271A.K. Srivastava (ed.), Advances in Citrus Nutrition, DOI 10.1007/978-94-007-4171-3_19, © Springer Science+Business Media B.V. 2012

19.1 Introduction

Production systems that combine grove design and irrigation management to increase yield and grove operational ef fi ciency have been studied in many citrus-producing regions of the world and have many economic advantages (Roka et al. 2009 ) . These production systems together with

Abstract

Advanced citrus production systems that combine grove design, size limiting rootstocks, irrigation and nutrient management, and mechanical harvesting have the potential to make citrus more ef fi cient and economically competitive. Recently, the open hydroponic system (OHS) of citrus production has been combined with orchard design to achieve these ef fi ciencies. Open hydroponics are de fi ned as a system of management practices aimed at increased productivity of citrus orchards by continuously applying a balanced nutrient mix-ture through the irrigation system, limiting the root zone by restricting the number of drip-pers per tree and maintaining the soil moisture in the rooted zone near fi eld capacity. Concepts of OHS are to maximize water and nutrient use ef fi ciency through improved nutrient availability to concentrate roots in the irrigated zone. These concepts are accom-plished through intensive water and nutrient management and results in increased early growth, sustained yields, and reduced nutrient leaching. Additional horticultural principles employed include higher tree density with size-controlling rootstocks grown on soil ridges, if needed, for improved drainage. Limited published information is available for citrus grown under OHS conditions. However, one study reported that orchards on OHS have outgrown trees on conventional production systems and appear to be more productive. Canopy volumes increased by approximately three times and fruit volume by more than fi ve times per unit of N after 4 years of intensive management compared with conventionally grown trees in replicated trials. Yield increases approaching 30% have been reported from several studies in many citrus-producing regions of the world. Use of this advanced produc-tion system may maintain higher levels of productivity through improved water and nutrient use ef fi ciencies resulting in improved short- and long-term economic returns, particularly in citrus industries infected with diseases such as Citrus Greening.

Keywords

Rootstock • Size limiting • Intensive irrigation and nutrient management • Root zone • Nutrient use ef fi ciency

Open Field Hydroponics: Concept and Application

Kelly T. Morgan and Davie Kadyampakemi

19

K. T. Morgan (*) • D. Kadyampakemi Soil and Water Science Department, Southwest Florida Research and Education Center , University of Florida , 2685 State Road 29 North , Immokalee , FL 34142 , USA e-mail: conserv@u fl .edu ; dkadyampakemi@gmail.com

272 K.T. Morgan and D. Kadyampakemi

mechanical harvesting have the potential to make citrus production more ef fi cient and economically competitive. The two basic grove production components are the Advanced Production System (APS) and the Open Hydroponic System (OHS) (Stover et al. 2008 ) . The fundamental concepts of APS/OHS for citrus production combine high tree density with intensive management to optimize tree performance. These concepts are designed to more fully and ef fi ciently exploit a citrus tree’s potential by providing optimal water and nutrient conditions. Those improvements are expressed in maximizing water and nutrient use ef fi ciency and concen-trating root within irrigation zones which should lead to less nutrient leaching. The OHS is an integrated system of prac-tices, including irrigation, and nutrition practices that was developed in Spain in the early 1990s to contend with gravel based soils and the problem of low fertility ( Martinez-Fuentes et al. 2004 ; Falivene et al. 2005 ) . Tight control over water and nutrient-mediated plant growth and development by the OHS using irrigation to train the root system into a limited area and fertigates with daily requirements of nutri-ents (Stover et al. 2008 ) .

Falivene ( 2005 ) de fi ned the OHS as a system of manage-ment practices aimed at increased productivity of citrus orchards by continuously applying a balanced nutrient mix-ture through the irrigation system, limiting the root zone by restricting the number of drippers per tree, and maintaining the soil moisture near fi eld capacity in a limited wetted zones. The combination of these practices is claimed to provide a greater control and manipulation of nutrient uptake at speci fi c physiological stages and improved water uptake (Yandilla 2004 ) . Many concepts associated with OHS has been suc-cessfully used in the production of peaches, almonds, grapes, citrus, avocados, and several vegetable crops in Spain, South Africa, Chile, Argentina, Morocco, and California (USA). In South Africa, commercial growers have adapted the OHS through use of drip fertigation on a daily basis during day-light hours (Pijl 2001 ; Schoeman 2002 ) resulting in increased citrus yield and fruit size (Kruger et al. 2000a, b ; Kuperus et al. 2002 ) . OHS principles were introduced in Australia as an intensive fertigation practice in citrus orchards (Falivene et al. 2005 ) but are somewhat less intensive than the original OHS developed in Spain to better meet the conditions and needs of the Australian industry. Thus, OHS has been modi fi ed to meet local cultural, weather, and soil conditions. In Florida, OHS-related management practices are being applied to higher density citrus plantings. The combination of OHS with high tree density is being promoted under the name APSs (Stover et al. 2008 ; Morgan et al. 2009b ; Roka et al. 2009 ) . The use of mechanical harvesting in these high-density low canopy volume orchards will not be a subject of this review, but contributes to the overall ef fi ciency of the production system (Roka et al. 2009 ) .

