The fate of fertilizer-N applied to a Florida citrus soil

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Title:
The fate of fertilizer-N applied to a Florida citrus soil
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Hagillih, Daniel Apollo, 1946-
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Nitrogen fertilizers -- Florida   ( lcsh )
Citrus fruits -- Fertilizers -- Florida   ( lcsh )
Soils -- Florida   ( lcsh )
Soils -- Nitrogen content   ( lcsh )
Horticultural Science thesis Ph. D
Dissertations, Academic -- Horticultural Science -- UF
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Thesis:
Thesis--University of Florida.
Bibliography:
Bibliography: leaves 91-102.
Statement of Responsibility:
by Daniel Apollo Hagillih.
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Typescript.
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Vita.

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Full Text




















THE FATE OF FERTILIZER-N APPLIED TO
A FLORIDA CITRUS SOIL







By

Daniel Apollo Hagillih


A DISSERTATION PRESENTED TO THE
GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA


1980


_f _












ACKNOWLEDGEMENTS


The author wishes to express his gratitude and appre-

ciation to Dr. R.C.J. Koo, the chairman of his supervisory

committee, for his patient guidance in the execution of

this research and the preparation of the manuscript.

Throughout the graduate training of the author, Dr. Koo

strived to imprint on the author an appreciation of the

scientific method in research.

The author is equally indebted to Dr. A.H. Krezdorn,

Professor Emeritus of Fruit Crops, who had been the Chair-

man of his supervisory committee. Even in retirement, Dr.

Krezdorn continued to serve on this committee and to will-

ingly give of his time, counsel and support. To Dr. W.S.

Castle, a member of his committee, the author owes a debt

of gratitude and appreciation for his valuable contribu-

tion in suggestions, counsel and involvement in all phases

of this study.

Sincere appreciation is extended to Dr. N. Gammon,

Jr., Professor Emeritus of Soil Chemistry, for his construc-

tive criticism and counsel particularly in the soil aspect

of the research. The author also wishes to thank Dr. D.A.

Graetz for his assistance in the analysis of soil samples.





ii









The graduate training of the author was sponsored by

the Agricultural Research Corporation of the Sudan. Towards

the end of his training, the Department of Fruit Crops

offered the author a graduate assistantship. He grate-

fully acknowledges the financial support from both.

Finally, the author wishes to thank Ms. Janice L.

Cambridge for her companionship and encouragement through-

out the duration of this study.


iii















TABLE OF CONTENTS


ACKNOWLEDGEMENTS . .

ABSTRACT . . .

INTRODUCTION . . .

LITERATURE REVIEW . .

Nitrogen Balance and Its Major Components

Considerations for Increasing the
Efficiency of N Usage by Citrus Trees


Page

. ii

. v

. 1

. 3

. 3


Analytical Procedures for N Balance
Components. . ... 10

Isotope Approach to N Utilization
by Citrus Trees . . 15

MATERIALS AND METHODS. . ... 18

Experimental Site . .. 18

Experimental Design . 18

Field Sampling Procedures . .. .24

Sample Preparation and Analysis . 27

Statistical Analysis. . 29

RESULTS AND DISCUSSION . 30

Experiments 1 and 2 ..... .. 30

Experiment 3. . .. 70

CONCLUSIONS. .... ... 87

LITERATURE CITED ..... . 91

BIOGRAPHICAL SKETCH. . .... .. 103


. .














Abstract of Dissertation Presented to the
Graduate Council of the University of Florida in
Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy



THE FATE OF FERTILIZER-N APPLIED TO
A FLORIDA CITRUS SOIL

By

Daniel Apollo Hagillih

March 1980



Chairman: Dr. R.C.J. Koo
Major Department: Horticultural Science



The fate of fertilizer-N applied to Astatula fine sand

was studied by the destructive sampling of 1-year-old 'Pine-

apple' orange (Citrus sinensis [L.] Osbeck) trees on 'Alemow'

(C. macrophylla Wester), and through soil chemical analysis,

using the difference method. The fall-winter study involved

unfertilized control, isobutylidene diurea (IBDU) and NH4NO3

sources in a factorial combination with no irrigation, low

and high irrigation levels. Nitrogen source and irrigation

treatments lasted for 14 and 6 weeks, respectively. The

spring study involved only the 3 N sources and lasted for

6 weeks. Leaf and fruit sampling of bearing 'Pineapple'

orange trees, fertilized with the same N sources, and soil









chemical analysis of the unreplicated tree plots were also

used. Soil water content in all experiments was determined

by a neutron probe.

The results indicated that growth parameters of the

young trees, including percent trunk diameter increase, dry

weights of component parts and the entire tree, showed no

statistical differences attributed to N source, apparently

because of the short duration of the studies. There was,

however, a trend for IBDU and NH4NO3 fertilization to re-

sult in greater dry matter, particularly in the more succu-

lent aerial parts. This was apparent in the leaves of the

fertilized trees which were statistically greater than those

of unfertilized trees.

