caused by
saline irrigation water are not usually permanent. However, affected trees may remain stunted
compared to trees not receiving salinized water, especially if the trees are young when they are injured.
It has been long known that citrus rootstocks differ in their salinity tolerance (Oppenheimer, 1937;
Cooper and Gorton, 1952). Field studies in Texas and California tested salinity tolerance of rootstocks
according to their ability to exclude Cl from leaves. These results were corroborated lately for many
rootstocks under field conditions (Garcia Lidon et al., 1998; Levy, Columbus et al., 1999; Levy and
Lifshitz, 1999; Levy, Lifshitz et al., 1999; Levy and Shalhevet, 1990; 1991). In general, the decreasing
order of salinity tolerance (most tolerant to most sensitive) is: Cleopatra mandarin (C. reshnii Hort ex
Tan.), Rangpur lime (C. limonia Osbeck.), SB812 (C. sunki x P. trifoliata L.), X639 (C. reshni
Hort. ex Tanaka x P. trifoliata L), Gau Tou, Volkameriana (C. volkameriana Chapot), sour orange
(C. aurantium L.), Swingle citrumelo, rough lemon (C. jambhiri Lush.), Carrizo and Troyer citranges
(C. sinensis x Poncirus trifoliata), C35 citrange, citron (C. medica L.). The above ranking may
change somewhat by the effect of scion, and conditions of incompatibility, which can be physiological or
pathological (viruses, viroids, root infections).
It is important to remember that growth and yield of trees on all rootstocks can be reduced by excessive
salts. The comparatively high tolerance of sour orange to salinity puts growers all over the world at a
dilemma since the threat of tristeza prevents its use, and many tristeza-tolerant rootstocks (such as rough
lemon, trifoliate orange, Carrizo citrange, Swingle citrumelo, etc.) are sensitive to salinity. Recent
research indicates that drip irrigated trees on sour orange may be more susceptible to salinity than
mature trees (Levy et al., 1999). The use of rootstocks that are susceptible to lime induced chlorosis in
calcareous soils present the grower with a dilemma: Deficit irrigation can cure lime induced chlorosis,
however, these rootstocks are also susceptible to salinity, which may increase, due to reduced leaching
under deficit irrigation.

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The critical salinity level will vary with the buffering capacity of the soil (soil type, organic matter),
climatic conditions, and the soil moisture status. Many salinity-induced symptoms such as reduced root
growth, decreased flowering, smaller leaf size, chlorosis and impaired shoot growth are often difficult to
assess but occur prior to ion toxicity symptoms in leaves. Often, Na toxicity symptoms seldom
distinctly appear but rather an overall leaf "bronzing" appears along with reductions in growth (Fig. 11).
As with Cl, high leaf Na can cause nutrient imbalances at much lower concentrations than those required
for visible symptoms.
Direct Foliar Damage Through Sprinkler Spray
Sprinkler irrigation wets the foliage, either partially or fully. Severe damage to the leaves in the skirt of
the trees of undertree sprinkler-irrigated orchards has been described. Leaf chloride and sodium
toxicity due to direct contact with saline water has different symptoms than toxicity of chloride that was
absorbed by root. Contact damage, consisting of burned necrotic, or dry-appearing tips on leaves, is
one of the most common visible salt injury symptoms. There are reports of chloride and sodium
concentrations of the lower leaves that were about four times greater than those of the upper leaves
(grapefruit, Valencia and Washington navel). The lowest concentration of either sodium or chloride
generally associated with leaf burn is about 0.25%. Controlled experiments showed that citrus leaves
easily accumulate chloride and sodium from direct contact with water drops (Eaton and Harding, 1959,
and Ehlig and Bernstein, 1959). The accumulation is greater from intermittent than continuous wetting
and from daytime than nighttime irrigation. Accumulation is a function of the rate of evaporation, which
results in increased salt concentration of the water film on the leaves. Similar damage can also develop
from wind blown salt near the sea. The sensitivity of citrus scion rootstock combination to injury through
direct foliar contact bears no relationship to its general tolerance to soil salinity.
Nutrition and salinity
The frequency of injecting nutrients or of applying granular fertilizer has a direct effect on the
concentration of TDS in the soil solution. A fertilization program that uses frequent applications with
relatively low concentrations of salts will normally result in less salinity stress than programs using only
two or three applications per year. Controlled-release fertilizers and frequent fertigations are ways to
economically minimize salt stress when using high salinity irrigation water.
Selecting nutrient sources that have a relatively small osmotic effect in the soil solution can help reduce
salt stress. The osmotic effect that a material adds to a soil solution is defined as its salt index relative to
sodium nitrate, taken to be equal to 100. Since sources of phosphorus (P) generally have a low salt
index, they usually present little problem. However, the salt index per unit (kg) of nitrogen (N) and
potassium (K) should be considered. The salt index of natural organic fertilizers and slow-release
products are low compared to the commonly used soluble fertilizers. High-analysis fertilizers may have
a lower salt index per unit of plant nutrient than lower-analysis fertilizers since they may be made with a
lower salt index material. Therefore, at a given fertilization rate, the high-analysis formulation may have
less of a tendency to produce salt injury.