19.2 Open Hydroponic System Concepts

The goals of OHS are to (1) increase initial tree growth rate, (2) establish early sustained fruit production, (3) maximize ef fi ciency of production inputs, and (4) improve return on investment to achieve pro fi ts in as short a period of time as possible. To accomplish these goals within the framework of environmental needs, four fertigation concepts have been developed: (1) maximization of water and nutrient use ef fi ciency, (2) improvement in nutrient availability, (3) con-centration of roots in the irrigated zone, and (4) reduction in nutrient leaching. These goals and concepts have been incor-porated in the APS that utilizes OHS along with tree planting density, tree size control, and horticultural manipulation (pruning and girdling).

19.2.1 Maximize Water and Nutrient Use Ef fi ciency

Several studies conducted over many years have revealed that it is possible to increase yields and ef fi ciency of water use and nutrient use through water-saving irrigation meth-ods. In a study on water use ef fi ciency and nutrient uptake on low volume irrigated citrus in New South Wales in Australia on Tiltao sand, Falivene ( 2005 ) found that water uptake was limited by water availability rather than root density. Soils maintained at drained upper limit ( fi eld capacity) in the root zone resulted signi fi cantly in greater tree water use. Also, fertilizer injection with the microsprinkler system signi fi cantly increased the ef fi ciency of N and P uptake com-pared with surface application, whereas leaf K levels were lower under low-volume irrigation. Multiple applications of N in relatively small amounts with drip irrigation resulted in lower soil residual mineral N concentrations and enhanced N uptake ef fi ciency by the citrus roots (Alva and Paramasivam 1998 ; Paramasivan et al. 2001 ).

Alva and Paramasimam ( 1998 ) found groundwater N concentrations below citrus trees fertilized with soluble granular fertilizers ranged from 6.5 to 16.5 mg L −1 compared with groundwater N concentrations ranging from 1.4 to 6.9 mg L −1 below fertigated trees. In an investigation of soil N solutions below fertigated citrus trees, Paramasivan et al. ( 2001 ) found NO

3 -N concentrations occasionally peaked at

12–100 mg L −1 at depths above 120 cm below the soil sur-face, but NO

3 -N concentrations mostly remained below

10 mg L −1 at 240 cm. These studies indicate that, with proper irrigation scheduling, soil N can be maintained in citrus tree root zones and not leached to groundwater. In a 3-year study, Bryla et al. ( 2005 ) found that peach trees irrigated by surface and subsurface drip produced 19% higher prune weights and

27319 Open Field Hydroponics: Concept and Application

up to 24% greater marketable yield than those irrigated by microjets and furrow irrigation on a Hanford fi ne sandy loam in California. Drip irrigation systems, in particular, are known to improve irrigation and fertilizer use ef fi ciency because water and nutrients are applied directly to the root zone (Camp 1998 ) . The bene fi ts of frequent fertigation and/or irrigations in achieving high water and nutrient use ef fi ciency offered by drip irrigation can be negated by improper water placement as shown with total leaf area, and fi brous root length were reduced by 44–55% in grapefruits (Zekri and Parsons 1988 ) . Therefore, careful placement of water in the root zone is important in fruit production to ensure that water and nutrient uptake are optimized.

19.2.2 Improved Nutrient Availability

Several fundamental aspects of citrus physiology and cultural management must be taken into consideration when imple-menting OHS. Elements to be considered include fertilizer requirements and irrigation scheduling, and have been researched using many tree sizes and soil conditions. The following section reviews reports to help guide us in inter-preting likely responses of citrus to OHS. Highly intensive nutrition program of OHS with the goal of rapid tree growth and productivity typically results in a higher level of vigor. Considering current best management practices (BMPs) and potential improvements in nutrient use ef fi ciency, a goal for successful adoption of OHS will have to be more ef fi cient use of applied nutrients. To determine whether this is feasi-ble, we must review studies on fertilization practices on cit-rus, which sometimes appear confusing and contradictory, due to different tree ages, soil types and application practices compared. Obreza and Rouse ( 1993 ) showed that an increase in fertilizer rate from 0.32 to 0.64 kg N tree −1 in the third year after planting resulted in a decrease in total soluble solids concentration and soluble solids to acid ratio. Koo and Smajstrla ( 1984 ) made similar observations with annual N rates greater than 224 kg ha −1 using trickle irrigation and fer-tigation on 26-year-old “Valencia” orange on an Astatula fi ne sand in Florida. Furthermore, Koo ( 1980 ) , in trials on sandy soil, found no signi fi cant differences due to fertigation frequencies (3 or 10 times a year) on 13-year-old “Valencia” orange. Similarly, Syversten and Jifon ( 2001 ) studied ferti-gation in 6-year-old “Hamlin” oranges in Florida at 12, 37, and 80 times per year and found that fertigation frequency did not affect leaf nutrient concentration, canopy size, fruit yield, or juice quality.

Morgan et al. ( 2009a ) examined the effect of N fertilizer rates and methods of applying N on growth and productivity of young (3–5 years old) and maturing (8–10 years old) citrus trees on well-drained sandy Entisols of central Florida. They

observed that in young trees controlled release fertilizer applied once a year and fertigation 30 times annually pro-duced higher average yields (20.67 Mg ha −1 ) and larger trees (7.9 m 3 tree −1 ) compared with fertigation or dry granular fer-tilizer applied four times annually (18.77 Mg ha −1 and 7.5 m 3 tree −1 ). In maturing trees, however, the dry granular fertilizer applied 4 times a year and fertigation 30 times annually produced similar yields (52.0 and 53.7 Mg ha −1 , respectively) and total soluble solids (9.14 and 9.15°Brix, respectively). However, canopy volumes for the same trees were signi fi cantly greater with fertigation treatment (25.0 m 3 tree −1 ) compared with the dry granular fertilizer (23.2 m 3 tree −1 ). They observed that increased number of split applications would likely promote tree growth, albeit, little increase in fruit yield may be obtained in mature citrus.