There was a highly significant relationship between

N source and N concentration and total N of tree parts,

and total N of the entire tree. In every case, the order

of N absorption was NH4NO3- > IBDU-fertilized > control

trees. This confirms that a soluble source such as NH4NO3

is more readily absorbed in the short-term than a controlled

release form of N. It is suggested that this trend may not

prevail in the long-term.

Irrigation level did not statistically affect tree

growth and N content parameters, and soil N content in the

fall-winter study. The narrow range and short duration of

the irrigation treatments were probably not sufficient to

elicit treatment responses.









Nitrogen balance data showed that 66 and 30% of ap-

plied N from IBDU and NH4NO3, respectively, were retained

in the 0- to 60-cm soil profile. Twenty-five and 45% from

the same respective sources were accounted for in the young

trees. Total apparent N recovery in the soil-plant sys-

tem was 91 and 76% for IBDU- and NH4NO3-N, respectively.

The deficit was assumed to have been leached and/or lost to

the atmosphere.

There was no difference in fruit yield between IBDU-

and NH4NO3-fertilized bearing trees. Fertilized trees,

however, yielded 4 times as much as the control. Fruit

N concentration and removal and leaf N concentration were

in the order of NH4NO3- > IBDU-fertilized > unfertilized

tree. The absence of replication precluded the establish-

ment of a plausible N balance. Nevertheless, 40% of the

applied N was estimated to be contained in the fruits and

leaves.

High N recoveries on experimental plots involving young,

nonbearing, and bearing trees suggest that substantial leach-

ing of N may not be a major route for N loss from citrus

groves established on deep, well-drained soils when reason-

able rates of fertilizer-N are applied over the root zone.

The potential for a single application of IBDU to

young citrus trees, which are normally fertilized several

times a year, needs further study.


vii














INTRODUCTION


Florida's sandy soils planted to citrus are character-

ized by low native fertility which necessitates the regu-

lar addition of fertilizer nutrients to maintain a com-

mercial level of production. Such an edaphic environment

is also conducive to the substantial leaching of mineral N

by rainfall and irrigation (114). Soil tests which measure

N carry-over from previous N additions are of little value

in these soils. As a result, the replacement of nutrients,

including N, lost from the soil is generally dependent up-

on an evaluation of the need of the crop.

The N requirement of commercial citrus cultivars is

based on many long-term investigations in which tree growth

and yield were related to fertilizer rate, source, time of

application and leaf N content. Nitrogen uptake by the

citrus tree is considered to be a small part of the total

N loss from the deep, sandy soils of central Florida (100).

The efficiency of tree N utilization, i.e., the propor-

tion of N used by the tree compared to that applied, has

been estimated to be 25-30%. As a result, there is public

and grower concern regarding the quantitative extent of the

leaching loss of N applied to citrus. Leached N represents

an irretrievable loss of a costly resource and may









contribute to the NO3 pollution of underground water and

to lake eutrophication.

No meaningful N balance studies, i.e., accounting

quantitatively for the fate of N applied to citrus soils

as a fertilizer, have been done in Florida as they have in

California (29, 68, 69). In order to enhance our under-

standing of N use in Florida soils planted to citrus, it

is essential that the fate of fertilizer-N be studied.

The primary objective of this research was to make a

quantitative accounting of fertilizer-N applied to bearing

and young trees on a deep, well-drained sandy soil of the

Florida ridge area, where much of the citrus in Florida is

grown. A second objective was to evaluate isobutylidene

diurea (IBDU) as a controlled-release N fertilizer source

for citrus.


_ 1














LITERATURE REVIEW


Nitrogen Balance and Its Major Components


Preliminary Remarks

Important sources for the N in the soil solution in-

clude fertilizer-N additions and net mineralization. Other

sources also likely to add some N to the soil solution in-

clude rainfall, irrigation water, biological fixation, dif-

fusion of atmospheric ammonia into the soil surface, direct

absorption of atmospheric ammonia by plant leaves, and

transformations involved in the N cycle (80, 125). Major

sources of N removal from the soil solution are plant up-

take, leaching, net fixation in the soil and denitrifica-

tion. Smaller losses of N in most agricultural soils in-

clude gaseous losses through ammonia volatilization and

physical removal by wind and animals.

Attempts to account for the fate of applied N in the

soil-plant-atmosphere continuum have yielded some useful

information, but it is rare that a complete quantitative

recovery of applied N is obtained (2, 50, 97). Calculation

of a N balance for a field system is particularly diffi-

cult because of the many possible fates of N in soils (65),

and because of the physical difficulties encountered in









obtaining quantitative data for some of the major compo-

nents of a N balance such as leaching, plant uptake and

residual N (8, 97).