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Selecting nutrient sources that do not add a potentially harmful ion to already high levels in irrigation
water can also avoid the compounding of salinity problems. The Cl in KCl or Na in NaNO3 materials
add more toxic salts to the soil solution. High rates of salt application can alter soil pH and thus cause
soil nutrient imbalances. Specific ions can also add to potential nutrient imbalances in soil and trees.
For example, Na+ displaces K+, and to a lesser extent Ca++, in soil. This can lead to K deficiencies
and, in some cases, even to Ca deficiencies in leaves when irrigating repeatedly with water high in Na.
Such nutrient imbalances can compound the effects of salinity stress. Problems can be minimized if
adequate nutritional levels are maintained, especially those of K and Ca.
Preliminary results suggest that continuos application of nitrates, such as KNO3 under saline conditions
can reduce chloride uptake by susceptible rootstocks, and increase yield (Bar et al., 1997; Levy and
Lifshitz, 1999)
Managing irrigation and fertilization with high salinity irrigation waters requires routine evaluations of the
electrical conductivity of the irrigation water and soil solution with an EC meter. If excess salts
accumulate in the soil, it is best to the keep the soil moist so as not to further concentrate its salts.
Excess irrigations to leach accumulated salt may become necessary and should be made no less
frequently than every other week during the peak irrigation season. Heavy soils and areas with
compacted soils or poor drainage may need special attention concerning salinity management.
Irrigation with saline water makes it necessary to monitor the accumulation of salts in the soil, with
suction caps or by taking soil samples for laboratory analysis. Excess irrigation water in order to leach
the salts from the root zone (leaching fraction) may be necessary. Paradoxically, the use of tensiometers
to decide water needs can be dangerous, since water uptake will be reduced as soil salinity increases,
and reducing the irrigation based on the soil moisture measurements will aggravate the salinity condition
by reducing leaching. It is advisable to always monitor soil salinity when basing the irrigation regimes on
soil moisture measurement devices.
In arid climates, especially under drip-irrigation, special care should be given to prevent salt damage
caused by the first rains after a long drought period. Rain may leach the salts that accumulated on the
soil surface or the periphery of the wetted zone, into the root zone. Irrigation should thus be started
immediately when the rain begins; postponing irrigation for even a short period may result in severe
damage, typically defoliation.
With high salinity irrigation water, fertilizer formulations should have low salt index per unit of plant
nutrients. It may be necessary to increase the frequency of fertilizations, thereby making it possible to
reduce the salt content of each application and aid in preventing excess salt accumulation in the root
zone. Fertilization rates should be based on the long-term production from the grove. Application
rates can usually be lower for trees with high salinity than for trees irrigated with good quality water
since production levels will probably be lower. Leaf tissue analysis should be used to detect excessive
Na or Cl levels or deficient levels of other elements caused by nutrient imbalances from the salt stress.