Alva et al. ( 2003 ) found that total annual N applications could be reduced by >10% by applying approximately half the annual N (224 kg ha -1 ) in three foliar applications and the remaining half as fertigations with no signi fi cant reduction in yield. Thus, they proposed a combined use of foliar fertilizer application and fertigation as a BMP for N because these were effective in reducing nitrate leaching to sur fi cial ground-water. Nevertheless, even the most intensive practices in the studies reviewed provide many fewer seasonal applications of fertilizer than a typical OHS in which three or more fertiga-tion events per day are standard (Falivene et al. 2005 ) . Proper irrigation system design is important in OHS to ensure that the system does not leak and/or fail at some point or over-irrigation causing nutrient leaching. There are two main types of irrigation scheduling programs in OHS: pulsing irrigation and continuous (Falivene et al. 2005 ) . A pulsing irrigation management program involves short pulses (e.g., 20–30 min each) of irrigation provided to the trees at a number of times throughout the day while a continuous irrigation management program uses low output rates to match water use conditions in summer. The number and timings of pulses are based on a calculation of readily available water and average tree water use along with monitoring of irrigation scheduling devices like tensiometers, capacitance probes, and trunk diameter measuring devices. In a restricted root zone situation, up to nine or more pulses of irrigation needed to be scheduled throughout the day in summer (Falivene et al. 2005 ) .

19.2.3 Concentrate Roots in Irrigated Zone

Michelakis et al. ( 1993 ) , studying avocado water use in a Mediterranean climate in Greece under drip irrigation, found that root density was generally higher in the upper 50-cm soil layers and within 2 m from the drip line, with about 70% of the roots located in this region. They attributed the higher root percentage in the upper soil layers to biological factors

274 K.T. Morgan and D. Kadyampakemi

and to the higher oxygen diffusion rate. In the study, Michelakis et al. ( 1993 ) applied irrigation water to each treatment using one drip lateral per row of trees with drippers of 4 L h −1 discharge rate placed 70 cm apart. Coleman ( 2007 ) also observed that root length density in cottonwood, American sycamore, sweet gum, and loblolly pine was dependent upon depth and position relative to drip emitter when fertilizers were applied and is greatest at the surface and in proximity to the drip line. The factors controlling root length density in the woody species studied included age, depth, and proximity to the drip emitter. The principles underlying the restriction of the roots to the wetted zone using drip irrigation are also applicable to OHS.

19.2.4 Intensive Water and Nutrient Management

Kalmar and Lahar ( 1977 ) irrigated avocados with sprinklers at 7, 14, 21, and 28-day intervals on a grumusol with more than 60% clay from the soil surface to a depth of 150 cm and found that most water was absorbed from upper 60-cm soil layer, suggesting that this was where greater than 70% of roots were concentrated. Bryla et al. ( 2005 ) compared the effect of furrow and microsprinkler irrigation scheduled weekly or biweekly and surface and subsurface drip irriga-tion scheduled daily on production and fruit quality of peach on a Hanford fi ne sandy loam in California. They found that daily drip irrigations maintained signi fi cantly higher soil water content through the season, with signi fi cant reduction in soil water stress (−1.4 MPa average midday stem potential between drip irrigation events compared with −1.8 MPa between furrow irrigations). As a result of lower plant water status, higher marketable yields with larger fruits were pro-duced. Schoeman ( 2002 ) also made similar observations in citrus using daily drip fertigation in South Africa. Schumann et al. ( 2003 ) compared fertilizers sources and rates in “Hamlin” orange on a Candler fi ne sand in Florida. The results showed that optimal soluble solids production for fer-tigation was obtained at a N level of 145 kg ha −1 while the N optima for dry granular and controlled release fertilizers were 180 and 190 kg ha −1 , respectively. The greater ef fi ciency of fertigation amounted to a N saving of 35–45 kg ha −1 year −1 and approximately 20% more soluble solids yield than the other fertilizer sources. Also, leaf N concentrations were signi fi cantly higher per unit of N applied for fertigation > dry granular > controlled release. Thus, Schumann et al. ( 2003 ) concluded that fertigation was the most ef fi cient fertilizer source because of optimal placement in the root zone and optimal temporal distribution over the season.

Morgan et al. ( 2009a ) made similar observations in young citrus trees on the sandy soils of central Florida. In theory,

the growth and yield of citrus trees should be maximized if the demand for nutrients and water by the roots is always matched with an adequate supply from drip fertigation, thus avoiding even transient de fi ciencies. The daily timing for both water and nutrient delivery should coincide with the time of maximum transpiration fl ow, which is during the daylight hours. Thus, the common strategy in OHS systems worldwide is to pulse-fertigate during daytime hours and hopefully attain a high percentage of immediate uptake by the roots instead of temporarily storing the water and nutri-ents in the soil as with conventional fertilization and irriga-tion systems. Storage of water and nutrients in the soil before uptake by roots will increase losses and inef fi ciencies due to evaporation, leaching, adsorption, precipitation, and volatil-ization mechanisms as well as immobilization by microbes. All those processes are kinetically regulated, thus minimizing the duration of soil contact is one of the underlying princi-ples of hydroponics.