Leaching of N

Leaching has been defined as the movement of solutes

from one soil zone to another by percolated water (107).

Leaching of N is important because it represents the loss

of a costly resource and because it may contribute to en-

vironmental pollution.

Few measurements have been reported giving accurate

data concerning nutrient loss from agricultural lands by

leaching (34). Most of the mineral N lost by leaching in

citrus orchards is in the NO3 form (42, 51). The extent

of NO3 loss from the soil-plant system depends on the fer-

tilizer material, soil type, plant species, rainfall and

management practices. High infiltration rates and low

water storage capacities of sandy soils make them espe-

cially subject to NO3 leaching. Other factors which in-

fluence the magnitude of NO3 leaching include evaporation,

nutrient concentration, depth of rooting, N reserves, min-

eralization of the soil, nitrification of the NH4-N and

soil temperature (113).

In California, the proportion of N applied to citrus

leached annually as NO3 ranged from 45 (8) to 90% (51).

These high NO3 leaching losses, however, occurred under

conditions of excessive irrigation on well-drained soils









atypical of many irrigated areas in arid zones. In a deep

sandy Florida soil, split application of 200 kg/ha of N

annually to a mature citrus grove did not result in NO0

leaching even when the rainfall was high (35). Studies in-

volving a single annual application of N to evaluate the

magnitude of NO3 leaching loss from citrus plantings in

Florida have not been done.


Plant Cycling of N

Information on the N content of an entire citrus tree

is meager. The size of the mature tree makes the destruc-

tive biomass sampling a difficult and costly task (111).

In forest tree studies, where there is a commercial inter-

est in the wood, biomass sampling procedures for the tree

are more highly developed (9, 66, 91, 126). Also, with

citrus, leaf N content is considered an adequate index of

the N status of the tree. The commercial adoption of leaf

analysis as a guide to fertilization of citrus has report-

edly reduced N usage by about 50% in California (31).

A few studies on total citrus tree N content have been

reported, but very often these studies were lacking in ade-

quate replication (5, 63, 100, 121). In Florida, it was

estimated that a 15-year-old orange tree absorbed only 25-

30% of applied N (100). Of this amount, 33% is returned

to the soil during the year through shedding of old leaves,

flowers and young fruits, 50% is removed in the crop of

fruit, and about 15% goes into permanent structures in the









form of new twigs and trunk enlargement. Data from a

single 19-year-old grapefruit (Citrus paradisi Macf.) tree

in Florida (5) indicated that the N content of component

tree parts on a % dry weight basis in descending order was

leaves, immature fruit, twigs less than 1.3 cm diameter,

fibrous roots, large roots, limbs up to 1.3 cm diameter

and trunk.

A series of studies to determine the N content of

'Valencia' orange trees have been reported from California

(19,20, 121). One study showed that a 10-year-old tree

contained 200 g of N in the leaves (19), while another

study indicated that nearly one-half of the N content of

the tree was in the leaves (20). Wallace et al. (121)

reported that a 14-year-old 'Valencia' orange tree contained

673 g of N. The authors did not, however, indicate how

much N was applied so that it is difficult to relate this

figure to trees of similar age elsewhere. Apparently, a

significant amount of N added to the soil is removed by

citrus fruits (59, 131). Zidan and Wallace (131) reported

that a mature 'Washington' navel orange fruit with a fresh

weight of 192 g contained 394 mg of N while a 'Valencia'

orange fruit of 165 g contained 327 mg.


Residual Soil N

A part of the N balance which is the most difficult to

account for because of spatial variability is the amount of

N remaining in the soil at any given sampling time.









Sampling and analysis of the soil material for NO3N (1, 3,

51, 61, 81, 82) and/or total N (36, 84, 102) is commonly

used to measure soil N content but large samples are usu-

ally required to achieve an acceptable degree of accuracy.

The variation in residual soil N content, both within and

among experimental units, has been reported to decrease

with increasing depth in the soil profile (79). Also, the

variability of virgin land has been reported to be less for

N than similar areas of cultivated land (7).

In a Florida study involving an orchard of mature

citrus trees, the soil nutrient content was the least vari-

able in samples collected below the tree dripline as com-

pared to other sampling locations (11). Nevertheless, it

was demonstrated that a large number of samples was re-

quired to reduce the variation regardless of the sampling

location.



Considerations for Increasing the
Efficiency of N Usage by Citrus trees


General Concepts

Optimizing N use by citrus trees appears to involve

the development of management programs which prevent the

excessive loss of NO3 by leaching without sacrificing fruit

yield, size, and quality (29). It has been suggested that

optimum management could be achieved by applying only the

amount of N needed to sustain high fruit production, with









leaf analysis being used as a guide to determine the proper

rate (32, 90). Differences in citrus cultivar N require-

ments and soil types, however, dictate an approach in which

timing, rate, placement, N source and water regime must

be considered in the efficient use of fertilizer-N.