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Other Uses for Irrigation
Evaporative Cooling
Heat spells can cause damage to flowers and young fruitlets and increase "June drop". Washington navel
orange is especially prone to such damage. Reduction in net photosynthesis by temperatures above
30oC may cause low-carbohydrate-induced fruit abscission (Palmer et al., 1977). Evaporative cooling
by overhead irrigation can lower the air and canopy temperature considerably. In Florida, overhead
irrigation lowered midday air temperatures from 34oC to 28oC and increased relative humidity from 40
to 60%, relative to the non-irrigated control (Koo and Reese, 1975). In the San Joaquin Valley Calif.,
intermittent application of overhead sprinklers reduced leaf temperature from 52oC to 29oC when the
ambient temperature was 38oC (Brewer, 1977). Overhead sprinkling may increase the hazard of
sodium and chloride damage to leaves and should be practiced only with water of low salinity.
Freeze Protection
Various forms of irrigation have been used for frost and freeze protection for many years. When used
properly, water can provide partial or complete cold protection for a number of crops. On the other
hand, improper use of water can increase evaporative cooling or ice loading and cause greater damage
than if no water were used at all. Irrigation can be used effectively for citrus frost and freeze protection
if it is properly managed. Microsprinkler irrigation has been shown to be effective in providing good
protection for young trees and partial protection of mature trees (Fig. 12). In general, the higher the
volume of water provided by microsprinklers, the more effective the cold protection will be.
The heat that is released when liquid water freezes to solid ice is called the heat of fusion. As long as
enough water is continuously applied to a plant, the heat generated when water freezes can keep the
plant at or near 0oC. At least 6-9 mm/hour is generally required for cold protection. At very low
temperatures, low humidity, or high winds, more water must be applied to get adequate protection.
The energy released when the irrigation water drops from its initial temperature to 0oC contributes
sensible heat during a freeze event. When temperatures drop below freezing, the latent heat of fusion is
released when the water freezes. Depending on the amount of ice that forms, the heat released can
raise temperatures in the lower part of the canopy. Water has a high heat capacity and can store a fair
amount of heat. Therefore, a moist soil can hold more heat than a dry soil.
Microsprinklers can also raise the dew point or frost point temperature in the grove. When the
temperature drops to the frost point, heat is released as the water vapor is converted to ice crystals.
When the grove air temperature reaches the dew point temperature, the rate of cooling slows down
because heat is released as the water vapor in the air condenses. It has been suggested that
microsprinklers can provide some protection above the spray zone because moist air rises and
condenses higher up in the canopy. The heat of condensation may help warm the upper canopy and
protect more of the tree.

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Depending on the dew point temperature, microsprinklers can sometimes create fog on cold nights.
Fog is beneficial for frost protection and if the fog is dense enough and the droplets are of the proper
size, the rate of cooling can be slowed since fog can act like a blanket and reduce the rate of radiation
loss. Similarly, clouds (which consist of water droplets) can act like a blanket and slow radiation loss.
If the water application rate is high enough on the trunk of a young tree, it will be protected by the ice
formation. However, on the edge of and outside of the iced zone, temperatures will not be maintained
at 0oC, and those parts will probably be damaged or killed. Therefore, usually the tops of young trees
or branches above the iced zone are more severely damaged after a freeze.
Microsprinklers can provide some protection to leaves and wood, particularly on the lower and inner
part of the canopy. A dense canopy tends to retain heat from the soil and provide better protection
than a thin canopy. Damage will commonly be seen on the outer and upper parts of the tree after
severe freezes. Since fruit is more sensitive to cold temperatures than leaves or wood, microsprinklers
generally do not protect the fruit. At higher volumes, spray jets will help protect fruit a little better than
no irrigation, but generally microsprinkler irrigation is best for tree protection rather than fruit protection.
There is a limit to the effectiveness of microsprinklers. Factors such as tree health and cold acclimation
affect tree survival. Depending on volume of water applied, the lower limit of effectiveness for
microsprinkler irrigation is around -8oC in Florida. It is common for protection to be seen only in the
iced zone on young trees. Damage commonly occurs, particularly in severe freezes, above the iced
zone. Young trees are usually more tender and do not retain heat as well as mature trees.
Microsprinklers have been effective in protecting the bud union and lower portion of young trees. In
young trees, the microsprinkler protects the lower trunk by the direct application of water. When water
freezes, it releases heat. If the application rate is high enough, the freezing water will maintain the trunk
at a temperature near 0oC. The microsprinkler must be close enough to the young tree so that water
sprays directly on the trunk and lower part of the tree. Recommended distances between the trunk and
the microsprinkler are 0.4 to 0.7 m. If the microsprinkler is too far away from the young tree, wind can
blow the water away. If the water freezes before it hits the tree, milky white ice can form on the tree.
Protection under milky ice is usually not as good as under clear ice. It is best to put the microsprinkler
on the upwind side of the tree. In this position, the wind will carry the water into the tree and not blow it
away from the tree.
Insulating tree wraps placed around the trunks of young trees slow the rate of temperature fall. Tree
wraps alone provide some trunk protection. Tree wraps in combination with microsprinkler irrigation
provide even better cold protection insurance. If the irrigation system fails during the night, the tree
wrap (particularly if it is a good insulator or has enclosed water pouches) can slow the temperature drop
and protect the tree longer.
Spray pattern and application rate influence effectiveness. Emitters that produce a 90o or 180o pattern
concentrate the water on the young tree and provide better protection than 360o patterns. In one study,
lower leaves in the direct water spray zone stayed above freezing and were as much as 7.8oC warmer
than dry leaves in nonirrigated plots (Parsons et al., 1981). Other work showed that microsprinklers