Studies on tree root density distribution have been in done in Florida and other parts of the world. Castle and Krezdorn ( 1975 ) described two general types of citrus root systems, one characterized by extensive lateral and vertical development and the other by intensive higher fi brous root density near the soil surface. Trees on rough lemon ( C. jambhiri Lush), Volkamer lemon ( C. volkameriana Pasquale), and Palestine sweet lime ( C. limettioides Tan.) rootstocks are typical of cit-rus trees with extensive root structure where 50% of the fi brous roots were at soil depths > 0.7 m. Large, high-yielding trees with extensive root systems dominated the citrus indus-try in Florida when trees were irrigated less intensively and planted at much lower densities. Unfortunately, rough lemon has been virtually eliminated as a commercial rootstock due to citrus blight disease (unknown etiology) in the 1970s and 1980s (Castle 1980 ) . Carrizo citrange and Swingle citrumelo are examples of the intensive-type root systems with few fi brous roots below 0.7 m and less lateral development (Castle and Krezdorn 1975 ) . These rootstocks now dominate the Florida citrus industry and are well suited for high-density, intensively managed plantings (Castle 1978 ) . Morgan et al. ( 2007 ) found that average fi brous root length density (FRLD) decreased from 1.11 cm cm −3 at the soil surface to 0.27 cm cm −3 at 90 cm depth and decreased from 0.52 cm cm −3 near the tree trunk to 0.16 cm cm −3 at greater than 175 cm, resulting in mature trees with bimodal root systems. Also, the FRLD varied as a function of rootstock in which trees on Swingle citrumelo developed higher FRLD near the soil surface (1.39 cm cm −3 ) than trees on Carrizo citrange (0.84 cm cm −3 ) with similar FRLD below 0.3 m. Abrisqueta et al. ( 2008 ) studied root dynamics of young peach found that partial root zone drying and continuous de fi cit irrigation in Spain reduced root density in the nonirrigated zones by 42% and 73%, respectively. In the study, higher root length densities were

27519 Open Field Hydroponics: Concept and Application

recorded in non-limiting irrigation conditions than under de fi cit irrigation where root growth was reduced.

The use of OHS can limit root growth to within the irri-gated zone. Research studies into restricted root zones using physical constraints have shown a reduction in yield in fruit and vegetables (Ismail and Noor 1996 ; Bar-Yosef et al. 1988 ; Boland et al. 2000 ) . These studies attributed the yield reduc-tion to reduced canopy growth. Reduced canopy growth or a reduction in yield per tree has not been observed to date in OHS (Boland et al. 2000 ; Falivene 2005 ) . The wetted soil volume in OHS is considerably greater than the restricted root zone studies mentioned above where signi fi cant reduc-tions in vegetative growth and yield have been reported (Falivene 2005 ) . The study by Boland et al. ( 2000 ) on peach in Australia showed a signi fi cant reduction in growth and yield when the root zone was restricted to 3% of its potential. In contrast, the wetted soil volume in OHS is approximately 8–15% of the potential root volume (Falivene 2005 ) . These studies envisage that in an OHS situation, the roots are redi-rected to grow more densely in a smaller volume of soil, but the soil volume is suf fi ciently large enough to support active root growth and a productive tree.

19.2.5 Reduced Nutrient Leaching

Many researchers have attempted to study nutrient leaching to sustain environmental quality (Warrick 1986 ). Paramasivan et al. ( 2001 ) found that nitrate-nitrogen leaching losses below the rooting depth accounted for 1–16% of applied fertil-izer N and increased with increasing rate of N application (112–280 N ha −1 year −1 ) and the amount of water drained. Paramasivan et al. ( 2001 ) also noted that the leached nitrate-nitrogen at 240 cm remained well below the maximum con-taminant limit of 10 mg L −1 . They ascribed their observations to careful irrigation management, split fertilizer applications, and proper timing of the application. Thus, it should be pos-sible to reduce nutrient leaching with an OHS, because in both scenarios, water and nutrients are applied in quantities approx-imating plant needs and close to the plant with less waste and at speci fi c physiological stages of the plants (Mason 1990 ) .

19.3 Principles Used in Advanced Production Systems

Certain principles of irrigation, nutrient, and horticultural man-agement must be followed in a systematic approach to achieve the goals of OHS. The principles of production used in APS are currently being followed by some citrus growers, but require some modi fi cations and more intensive management. The prin-ciples added to OHS to develop the APS are (1) higher tree

densities, (2) size-controlling rootstock selection, (3) restricting root zones with drip irrigation, (4) intensive irrigation and nutrition management, and (5) horticultural manipulation.

19.3.1 Higher Tree Density (>725 trees ha −1 )

The ideal grove is one in which there is dense planting of rapidly developing trees to bearing volume with suf fi cient bearing volume to support high levels of cropping (Fig. 19.1 ). The rapid growth of these trees and fi nal size are critical to improvement in water and nutrient use ef fi ciency. The use of planting ridges is likewise important, particularly in loam or clay loam soils (Fig. 19.2 ). The ridges allow for proper water

Fig. 19.1 Densely spaced citrus trees grown on ridges. Trees grow into hedgerows within 3 years to maximize early yield (Photo by K. T. Morgan)

Fig. 19.2 Trees planted on ridges allow for good water drainage and air in fi ltration (Photo by K. T. Morgan)

276 K.T. Morgan and D. Kadyampakemi

drainage and air in fi ltration to maintain aerobic conditions in the drip irrigation zones. Such groves provide certain known advantages related to production, harvesting, and returns, but to be successful, smaller-sized, closely planted trees are essential (Roka et al. 2009 ) . Changes in orchard design have occurred primarily in the deciduous fruit industries. Robinson et al. ( 2007 ) published results of planting densities ranging from 850 to 5,445 trees ha −1 for apple orchards in New York. They found that the optimum economic density was between 2,500 and 3,000 trees ha −1 . The optimum density achieved improved yield by >20% and quality coupled with lower costs of production. The practices and concepts that constitute the OHS are an excellent match with higher planting densities.