One approach to increasing the efficiency of N use

has been to split the annual application of N (17, 35, 88).

This is supported by a Florida experiment conducted under

high rainfall conditions (35). In terms of citrus fruit

productivity, however, other studies (17, 88) indicated

that a single application of fertilizer-N applied during

the drier portion of the year, in winter, gave the same

yield response compared to 2 applications per year.

Another approach is the foliar application of N in

the form of low-biuret urea. Studies from California (29,

30) indicated that foliar application of N was associated

with lower NO3 leaching potential than was soil applica-

tion. In general, foliar-applied N, under the low rainfall

conditions of California, was found to be as effective as

soil-applied N for fruit production (29, 30). This approach

may not be suitable in Florida where frequent rains may

wash away the N applied to the foliage. Efficient N use

may require the application of nominal rates to the soil,

supplementing this with foliar-applied N.

It has been suggested that in future studies, split

soil application of N be a treatment along with foliar-soil

combinations (54). A major limitation in foliar application









of N, however, is that it is energy intensive especially

if frequent applications are necessary.


Concept of Controlled Release N Fertilizers and IBDU

The concept of the controlled release of N to increase

fertilizer-N efficiency has been amply reviewed (62, 74,

76, 77, 85, 106). The basic approaches are the development

of compounds of limited solubility, use of coated granules,

and formulation of ammoniacal fertilizers with nitrifica-

tion inhibitors. The objectives of such fertilizer-N forms

are to minimize the immobilization, leaching and volatili-

zation losses of soil-applied N, and to release an adequate

amount of available N to satisfy the crop requirement. How-

ever, because of higher costs, these fertilizers are being

used primarily in turfgrass, ornamentals, home grounds,

nursery fields, and for other specialty situations.

Isobutylidene diurea is a condensation product of urea

and iso-butyraldehyde patented in Japan as a slow-release

N source. Hamamoto (41) described its preparation and

physical properties in greater detail. His review of the

studies conducted in Japan showed that the large granule of

IBDU was effective in preventing high N leaching loss when

applied to paddy soils. By using IBDU with green tea

(Camellia sinensis [L.] Kuntze) plants Hamamoto (41) re-

ported that one application was enough to get average yields

and thus save labor. The conversion of IBDU to plant avail-

able form appears to result from the dissolution of IBDU









granules. The rate of dissolution is strongly influenced

by particle size (41). Fine sized IBDU granules release

larger amounts of N than coarse granules. This was ob-

served in an incubation study (48), for turfgrasses (116,

117), and for flue-cured tobacco (Nicotiana tabacum L.)

(71). Hughes (48) confirmed Hamamoto's (41) findings that

IBDU-N release was greater at low pH values and high tem-

peratures. In evaluating IBDU for 'Merion' Kentucky blue

grass (Poa pratensis L.), Moberg et al. (72) indicated that

a uniform growth response was possible from 2 applications

of IBDU as compared to 3 of UREX (a urea-paraffin product).

Studies in citrus or other tree crops with IBDU have not

as yet been reported.



Analytical Procedures for
N Balance Components

Leached N Sampling and Analysis

Soil N losses from leaching have generally been stud-

ied in lysimeter experiments or in controlled field drain-

age plots by measuring the N content of the effluent water

(43). In citrus, lysimeter studies have largely been lim-

ited to young trees in the greenhouse. Such studies can

provide useful basic information, but they are limited in

scope because of the difficulty of extrapolating the data

to orchard conditions (39). Moreover, a major shortcoming

of lysimeters is the retention of more moisture than









corresponds with field capacity, resulting in the danger of

anaerobiosis and denitrification (23).

In mature citrus orchards, several approaches have

been made for estimating the leaching of fertilizer-N.

Wander (123) analyzed water samples from the lower edges

of citrus plantings located on slopes in Florida. Be-

cause the actual amount of water used by mature citrus

trees and the exact rainfall in the local area was not

known, however, it was difficult to translate such water

analyses into figures representing actual amounts of N lost

by drainage.

Various systems of drain lines and tiles have been

devised in which data obtained by effluent water analysis

and water discharge rates are used to calculate the amount

of N leached. The amount of NO3-N in the drainage from

citrus groves has been determined by this procedure in

California (26, 60) and Florida (16, 18). However, the

reliability of this approach for monitoring subsurface

NO3-N losses has been questioned (108).

Several investigators have used porous ceramic cups to

collect soil solution samples from citrus groves (10, 26,

35, 60). This procedure was found to be useful for esti-

mating the potential for NO pollution in a Florida citrus

grove (35). The procedure also gave a reasonable estimate

of the amount of NO3-N leached from a millet (Pennisetum

typhoides L.) plot (37). However, this procedure has been









criticized because of the difficulty of providing enough

water samples for chemical analysis and the requirement

for a suction device. A new vacuum extractor was developed

to surmount these difficulties but its usefulness has been

limited by operational problems (27).