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 28 of 41
maintained trunk temperatures near -1.1oC when minimum air temperature reached -9.4oC in a freeze
with winds exceeding 32 km/hr (Davies et al., 1984). While ice loading can bend over one-year-old
trees, trunk breakage is minimal and young tree survival is usually excellent.
Placement of the emitter in relation to the tree is important, particularly in a windy freeze. The
microsprinkler must be located upwind of the tree. Emitters on the downwind side of young trees
caused significantly more damage than no irrigation (Parsons et al., 1985). With the emitter on the
downwind side, the variable wind periodically blew away the water spraying on the tree. This irregular
wetting and drying of the trunk caused more damage due to evaporative cooling. Similar damage
caused by evaporative cooling was observed on mature trees when undertree impact sprinklers froze
and stopped applying water during a freeze (Parsons and Tucker, 1984).
While microsprinklers are best at protecting young trees, they also provide partial protection of mature
trees. The greatest benefit has been observed in calm frosts, but there have been several examples of
improved recovery of mature trees when microsprinklers have been used in windy freezes. In a
relatively calm frost, microsprinklers benefited trees over 4 m tall. Tree recovery and leaf canopy
density 3 months after a freeze were better where microsprinklers were operated in a grove compared
to where they were not operated (Oswalt and Parsons, 1981). Emitters with greater output provide
more protection.
Microsprinkler irrigation is more effective for cold protection when high volumes of water are available.
Approximately 19 m
3/ha/hr are needed to provide reasonable protection. This can be accomplished
with one 76 liter/hr emitter per tree in a grove with 247 trees/ha. Rates below this level will provide
some protection but not as much as higher rates. Application rates of 28 m3ha/hr or more can provide
even more cold protection (Parsons, 1984).
It is generally thought that microsprinklers provide little or no protection for fruit, although the
percentage of marketable fruit was slightly higher in one study when the air temperature dropped to
-8.3oC. At temperatures below this, microsprinkler systems were not effective for leaf or fruit
protection (Buchanan et al., 1982). Depending on climate and cold acclimating conditions, citrus fruit
will generally not be damaged until the temperature drops to below -2.2oC for 4 or more hours. Since
microsprinklers can raise air temperature slightly, they can modify both the duration and minimum
temperature that occurs in a grove. Hence in theory, microsprinklers could potentially reduce fruit
damage somewhat in the lower part of the canopy during borderline frost conditions.
With mature trees, several factors may contribute to the effectiveness of microsprinkler irrigation. In
addition to the latent heat of fusion from the formation of ice, other factors may include a) sensible heat
from the temperature of the water, b) fog or mist formation, c) increased humidity, and d) increased soil
heat capacity and thermal conductivity (Parsons et al., 1982). Well water can have temperatures of up
to 20oC. As this water cools, it releases sensible heat. Fog does not always form on freeze nights, but
when the dew point is relatively high, microsprinklers can create fog. Water droplets, particularly those
in the 10 µm diameter size range, reduce long wave radiation, but the contribution of microsprinkler
generated fog to freeze protection has been questioned by some (Davies et al., 1984). Increased
humidity can also reduce radiation loss and may contribute to warming. Any irrigation system that wets

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 29 of 41
the soil, including microsprinklers, increases soil thermal conductivity and the rate of energy transfer
from the soil to the atmosphere near the soil. High density plantings may also help slow the loss of heat
from microsprinkler-irrigated groves.
Using Drought Stress to Induce Bloom
A period of dormancy (technically quiescence in Citrus) markedly enhances flowering in citrus trees,
and flowering generally increases with duration of dormancy. In subtropical citrus areas, dormancy is
primarily induced by low wintertime temperatures (less than 13oC). Water deficits can also be used to
induce dormancy, and this is widely used in the tropics but can also be used to enhance flowering in
other citrus areas. An insufficient dormant period may result in a weak spring bloom, increasing the
likelihood of significant off-bloom, and markedly reducing yield in many varieties.
Flower bud initiation occurs during this dormant period of minimal vegetative growth. Once the
dormancy period has ended, an adequate water supply is necessary for the trees to bloom well. Severe
water deficits following the break of dormancy will delay flowering, and when bloom does occur,
flowering may be excessive resulting in an over-abundance of small fruit or even excess competition for
resources leading to poor set.
Citrus trees growing in tropical climates can bloom continuously year round, and rainfall or irrigation
after a period of drought can trigger a flush of flowering. In subtropical climates, some citrus species
(notably lemon, pummelo, mandarins and tropical orange cultivars, such as Pera) are capable of
flowering all year round. Irregular irrigation can also cause out-of-season flowering by inducing short
periods of dormancy. The fruit that may set is usually undesirable and heavy out-of-season flowering
can reduce development of main season fruit.
A traditional orchard management practice in lemon culture in Sicily is forzatura (forcing), withholding
irrigation for periods of a month or longer in order to cause wilting and induce summer bloom. The fruit
that will set from such flowers is picked the following summer as green (verdelli) fruit. Lemon cultivars
differ in their response to this treatment. Good responders are the cultivars Eureka, Femminello,
Villafranca, Verna and Fino as compared with cultivars which tend to flower only in the spring: Lisbon
and Interdonato. The amount of drought stress necessary to induce flowering is important since
excessive stress can be harmful. Some reports indicate that about 60% of the flowers abort after
excessive drought stress compared with only 20% after moderate drought (Torrisi, 1952). Excessive
drought stress also harms the development of fruits already on the trees and will substantially reduce the
yield of regular winter lemons (Calabrese and Di Marco, 1981). Drought is utilized in Mediterranean
countries to stimulate flowering in lemons. By withholding water during certain periods, summer
flowering is initiated, and the resulting out-of-the-season fruit can be harvested at a time when prices are
higher.
In humid areas, methods to increase citrus flower bud induction may be useful to enhance yield during
winters with above normal temperatures. If little rain occurs, drought stress may be developed by
discontinuing irrigation. Withholding irrigation should be continued until at least 30 to 40 days of stress
have occurred. Visual observation of leaf wilting is a good indicator of proper stress. Wilting by 10 or