With the advent of the OHS for citrus, some data have dem-onstrated the performance of groves of closely spaced citrus trees have been managed with the OHS. Yields of Nova, Marisol, and Delite mandarins in Spain, planted at higher den-sity (1,012 trees ha −1 ) and grown using the OHS, were about 65–75 tons ha −1 in the sixth year which is higher than for a conventional orchard using low- to medium-density plantings (375–575 trees ha −1 ) (Martinez-Fuentes et al. 2004 ; Falivene et al. 2005 ) . In Florida, higher planting density (889 trees ha −1 ) produced higher >33% early (4–8 years after planting) pro-duction compared with lower tree densities (370 trees ha −1 ) (Wheaton et al. 1995 ; Parsons and Wheaton 2009 ) . However, average annual fruit yields for the same high-density plantings were similar in at 9–13 years after planting when trees were maintained at a height of 5.5 m. When the trees were main-tained at a 3.5 m height, the average annual yields were reduced by 50–60%. Those studies demonstrated the feasibility of higher density plantings for citrus, but the trials were con-ducted under lower tree planting densities than proposed for the future with OHS and under less-intensive management. Thus, there is a possibility of further increasing yield per unit area using OHS with densely planted citrus trees.

19.3.2 Size-Controlling Rootstock Selection

Rootstock selection along with tree planting density is a key element in the APS/OHS approach to the future. Citrus trees, like humans, need a certain amount of space to develop and fl ourish. When the allocated space is fi xed, e.g., 1 ha of land, tree size becomes critical because the productive unit is the canopy and only a certain volume of canopy can be grown per unit land area. Vigorous, large trees are neither compatible with close spacing nor productive in their younger years (Fig. 19.3 ). Thus, in a world of economic necessity dictated by early and robust returns, small, closely spaced trees become a required component of the new production concepts (Stover et al. 2008 ; Morgan et al. 2009b ) . When that combination is achieved, the higher density grove will outperform the more conventional one especially in the early years.

19.3.3 Restricted Rootzones and Intensive Irrigation and Nutrient Management

Management of water and nutrients in the root zone of citrus is critical to establishing rapid growth and early sus-tained fruiting (Morgan et al. 2009b ) . Root density can be restricted to the wetted zone of each drip emitter (Fig. 19.4 ). Restriction of the roots in this manner allows for the soil surrounding the roots to be maintained at nearly fi eld capac-ity. If the intensive management is provided, by pulsing drip applications at regular intervals on a daily basis, nutri-ents within the root zone will not be leached by excessive irrigation. The fertigation system must contain four basic

Fig. 19.3 The size of vigorous, rapidly growing trees must be con-trolled by hedge if nutrition is not altered to reduce vegetative growth (Photo by K. T. Morgan)

Fig. 19.4 The roots can clearly be seen beneath these citrus trees. The irrigation zone of each dripper is outlined in white delineating the high-est root density with lower root densities outside these zones and nearer the soil surface (Photo by K. T. Morgan)

27719 Open Field Hydroponics: Concept and Application

parts of (1) fi ltration, (2) fertilizer mixing and/or storage, (3) fertilizer injection, and (4) fertigation control. Water fi ltration depends on the water quality and water source. For surface water, the system must contain a sand media fi lter adequately sized to remove biological materials as well as particulates depending on water quality. If the water source is a well, a screen or disk fi lter (Fig. 19.5 ) with an opening size of 0.05 mm or less is required. Fertilizers can be stored dry and mixed as needed in multiple tanks depend-ing on requirements of the system (Fig. 19.6 ). A minimum of two tanks are recommended for best results. One tank would contain the nitrogen, potassium, and phosphorus components with the additional tanks containing desired

Mg, Ca, and micronutrients. A manifold of valves from each fertilizer storage tank to the injection device must be provided (Fig. 19.7 ). The fi nal element of the fertigation system is a controller or custom computer operating system depending on the size of the system to be used. The control-ler must be adequate to operate multiple injections with multiple drip pulses per day.