A new approach to calculating the leaching of NO3 from

the root zone of crops is based on a comparison of the be-

havior of NO and Cl ions in the drainage or percolating

waters that have moved below the root zone (82, 83).

Changes in the ratio of these ions were used to calculate

the amount of NO3 leached. In unsaturated soils where the

determination of NO3 and Cl concentrations in the percola-

ting water is difficult these concentrations were deter-

mined in a saturation extract. In both cases NO3 and Cl

ratios were found to be satisfactory in estimating NO3

leached below the root zone of crops.


Residual Soil N Sampling and Analysis

Soil samples from citrus orchards are obtained using

various sampling techniques. For cores greater than 6 m

in length, a power-driven 45-cm diameter auger is often

used (82). When samples are taken to only 6 m, a truck-

mounted hydraulically powered sampler is frequently pre-

ferred. Most sampling in citrus orchards is conducted to

shallower depths, generally not exceeding 4 or 5 m. In

such cases, conventional sampling soil tubes or augers are

used.









Several analytical procedures have been adopted to

determine residual soil N. For total N determination, 2

methods have gained general acceptance: the Kjeldahl

method which is essentially a wet-oxidation procedure, and

the Dumas method which is fundamentally a dry-oxidation pro-

cedure (12). Residual soil N is also determined by analyz-

ing the soil for exchangeable NH4 and NO3 ions using steam

distillation procedures (13). Recent trends in quantita-

ting residual soil N, however, have involved the determina-

tion of NO3 (67) and NH (4) ions in saturation extracts

using specific ion activity electrodes.


Soil Water Measurement

Nitrogen is absorbed by plant roots from the soil solu-

tion. The soil water content is important, therefore, not

only because of its function as a solvent but also because

its mass movement is directly related to the leaching of the

soil profile.

Soil water content is measured by several techniques,

each with advantages and disadvantages. One technique used

extensively in loam- to clay-textured soils is based upon

the tensiometer (92, 93, 95, 105). The information fur-

nished by tensiometers tends to be qualitative. Tensio-

meters are generally used for determining timing and dura-

tion of irrigation and have not been found to be particu-

larly useful for sandy soils (52, 56).









Electrical-resistance blocks are not extensively used

on sandy soils (64, 105) because gypsum blocks operate most

reliably in the drier portion of the soil water content

spectrum. In the sandy Florida soils, the available water

is relatively small, and soil water should be maintained

above 65% of field capacity. Under these conditions gyp-

sum blocks are not particularly suitable.

For direct quantitative measurement of soil water by

volume, 2 methods are used. The gravimetric procedure in-

volves the calculation of a volumetric water content from

the weight of the water in a soil sample and the soil bulk

density (46, 64, 94). Rapid, large-scale soil water con-

tent sampling by this technique is laborious and time-

consuming.

The neutron scattering method is a faster and equally

accurate method as compared to the gravimetric procedure

(14, 15, 40, 104). The method has been found to be useful

in studying patterns of soil water depletion in Florida

citrus groves (53).


Plant Tissue Sampling and Analysis

The leaf is the most commonly sampled tissue to deter-

mine the N status of a citrus tree (31, 32, 99, 101). As

a result, techniques in leaf sampling and analytical pro-

cedures have been highly developed. The entire mature tree,

however, is rarely sampled to determine its total N content.









One approach for determining the N content of the tree is

to uproot the entire tree. The tree is then fractionated

into various component parts and the N content of sample

parts determined after oven-drying and grinding. This

approach has been tried in Florida (5) and in California

(19, 20). Another approach has involved making a. periodic

weight and chemical determination of all the blossoms,

leaves and fruits that fall from citrus trees. Wallace

et al. (121) used this approach to obtain data which were

then combined with similar information for harvested fruits,

new leaves and twigs, to obtain a measure of the nutrient

content of the tree.



Isotope Approach to N
Utilization by Citrus Trees


The use of isotopes in tree nutrition research was

discussed by Walker (118) and Hauck (44). The advantages

of this technique include the direct measurement of trans-

port velocities and separation of newly absorbed nutrients

from those already present in the various components of the

ecosystem (96).

Nitrogen occurs in nature in 2 stable isotopes, N14 and

N5. Only recently has it been possible, as the result of

advances in cryogenic techniques whereby nitric oxide gas

is liquefied and then distilled, to separate the 2 isotopes

with relative ease (21). Nitrogen15 fertilizer formula-

tions are those which have been enriched with excess N15









while Nl -depleted fertilizers, also known as N14 ferti-

lizers, are those from which much N5 has been removed.