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 30 of 41
11 am and recovery overnight is ideal. Drought stress may be difficult to develop on deep sandy soils
due to the deeper rooting zone. Two or three weeks may be required to reach an adequate level of
stress to begin flower bud induction. If winter rain thwarts attempt to maintain drought stress, the
associated cool weather behind the weather front (a typical occurrence) often provides some cool
temperature but may not be sufficient to provide adequate bloom.
Chemigation and Fertigation
Chemical application through irrigation systems is called chemigation. Chemigation has been practiced
for many years especially for fertilizer application (fertigation), but care must be taken to prevent any
contact with drinking water and all precautions taken to prevent contamination of water sources or
ground water. Other chemicals are also being applied through irrigation systems with increasing
frequency. The primary reason for chemigation is economy. It is normally less expensive to apply
chemicals with irrigation water than by other methods. The other major advantage is the ability of
applying chemical only when needed and in required amounts. This "prescription" application not only
emulates plant needs closer than traditional methods, but also minimizes the possibility of environmental
pollution. Through chemigation, chemicals can be applied in amounts needed and thus large quantities
are not subject to leaching losses if heavy rainfalls follow applications. Thus, reducing adverse
environmental impacts in addition to saving the time and money needed to reapply the materials. Micro
irrigation is suited for applying herbicides to orchards in arid areas, where weeds do not develop outside
the irrigated area. Compared with conventional application methods, herbigation with Bromacil plus
Diuron gave better control of Bermuda grass and other weeds with lower quantities of herbicides. In
Spain, good results were reported for drip herbigation when herbicides were continuously applied in
low concentrations. Under-the-canopy sprinklers have been used successfully for applying copper
based fungicides for the control of brown rot on fruit. Systemic pesticides can be applied through the
irrigation system, provided such application passes health and safety regulations.
Injection Methods
There are several methods of chemical injection into an irrigation system. The choice of appropriate
methods and equipment will depend on several factors. Injection methods include centrifugal pumps,
positive displacement pumps, water operated piston pumps, pressure differential methods, and the use
of the venturi principle. Some injectors use a combination of these methods. In most setups these
injectors are coupled to the irrigation controller or computer.
If solid materials will be injected, they will need agitation and mixing at the pump site. Liquid fertilizers
and agricultural chemicals, on the other hand, can be injected directly from their storage tanks. Injection
of most fertilizer materials can normally be accomplished without great concern for workers exposed to
the materials. However, when handling and injecting acids and toxic pesticides, worker safety is of
great concern.
Some installations may require more than one injector because of vastly different flow rate requirements
for the materials used. For instance, fertilizer injections are normally at a rate of at least 0.1% of the
system flow rate. The injection rate for acids, water conditioners, chelates, and some pesticides may be