19.4 Horticultural Manipulation

High early production is essential for higher-density, shorter-cycle citrus production to be economically sound (Roka et al. 2009 ) . Early cropping not only front-loads eco-nomic returns but also importantly competes with vegeta-tive growth and helps keep trees smaller (Erner 1988 ; Takahara et al. 1980 ) . Several horticultural practices to enhance early cropping have been explored and documented in citrus and other fruit crops, and many have been widely used in recent high-density citrus plantings in South Africa and likely other regions where intensive plantings have been utilized (Perez-Madrid et al. 2005 ) . Not all scion/rootstock combinations will require horticultural intervention to accel-erate cropping. Increasing citrus production usually means that the trees are forced to break juvenility and begin repro-ductive growth earlier than would naturally occur (He 1997 ) . Trees can be manually manipulated using various horticul-tural techniques such as pruning and girdling to improve fruit set, yield, and quality (Fig. 19.7 ). These techniques have perhaps even greater potential to enhance fruit quality and also increase yield when applied to more easily managed small trees planted close together (APS) and intensively

Fig. 19.5 Adequate fi ltration must be provided for the drip systems. This photo illustrates a manifold of sand fi lters from a surface water source (Photo by K. T. Morgan)

Fig. 19.6 Fertilizers can be custom blended using dry materials and multiple storage tanks connected to the fertigation system (Photo by K. T. Morgan)

Fig. 19.7 Intensive management of fertigation demands the use of many injection valves with fi lters from the fertilizer supply tanks (Photo by K. T. Morgan)

278 K.T. Morgan and D. Kadyampakemi

managed using the OHS approach. Stover et al. ( 2008 ) suggested that use of APS/OHS systems should help control vegetative growth, keeping trees in check and reducing cost of pruning while also providing earlier cash fl ow (Roka et al. 2009 ) .

The effects of girdling (Fig. 19.8 ) on crop performance depend on when it is done. Girdling in autumn enhances fl owering in citrus (Goldschimidt and Colomb 1982 ) , at full bloom improves fruit set in oranges (Monselise et al. 1972 ) , and in summer, girdling increases grapefruit size by >10% (Fishler et al. 1983 ) . In other cases, girdling was reported to limit nitrogen, phosphate, and calcium uptake in avocado in South Africa (Davie et al. 1995 ) and increased average fruit weight by 0.8 g but reduce the average soluble solids con-centration at harvest by 0.8°Bx in grapes in California (Harrell and Williams 1987 ) . However, Andrews et al. ( 1978 ) reported that girdling of peach trees in Florida increased fi rst harvest fruit yield by an average of 45%, enhanced ripening by 3–5 days, but resulted in severe necro-sis of leaves and gumming on the area of the cut. The prun-ing and girdling practices need to be carefully considered for use in high-density citrus plantings because bene fi ts will need to be substantial to justify the high labor costs associ-ated with these practices.

19.5 Results of OHS Applications

Limited information is available for citrus grown under OHS conditions. Kruger et al. ( 2000a, b ) found 19% yield increases in Clementine mandarin for drip-irrigated citrus using OHS (47.86 Mg ha −1 ) and microsprinkler-irrigated (40.22 Mg ha −1 ) blocks with soluble fertilizer in a

non-replicated OHS demonstration in South Africa. Similar results (25% increase) were found for drip-irrigated OHS-produced Midknight Valencia (55.3 Mg ha −1 ) and microsprin-kler-irrigated control (44.0 Mg ha −1 ). Yield increases were contributed to a 27% increase in fruit number, but fruit size was not reduced. Likewise, Martinez-Valero ( 2004 ) reported third year and cumulative yields of 8.5 and 206.8, and 17.6 and 240.6 Mg ha −1 for OHS 6-year-old Nova hybrid manda-rin ( C. reticulate Blanco × Tangelo Orlando) and Marisol clementine ( C. reticulata Blanco), respectively. No com-parisons of treatments with common growing conditions were available. A study currently being conducted in Florida is comparing the effects of drip irrigation using OHS nutri-ent management with small-area and large-area microsprin-kler irrigation systems on Hamlin and Valencia orange growth and production at three tree densities. Preliminary data after 4 years indicate that the OHS system improves growth (measured by increase in canopy volume) when drip-grown trees are compared with small-area and large-area microsprinkler-irrigated trees, respectively (Table 19.1 ; Morgan unpublished data). Yields in the same study increased by nearly 1.9 and 5.0 times when drip OHS is compared with small-area and large-area microsprinkler. During this study, water and fertilizer amounts were decreased by 20–50% using drip fertigation compared with microsprinkler.

19.6 Future Research

Key aspects of OHS include appropriate size-limiting root-stocks for selected scions grown at higher tree densities under soil and environmental conditions prevalent in each production area. This will require intensive long-term research in each production area of the world to determine the appropriate rootstock/scion combination for a range of in-row and between-row spacing. Determining this combi-nation on a region-by-region basis is critical. Likewise, the range of tree densities depends on the fi eld equipment used. Using large equipment currently available for wider-spaced trees will not be appropriate for closer-spaced trees. Thus, smaller equipment must be developed, and equipment that can spray, hedge, and harvest multiple rows simultaneously must also be considered. Such equipment will operate over the row and are available for many crops but not currently for citrus. While drip irrigation equipment is commonly used for citrus in many areas, microsprinkler irrigation is used in many areas, including Florida and Texas, for frost protection. Adaptation of OHS to use these microsprinkler systems will be necessary, as a dual drip fertigation and microsprinkler frost protection system is considered too expensive.

Fig. 19.8 This tree has been girdled each year for 3 years to force the tree into early reproductive growth (Photo by K. T. Morgan)

27919 Open Field Hydroponics: Concept and Application

19.7 Conclusion

Intensive production systems to improve citrus production ef fi ciencies will involve planting densities of 750 or more trees per ha on rootstocks that match fi nal tree size to soil characteristics and planting density. Tree irrigation and nutri-tion using this new system will be linked through the use of management systems that will apply the appropriate ratio of nutrients to roots concentrated within the irrigated zone. The production system adopted by citrus producers should be operated in a manner that maintains current nutrient and water quality standards. Finally, the system will rely on selective horticultural manipulation of tree growth and fruiting through mechanical pruning and girdling as needed. This combined system of production will result in higher young tree growth rates, earlier fruit production, and may maintain high levels of productivity compared with current cultural practices espe-cially in the presence of tree losses due to Citrus Greening.