Studies using N15 as a tracer are commonplace in agro-

nomic crops because the plants are relatively shallow-

rooted (6, 49, 50, 73, 112, 127, 128, 130). Deeper-rooted

forest and fruit trees do not lend themselves to N balance

studies using N15 (66, 75). The high cost of the tracer

has largely restricted its use to laboratory and greenhouse

studies with young trees and strongly affected the size of

the experiment and the experimental design. Furthermore,

the procedure requires the use of expensive instrumentation

to which many laboratories may not have access. Neverthe-

less, such studies have been reported for young apple

(Malus sylvestris Mill.) trees (24, 38, 45) and nonbearing

prune (Prunus domestic L.) trees (124).

Greenhouse studies with citrus using N15 were reported

by Wallace (119, 120) and Wallace et al. (122). Wallace

(119) found that growth complications and variability in

plant material resulted in conventional methods being in-

ferior to the isotopic technique in studying the influence

of temperature on N absorption and translocation. His qual-

itative investigation (120) showed that NO3-N was 2 to 5

times as readily absorbed from soil as was NH4-N by 8-week-

old cuttings of 'Eureka' lemon (C. limon L.). Nitrogen15

appeared in the leaves of a 3-year-old tree 4-7 days after

application to the tree (122).









After this initial interest, no studies in citrus

with N15 have been reported until recently. Kubota and

coworkers (57, 58) investigated the behavior of N supplied

in early spring and early summer on 9-year-old 'Satsuma'

mandarin (C. unshiu Marcovitch) trees. In the spring ex-

periment, N5 was detected in rootlets and leaves 3 and 7

days, respectively, after the application. Seventy-five per-

cent of absorbed N15 was found in the aerial parts of the

tree. In the summer study, N15 was detected in rootlets

and spring leaves the next day after application, indica-

ting a faster absorption of N in summer. Ninety-two percent

of the N15 was distributed in the above ground parts. A

review of N5 studies on citrus in Japan was recently

presented by Yuda (129).
15 15
Both N-- depleted and N- enriched fertilizer materials

have been reported to give accurate and precise measurement

of tracer N in plant tissue (28, 103). No studies with

N- depleted fertilizers, however, appear to have been re-

ported with citrus.














MATERIALS AND METHODS


Experimental Site


Experiments for this study were conducted on an Asta-

tula fine sand site 24 km from Lake Alfred, Florida. The

soil is a Typic Quartzipsamment of the Entisols (47).

Typically, it is well-drained with a low water and miner-

al nutrient retaining capacity.


Experimental Design

Experiment 1

Two factors, N source and irrigation, were examined

in this experiment. Levels of the N source factor in-

cluded no N control, IBDU-N, and NH4NO3-N, while those of

the irrigation factor were no irrigation, low and high

irrigations. It was desired to study these treatments in

a factorial arrangement; however, it was not possible to

physically establish the experiment in this manner because

of the limitation imposed by the irrigation treatment.

Therefore, the 1-year-old 'Pineapple' orange (Citrus sin-

ensis [L.] Osbeck) trees on 'Alemow' (C. macrophylla Wester)

rootstock were spaced at 3 x 3 m in 2 rows on July 13, 1978,

using a modified split plot design (Fig. 1). The field

layout consisted of 3 replications with irrigation level






































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as the main plot treatment. The size of each main plot

was determined by 6-m lengths of perforated sprinkler pipe

used to apply the irrigation treatments. Each main plot,

therefore, had 4 trees, 2 on each side of the pipe. Each

single tree plot received one level of the N treatment but

4 replications of each N treatment were randomly dis-

tributed over the 3 irrigation main plots. Thirty-six trees

were used in the experiment.

Fertilizer-N from IBDU and NH4NO3 sources at the rate

of 201.6 g/tree was hand broadcast in a 3 x 3 area around

each tree on September 28, 1978, 11 weeks after the trees

were planted. Trees in control plots were not fertilized.

Irrigation treatment was not started until November 27, 1978,

after rainfall had tapered off. Soil water content was

used to schedule irrigation. Approximately 0.75 cm of ir-

rigation water was applied whenever the average soil water

content fell below 4% by volume in the 0- to 60-cm soil

profile of the high irrigation plots. The high irrigation

plots were irrigated twice as often as the low irrigation

plots which also received 0.75 cm of irrigation water on

each application. A rain gauge at the experimental site

and an irrigation flow-meter were used to monitor water

supply. When the flow-meter malfunctioned 1500-ml cans

were placed in the plots for collection and measurement of

the amount of irrigation water delivered.

The experiment was terminated on January 9, 1979, be-

cause of freeze damage. Fertilizer-N and the first










irrigation had been applied 14 and 6 weeks, respectively,

prior to the termination of the experiment. The trees un-

der low irrigation had received a total of 2.20 cm of ir-

rigation water in 3 irrigations while those under high ir-

rigation had received 4.95 cm in 6 irrigations. For the

entire period of the experiment, the nonirrigated plots

had received 20.17 cm of rainfall; the low and high irriga-

tion plots had received a combined total of 22.37 and 25.12

cm of rainfall and irrigation, respectively.