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 31 of 41
less than 10% of that for fertilizers, making it impossible to use the same injection device for both
applications.
Normally, it is desirable to inject materials upstream of filters. The filters should trap any contaminants
or precipitates that occur as a result of the injections. However, due to their corrosive effect, acids and
corrosive fertilizers should normally be injected downstream of metal filters. It is also necessary to
discontinue injections during filter backwash (flushing) cycles. On filter systems with automatic
backwash controls, a controller should be installed to control both the backwash cycles and the
injectors.
Fertigation
Fertigation (applying fertilizers through the irrigation system) is very common with microirrigation
systems, and drip irrigation usually will not be successful without it The success of fertigation in
microirrigation may depend on the size of the active root system which, in turn, depends on the amount
of summer rain in the area. In humid areas (such as Florida) the active root system expands to cover
much of the orchard floor during the summer rainy season. In these areas, microsprinkler coverage
should be at least of 80% of the tree canopy area to provide adequate fertilizer distribution to the roots.
With drip irrigation, which usually covers much less ground area, fertigation applications may need to be
very frequent.
Fertilizer Solubility
Several dry fertilizer products used for making fertilizer solutions are marketed with or without a
protective conditioner. Whenever possible, the "solution grade" form of these products should be
purchased to avoid having to deal with the conditioners and the potential plugging problems they can
cause. Many dry solid fertilizers are manufactured with a coating (commonly with clay, diatomaceous
earth, or hydrated silica) to keep moisture from being absorbed by the fertilizer pellets. To avoid having
these materials create plugging problems, it is best to prepare a small amount of the mix to observe what
happens to the coating. If the coating settles to the bottom of the container, the clear transparent liquid
can be taken from the top portion without disturbing the bottom sediment. If a scum forms on the
surface, conditioners may need to be added to facilitating the removal of the scum.
When urea, ammonium nitrate, calcium nitrate, and potassium nitrate are dissolved, heat is absorbed
from the water as the solution cools. Consequently, it may not be possible to dissolve as much fertilizer
as needed to achieve the desired concentration. Oftentimes it is necessary to let the mixture to stand for
several hours and warm to a temperature that will allow all the mixture to dissolve.
Before injecting fertilizer solutions, a "jar test" should be conducted to determine clogging potential of
the solution. Some of the fertilizer solution should be mixed with irrigation water in a jar to determine if
any precipitate or milkiness occurs within one to two hours. If cloudiness does occur, there is a chance
that injection of that chemical will cause line or emitter plugging. If different fertilizer solutions are to be
injected simultaneously into the irrigation system, they all should be mixed in the jar. The jar test should
be conducted at about the same dilution rate that is used in the irrigation system.

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Nitrogen
Urea, ammonium nitrate, calcium nitrate, potassium nitrate, and ammonium sulfate are very soluble in
water. These nitrogen fertilizers are readily available on the market and are used extensively in the
preparation of single nutrient or multi-nutrient fertilizer solutions.
Phosphorus
Commercial fertilizers contain the guaranteed percentage of P2O5 on the label as water soluble and
citrate soluble phosphate. Phosphorus is not very mobile in many soils and is much less likely to be lost
when applied conventionally than nitrogen. Phosphorus fertilizer injection may cause emitter plugging.
Solid precipitation in the line occurs most often due to interaction between the fertilizer and the irrigation
water. Most dry phosphorus fertilizers (including ammonium phosphate and superphosphates) cannot be
injected into irrigation water because they have low solubility. Monoammonium phosphate (MAP),
diammonium phosphate (DAP), monobasic potassium phosphate, phosphoric acid, urea phosphate,
liquid ammonium polyphosphate, and long chain linear polyphosphates are water soluble. However,
they can still have precipitation problems when injected into water with high calcium concentration.
Problems occur when the polyphosphate injection rates are too low to offset the buffering effects of the
calcium and magnesium concentrations in the irrigation water.
The application of ammonium polyphosphate fertilizers to water that is high in calcium will almost always
result in the formation of precipitants which can plug the emitters. These precipitants are very stable and
not easily dissolved. Phosphorus and calcium, when in solution together, may form di- and tri-calcium
phosphates, which are relatively insoluble compounds. Similarly, phosphorus and magnesium can form
magnesium phosphates which are also insoluble and plug emitters. When irrigation water contains the
high levels of calcium, iron, or bicarbonate, phosphorus should not be injected unless significant
precautions are taken.
Phosphoric acid is sometimes injected into micro irrigation systems. It not only provides phosphorus,
but also lowers the pH of the water, which can prevent the precipitation problems previously mentioned.
This practice will be effective as long as the pH of the fertilizer-irrigation water mixture remains low. As
the pH rises due to dilution, phosphates precipitate. One approach that is sometimes successful is to
supplement the phosphoric acid injections with sulfuric or urea sulfuric acid to assure that the irrigation
water pH will remain low (pH < 4.0). Phosphoric acid injection will be effective only as long as the pH
of the fertigated water remains very low. Combined Ca and Mg should remain below 50 mg/L and
bicarbonate should remain less than 150 mg/L.
Potassium
Potassium fertilizers are all water soluble and injection of K through micro irrigation systems has been
very successful. The problem most often associated with potassium injection is solid precipitants that
form in the mixing tank when potassium is mixed with other fertilizers. The potassium sources most
often used in micro irrigation systems are potassium chloride (KCl) and potassium nitrate (KNO3).
Potassium phosphates should not be injected into micro irrigation systems. Potassium sulfate is not very