References

Abrisqueta JM, Mounzer O, Álvareza S et al (2008) Root dynamics of peach trees submitted to partial rootzone drying and continuous de fi cit irrigation. Agric Water Manage 95:959–967

Alva AK, Paramasivam S (1998) An evaluation of nutrient removal by citrus fruits. Proc Fla State Hortic Soc 111:126–128

Alva AK, Paramasivam S, Graham WD et al (2003) Best nitrogen and irrigation management practices for citrus production in sandy soils. Water Air Soil Pollut 143:139–154

Andrews CP, Sherman WB, Sharpe RH (1978) Response of peach and nectarine cultivars to girdling. Proc Fla State Hortic Soc 91:175–177

Bar-Yosef B, Schwartz S, Markovich T et al (1988) Effect of root volume and nitrate solution concentration on growth, fruit yield and temporal N and water uptake rates by apple trees. Plant Soil 107:49–56

Boland AM, Jerie PH, Mitchell PD et al (2000) Long-term effects of restricted root volume and regulated de fi cit irrigation on Peach: I. Growth and mineral nutrition. J Am Soc Hortic Sci 125(1):135–142

Bryla DR, Dickson E, Shenk R et al (2005) In fl uence of irrigation method and scheduling on patterns of soil and tree water status and its relation to yield and fruit quality in peach. HortScience 40(7):2118–2124

Camp CR (1998) Subsurface drip irrigation: a review. Trans ASAE 41:1353–1367

Castle WS (1978) Citrus root systems: their structure, function, growth, and relationship to tree performance. In: Cary PR (ed) Proceedings of the international society for citriculture, Grif fi th, NSW, Australia, vol 1, pp 62–69

Castle WS (1980) Fibrous root distribution of ‘Pineapple’ orange trees on rough lemon rootstock at three tree spacings. J Am Soc Hortic Sci 105(3):478–480

Castle WS, Krezdorn AH (1975) Effect of citrus rootstocks on root distribution and leaf mineral content of ‘Orlando’ tangelo tree. J Am Soc Hortic Sci 100(1):1–4

Coleman M (2007) Spatial and temporal patterns of root distribution in developing stands of four woody crop species grown with drip irri-gation and fertilization. Plant Soil 299:195–213

Davie SJ, Stassen PJC, van der Walt M (1995) Girdling for increased “hass” fruit size and its effect on carbohydrate production and storage. Proc World Avocado Congr 3:25–28

Erner Y (1988) Effects of girdling on the differentiation of in fl orescence types and fruit set in ‘Shamouti’ orange trees. Israel J Bot 37:173–180

Falivene S (2005) Open hydroponics: risks and opportunities. Land and Water Australia through the National Program of Sustainable Irrigation. http://www.arapahocitrus.com/ fi les/OHS_Stage_Publications.pdf

Falivene S, Goodwin I, Williams D et al (2005) Open hydroponics: risks and opportunities. stage 1: general principles and literature review. NSW Department of Primary Industries and Land and Water Australia, Brisbane NSW, Australia

Fishler M, Goldschimidt EE, Monselise SP (1983) Leaf area and fruit size in girdled grapefruit branches. J Am Soc Hortic Sci 108:218–221

Goldschimidt EE, Colomb A (1982) The carbohydrate balance of alter-nate bearing citrus trees and the signi fi cance of reserves for fl owering and fruiting. J Am Soc Hortic Sci 107:206–208

Harrell DC, Williams LE (1987) The in fl uence of girdling and gibberel-lic acid application at fruitset on Ruby seedless and Thompson seedless grapes. Am J Enol Vitic 38(2):83–88

He ShiJie (1997) Girdling technique for Hongjiangcheng orange culti-var. South China Fruits 26:25

Ismail MR, Noor KM (1996) Growth, water relations and physiological processes of starfruit ( Averrhoa carambola L.) plants under root growth restriction. Sci Hortic 66:51–58

Kalmar D, Lahar E (1977) Water requirements of Avocado in Israel. I. Tree and soil parameters. Aust J Agric Res 28:859–868

Koo RCJ (1980) Results of citrus fertigation studies. Proc Fla State Hortic Soc 93:33–36

Koo RCJ, Smajstrla AG (1984) Effects of trickle irrigation and fertiga-tion on Fred end-use quality of ‘Valencia Orange’. Proc Fla State Hortic Soc 97:8–10

Kruger JA, Tolmay CD, Britz K (2000a) Effects of fertigation frequencies and irrigation systems on performance of Valencia oranges in two sub-tropical areas of South Africa. Proc Int Soc Citric Congr 9:232–235

Table 19.1 Citrus tree growth (canopy volume) and yield for 5-year-old Hamlin and Valencia trees grown for 4 years under three irrigation and nutrient treatments

Canopy volume (m 3 ) Yield (Mg/ha)

Soluble solids (°Brix) Canopy volume (m 3 ) Yield (Mg/ha)

Soluble solids (°Brix)

Drip OHS 9.9 A (29.8 m 3 /gN) 12.6 B (0.22 Mg/kgN) 3.8 14.0 A (42.5 m 3 /gN) 14.5 A (0.26 Mg/kgN) 4.9 Small-area microsprinkler