Experiment 2


This experiment was initiated to clarify certain re-

sults obtained in Experiment 1. The total N content of

NH4NO3-fertilized trees was 42% higher than that of the

IBDU-fertilized trees. Moveover, the complicating effect

of the freeze damage on N absorption was not known. It

was decided to determine if the same trend in N absorption

would prevail during the warm spring weather.

Nine unused trees planted at the same time as those

in Experiment 1 had been left unfertilized from the sum-

mer of 1978 through March, 1979. The 3 replications of

single tree plots were arranged in a randomized complete

block design, having as treatments unfertilized control,

IBDU and NH4NO3 as N sources. No irrigation treatment was

applied, however. Natural precipitation and routine


I









irrigation of the adjacent mature citrus grove provided

the only water supply.

Data from Experiment 1 indicated that NH4NO3-ferti-

lized trees had higher N content than IBDU-fertilized

trees. The effect of the freeze damage on N absorption

from the 2 N sources was not known. It was necessary

therefore to establish if the same trend would be manifest-

ed in warm weather. Hence, the trees in Experiment 2 were

deliberately grouped on the basis of their trunk diameter;

IBDU was applied to the largest trees, and NH4NO3 to the

smallest trees. Control trees were intermediate. The

same rate of fertilizer-N in Experiment 1 was hand broad-

cast in a 3 x 3 m area around the tree on March 23, 1979.

The experiment was terminated on May 8, 1979, 6 weeks after

fertilizer application.


Experiment 3

Three 16-year-old 'Pineapple' orange trees in an

existing rate and fertilizer-N source study were used for

this experiment. The trees had been fertilized with the

same rate of NH4NO3 since April 5, 1973,until September 23,

1977. In the current experiment,one of the trees received

a total of 707.2 g of N per year, in 3 equal applications

on March 2, 1978, June 11, 1978, and October 25, 1978, as

IBDU. Another tree received the same rate of N as NH4NO3

on the same dates. The fertilizer was applied by hand

broadcasting around the tree. Other nutrients were









adequately supplied to the fertilized trees. The third

tree which did not receive any fertilizer-N at the speci-

fied dates was used in this experiment as the unfertilized

control.

In order to study N movement in uncropped land 1 cir-

cular fallow plot 6 m in diameter, which corresponded to

the tree canopy diameter, was fertilized with IBDU on

October 25, 1978. Only one-third of the total annual ap-

plication in the tree plots was used. Another similar

plot was fertilized with the same rate of N as NH4NO3. A

third plot was left unfertilized as a control. The 6 un-

replicated plots in the experiment were subjected to the

routine management of the adjacent grove. The experiment

was terminated on February 17, 1979, 17 weeks after the

fall application of fertilizer-N, at which time the plots

had received 25.90 cm of rainfall and irrigation since

October 25, 1978. Evapotranspiration for the duration of

the study was estimated from climatological data (55).



Field Sampling Procedures


Soil Water Sampling

Changes in soil water content were measured with a

Nuclear-Chicago neutron probe (P-19) and a scaler (53).

One aluminum access tube for the probe was installed about

45 cm from every other tree in Experiments 1 and 2. In









Experiment 3, one tube was installed just inside the drip-

line of each tree and at an equivalent distance from the

center of each fallow plot.

One-minute readings were taken at depth increments

of 15 cm, beginning at 15 cm. The readings were converted

to % water content by volume using a calibration curve

developed from previous sampling in the experimental area

(53). The water content of the surface 15 cm was deter-

mined gravimetrically. A composite of duplicate 2.5 x

15 cm cores of soil per plot in the 3 experiments was col-

lected and its gravimetric water content determined by

over-drying to constant weight at 1050C. The water content

was transformed to a volume percentage by multiplying the

% weight by the average soil bulk density (1.54 g/cm3).

Water content in the soil profile was calculated by multi-

plying the average volume ratio by the depth of the entire

profile and results expressed as cm of water.


Soil Sampling

A composite of duplicate 2.5-cm diameter soil core

samples was taken within 30 cm from the tree from 0-7 cm

and 7-15 cm, and thereafter at 15-cm increments down to

60 cm in Experiments 1 and 2. Soil samples were similarly

taken at the tree canopy dripline down to 120 cm in Experi-

ment 3. In Experiment 1, samples were taken when cumula-

tive rainfall exceeded 1.25 cm. After the initiation of

the irrigation treatment, samples were taken only at the









termination of the experiment. In Experiment 2, samples

were taken after 2.28, 9.14 and 17.87 cm of cumulative

rainfall and irrigation. Samples were taken following more

than 4.87 cm of rainfall and/or irrigation in Experiment 3.