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 33 of 41
soluble and may not dissolve in the irrigation water. Potassium thiosulfate (KTS) is compatible with urea
and ammonium polyphosphate solutions, however it should not be mixed with acids or acidified
fertilizers. When KTS is blended with urea ammonium nitrate (NH4NO3) solutions, a jar test is
recommended before mixing large quantities. Under certain mixing proportions, particularly when an
insufficient amount of water is used in the mix, potassium in KTS can combine with nitrates in the mix to
form potassium nitrate crystals. If this happens, adding more water and/or heating the solution should
bring the crystals back into solution.
Calcium
Fertilizers containing calcium should be flushed from all tanks, pumps, filters, and tubing prior to injecting
any phosphorus, urea-ammonium nitrate, or urea sulfuric fertilizer. The irrigation lines must be flushed to
remove all incompatible fertilizer products before a calcium-containing fertilizer solution is injected.
Calcium should not be injected with any sulfate form of fertilizer. It combines to create insoluble
gypsum.
Micronutrients
Several metal micronutrient forms are relatively insoluble, and therefore not used for fertigation
purposes. These include the carbonate, oxide, or hydroxide forms of zinc, manganese, copper, and
iron.
The sulfate form of copper, iron, manganese and zinc is the most common and usually the least
expensive source of micronutrients. These metal sulfates are water soluble and are easily injected.
However, using these materials for fertigation is not very successful in alleviating a micronutrient
deficiency since the metal ion has a strong electrical charge (2+) and becomes attracted to the cation
exchange sites of clay and organic matter particles where it tends to sit near the soil surface.
Consequently, the micronutrient usually does not reach the major plant root zone. If the soil pH is high,
manganese, iron, and copper are changed into unavailable forms and little or no benefit will be obtained
from their use, unless small volumes of the soil is acidified. If the metal sulfate solutions are acidified,
and the soil is acid and small portion of the soil volume is replaced by pear or volcanic tuff and acidified,
this application may be feasible (Horesh et al., 1986; Horesh et al., 1991).
The use iron chelates, Fe-EDDHA or Fe-EDDHMA is very common for alleviating lime-induced-iron
deficiency, care should be taken to apply chelates toward the end of the irrigation, and prevent
exposure of the chelate solution to light.
Fertigation Management
Care must be taken to ensure that injected materials do not react with dissolved solids in the irrigation
water in such a way to form precipitates or deposits in the irrigation system. The chemicals must be
soluble and remain in solution throughout the operating conditions of the irrigation system. The fertilizers
selected to be injected into the irrigation water need to be entirely soluble in water and should not react
with salts or chemicals in the water. Most nitrogen sources cause few clogging problems. The

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 34 of 41
exceptions are anhydrous ammonia, aqua ammonia, and ammonium phosphate, which increase the pH
of the water and cause precipitates with calcium and magnesium to form. Application of most forms of
phosphorous through the system can result in extensive clogging. However, phosphoric acid can be
safely injected in most waters since it acidifies the solution to a point where precipitation is prevented,
and may help dissolve lime deposits in the emitters. All of the common potassium fertilizers are readily
soluble and present no clogging problems.
Fertilizers can be highly corrosive and are a potential health hazard to skin and eyes. Therefore, all
system components including pumps, injection devices, lines, filters, and tanks should be inspected prior
to use. There should be a routine monitoring program of the fertigation process with particular emphasis
on the start-up and shutdown periods. Injection rates and times should be calibrated and rechecked
frequently to ensure proper operation of the system. Leaks, runoff, excess applications and application
to areas with open water should be prevented. All system components should be flushed with clean
water following each use.
When injecting fertilizers, the EC (electrical conductivity) of the irrigation water with the fertilizer in it
should be checked. Heavy dosages of fertilizers can cause leaf burn, even if relatively low salinity water
is used. It is generally preferable to inject small dosages of fertilizer frequently or continuously rather
than making fewer applications at a high rate.
It is essential that proper active backflow prevention devices be used in the irrigation system to prevent
fertilizers from being back-siphoned into the water supply, these devices are required by law in some
countries. The injection device itself should have a screen and check valve. It is recommended that
injection take place upstream of filters so that any contaminants or precipitates can be filtered out.
Fertigation rates and times should be calibrated for each area that is fertigated. Flushing time needs to
be at least as long as the travel time in the system from the injection point to the furthest emitter.
Fertilizer injections need to be at least this amount of time, and flush times need to exceed this travel
time so that nutrients will not remain in the lateral tubing and promote algal growth.
Interactions of Irrigation with Other Horticultural Practices
Fertilization
Proper water management is essential to achieve high uptake and utilization of applied fertilizers.
Research results have shown that if drought stress is a limiting factor, economic returns from fertilizer
applications will be reduced. Excess irrigation may result in leaching of nutrients, causing them to be
unavailable to trees, and may move nitrates into the groundwater. Insufficient irrigation may result in low
fertilizer uptake and lack of growth and yield resulting from drought stress. Fertigation (injecting water-
soluble fertilizers with the irrigation water) can be an economical and effective method of fertilizer
applications. With fertigation, small fertilizer amounts can be applied at frequent intervals or
continuously, thus making nutrients available to trees throughout the year.