7.7 B (14.0 m 3 /gN) 18.7 A (0.16 Mg/kgN) 4.1 11.4 B (20.7 m 3 /gN) 12.3 B (0.11 Mg/kgN) 5.0

Large-area microsprinkler

8.9 B (10.3 m 3 /gN) 10.1 B (0.04 Mg/kgN) 4.1 12.0 B (13.9 m 3 /gN) 11.6 C (0.05 Mg/kgN) 5.0

The drip and small-area microsprinkler treatments were irrigated daily and fertigated daily and weekly, respectively. Large-area microsprinkler-treated trees were irrigated at target soil water depletion and fertigated monthly

280 K.T. Morgan and D. Kadyampakemi

Kruger JA, Britz K, Tolmay CD et al (2000b) Evaluation of an open hydroponics system (OHS) for citrus in South Africa: preliminary results. Proc Int Soc Citric Congr 9:239–242

Kuperus KH, Combrink N, Britz K et al (2002) Evaluation of an open hydroponics system (OHS) for citrus in South Africa. Annual research report. Citrus Research International (Pty) Ltd., Nelspruit, South Africa

Martinez-Fuentes AC, Mesejo C, Juan M et al (2004) Restrictions on the exogenous control of fl owering in citrus. Acta Hortic 632:91–98

Martinez-Valero R (2004) Preliminary results in citrus grove grown under the MOHT system. In: Proceedings of the international society for citriculture, Cape Town, South Africa, pp 316

Mason J (1990) Commercial hydroponics: how to grow 86 different plants in hydroponics. Kangaroo Press, Roseville

Michelakis N, Vougioucalou E, Clapaki G (1993) Water use, wetted soil volume, root distribution and yield of avocado under drip irriga-tion. Agric Water Manage 24:119–131

Monselise SP, Goren R, Wallerstein I (1972) Girdling effect on orange fruit set and young fruit abscission. HortScience 7:514–515

Morgan KT, Obreza TA, Scholberg JMS (2007) Characterizing orange tree root distribution in space and time. J Am Soc Hort Sci 132(2):262–269

Morgan KT, Wheaton TA, Castle WS et al (2009a) Response of young and maturing citrus trees grown on a sandy soil to irrigation sched-uling, nitrogen fertilizer rate, and nitrogen application method. HortScience 44(1):145–150

Morgan KT, Schumann AW, Castle WS et al (2009b) Citrus production systems to survive greening: horticultural practices. Proc Fla State Hortic Soc 122:114–121

Obreza TA, Rouse RE (1993) Fertilizer effects on early growth and yield of ‘Hamlin’ orange trees. HortScience 28(2):111–114

Paramasivan S, Alva AK, Fares A et al (2001) Estimation of nitrate leaching in an Entisol under optimum citrus production. Soil Sci Soc Am J 65:914–921

Parsons LR, Wheaton TA (2009) Tree density, hedging and topping, HS1026. Cooperative Extension Service. University of Florida, Gainesville

Perez-Madrid G, Almaguer-Vargas G, Maldonado-Torres R et al (2005) Girdling and gibberellic acid sprays in production, fruit quality and nutrient level in ‘Monica’ mandarin. Terra 23:225–232

Pijl I (2001) Drip fertigation: effects on water movement, soil character-istics and root distribution. M.S. thesis, University of Stellenbosch, Stellenbosch

Robinson TL, Hoying SA, DeMarree A et al (2007) The evolution towards more competitive apple orchard systems in New York. N Y Fruit Q 15(1):3–9

Roka MF, Muraro R, Allen RA et al (2009) Citrus production systems to survive greening: economic thresholds. Proc Fla State Hortic Soc 122:233–239

Schoeman SP (2002) Physiological measurements of daily daylight fer-tigated citrus trees. M.S. thesis, University of Stellenbosch, Stellenbosch

Schumann AW, Fares A, Alva AK et al (2003) Response of ‘Hamlin’ orange to fertilizer source, annual rate, and irrigated area. Proc Fla State Hortic Soc 116:256–260

Stover E, Castle WS, Spyke P (2008) The citrus grove of the future and its implications for huanglongbing management. Proc Fla State Hortic Soc 121:155–159

Syversten JP, Jifon JL (2001) Frequent fertigation does not affect citrus tree growth, fruit yield, nitrogen uptake, and leaching losses. Proc Fla State Hortic Soc 114:88–93

Takahara T, Okudai N et al (1980) Studies on the promotion of fl owering and fruiting of juvenile citrus seedlings. I. Effect of girdling. Bull Fruit Tree Res Stn D Kuchinotsu 2:1–14

Warrick AW (1986) Soil water distribution. In: Nakayama B (ed) Trickle irrigation for crop production. Elsevier, Amsterdam

Wheaton TA, Whitney JD, Castle WS et al (1995) Citrus scion and root-stock, topping height, and tree spacing affect tree size, yield, fruit quality, and economic return. J Am Soc Hortic Sci 120(5):861–870

Yandilla (2004) Martinez Open Hydroponic Technology, Yandilla Park Limited, Renmark. http://www.yandillapark.com.au/Growers/ohs_main.htm

Zekri M, Parsons LR (1988) Water relations of grapefruit trees in response to drip, microsprinkler and overhead sprinkler irrigation. J Am Soc Hortic Sci 113:819–823