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 35 of 41
Tree Density
Groves that have been planted at high tree densities have a higher per-area demand for soil moisture
during the first several years. As the trees mature and hedgerows are formed, per-area water
requirements for higher density plantings are similar to lower density plantings that have become
hedgerows. Irrigation practices may need to be modified for initial years in high-density plantings to
compensate for greater root densities compared to conventional tree spacings.
Rootstock/Scion
Large trees require more water than smaller trees. Due to their generally larger size and higher yields,
grapefruit trees typically require more water than sweet orange varieties. During the first several years,
trees budded on fast growing scions such as rough lemon will require more water per tree than trees on
slower growing rootstocks. Intentional drought stressing of lemon to induce summer flowering can
reduce the average water consumption of this cultivar, compared to other citrus cultivars. Therefore,
irrigation rate and timing should be adjusted based on rootstock, scion, and tree planting density.
Some rootstocks (such as rough lemon and Rangpur lime) have an efficient and abundant fibrous root
systems that allow them to extract water more readily from the soil than less efficient root systems such
as those of sweet orange. In addition, the lemon rootstocks (Milam, Volkameriana, rough lemon) and
sour orange are most tolerant to flooding (not to be confused with their susceptibility to Phytophthora
root and foot rot) while Cleopatra mandarin, Carrizo citrange, and Citrus macrophylla are least tolerant.
Consideration should be given to the adequacy of drainage before selecting the rootstock on which the
trees will be planted.
Weed Control
An effective weed control program is essential for groves watered with microirrigation systems. Weeds
and high grasses disrupt the wetting pattern of microsprinklers and lower the use efficiency of applied
water. Weeds will waste water, increase irrigation system maintenance costs and reduce the
effectiveness of fertigation and chemigation applications. Excessive vegetation control in bedded groves
(wide herbicide bands) will result in erosion of soil from bed tops and reduce the effectiveness of water
removal through water furrows and swales.
Of special consideration is drip irrigation in soils that suffer from wind erosion, excessive cultivation
increases wind erosion, and the wind blown soil may collects in the wet zone, essentially burying the
dripper lines.
Water is important in the activation of the soil residual compounds in herbicides. Irrigation may be
needed to move the applied materials into the zone of germinating weed seeds or into the root zone of
actively growing weeds. Too much irrigation, however, may leach the herbicide beyond the weed’s
root zone, resulting in ineffective weed control.

Water Management in Citrus- July 14, 1990 Draft . . . . . . . . . . . . . . page 36 of 41
Disease/Insect Control
Without adequate disease and pest control, even proper and efficient irrigation may not improve
profitability. Some nematacides and insecticides can be applied efficiently through the irrigation system
(chemigation) providing a low-cost application method. In humid areas, adequate drainage is essential
to ensure that sprays for mites, insects, and fungal diseases can be applied at appropriate times. Without
efficient drainage, application of pesticides can be difficult or impossible during rainy periods.

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Figure 1. Severely water-damaged (top) and healthy (bottom) citrus root
systems.

Figure 2. Schematic of completed water table
observation well.

Figure 3. Components of citrus water budget for microirrigated citrus with deep,
unrestricted rooting.
Figure 4. Components of water budget for citrus with restricted rooting due to
shallow water table.

Figure 5. Flood (seepage) irrigation of bedded citrus in Florida.
Figure 6. Overhead irrigation on mature trees.

Figure 8. ¼-circle microsprinklers on young trees.
Figure 7. Portable volume gun irrigation.

Figure 10. Drip line on orange tree.
Figure 10. Double drip lines on young citrus trees.

Figure 12. Ice on young citrus as a result of
microsprinkler irrigation for freeze protection.
Figure 11. Leaf bronzing caused by excess salinity.