In order to prevent free movement of water from the

soil surface through the sample holes, each hole was filled

with white sand, obtained from elsewhere, and marked to

avoid future sampling in that immediate vicinity.


Plant Tissue Sampling

Tree growth measurements in Experiments 1 and 2 were

obtained by measuring the diameter of the trunks and the

destructive sampling of the tree. Tree trunks were painted

at a point 15 cm above the bud union and the initial and

final trunk diameter recorded. Trees were excavated by

digging a trench 60 cm from the tree and recovering the

root system as much as practical. Each tree was fraction-

ated in the laboratory into 7 components: fibrous and

lateral roots, taproot, rootstock trunk, scion trunk,

crotch branches, green twigs and leaves. The root system

was rinsed in running tap water and the fresh and oven-dry

weight (700C) of each component obtained.

In the mature tree experiment, fruits were harvested

on February 17, 1979, and the total fresh weight recorded.

One 30-fruit sample from each tree was ground in a com-

muniting machine, and 2 subsamples oven-dried at 700C for

dry weight determination and chemical analysis.









The number of leaves on each mature tree was esti-

mated by the following procedure. Leaves in a 30 x 30 cm

frame from 4 sides of the tree were stripped and the

average number in the frame determined. The leaves were

oven-dried for 48 hours at 700C and the total dry weight

determined. The total dry weight of the sampled leaves

was used to calculate the average dry weight of a leaf.

Tree height and tree width in 2 directions (north-south and

east-west) were also measured. The canopy surface area was

then calculated from the formula:

S = 27W/3h2[(W/16+h2) 15-(W/4)3

where h = height and W = width of the tree (98). The

number of leaves on a tree was then estimated from a know-

ledge of the canopy surface area.



Sample Preparation and Analysis


Soil Samples

All soil samples were air-dried and sieved through a

2 mm screen. Total soil N was determined by a semi-micro

Kjeldahl procedure using a salicylic-sulfuric acid mixture

(12). Soil samples from the 4 replications in Experiment

1 were combined for the determination of exchangeable NH4

and NO, by a steam-distillation procedure using Devarda

alloy and MgO (13). Samples were not sufficient for the

determination of these ions for the second sampling date









in Experiment 1 and for all samples in Experiments 2 and

3. Total N was expressed as % N on a dry weight basis and

NH4-N and NO3-N as ppm. When a quantitative account for

N was being made, these values were converted to g or kg

of N in the soil profile. This was obtained as the product

of area, depth, average bulk density (1.54 g/cm3) and aver-

age N concentration in the profile.

In Experiments 1 and 2, the N content of the profile

was calculated for the total area fertilized (90,000 cm2)

but since the root system of the young trees occupied an

area less than 60 cm diameter, the N content was also cal-

culated for the latter area (2828 cm2). The depth used in

the calculations was 60 cm. For the mature tree experiment,

the area and depth were 28.28 m2 and 1.20 m, respectively.

The recovery of fertilizer-N was obtained by subtracting

the total soil profile N content of the unfertilized plot

from that of the fertilized plot. This value was expressed

as the percentage of the amount of N applied.


Plant Tissue Samples

All plant parts were ground in a Wiley Mill and passed

through a 20-mesh screen. Total N was determined by a

semi-micro Kjeldahl procedure using 0.5 g subsamples (12).

Nitrogen values were expressed both as % of dry weight and

in absolute amounts. The latter were obtained by multiply-

ing % N by the total dry weight of the component part. The

recovery of fertilizer-N by the trees was obtained by









subtracting the total N content of the unfertilized tree

from that of the fertilized tree and the result expressed

as the percentage of the amount of N applied. In the ma-

ture tree experiment, the N content of fruits and leaves

from the unfertilized tree was similarly substracted from

those of the fertilized tree.



Statistical Analysis


In Experiments 1 and 2, data pertaining to tree growth,

tissue N content, and soil total N content were submit-

ted to analysis of variance. In Experiment 1, these data

were analyzed in a 3 x 3 factorial arrangement. Data for

Experiment 2 were treated as a randomized complete block

design. Significant differences were determined by Duncan's

multiple range test. Soil water data and those from the

mature tree experiment were not analyzed.














RESULTS AND DISCUSSION


Experiments 1 and 2


Preliminary Remarks

Experiment 1 was prematurely terminated 14 weeks af-

ter fertilizer application and 6 weeks following the first

irrigation because the trees were beginning to show the

effects of freeze damage. This was especially apparent in

the unfertilized control trees where extensive leaf drop

occurred. Since the complicating effect of the freeze

damage on N absorption was not known, and NH4NO3-fertilized

trees appeared to have higher N content in comparison to

IBDU-fertilized trees,Experiment 2 was started to deter-

mine if the same trend would prevail in warm weather.

Therefore, the trees in Experiment 2 were deliberately

grouped such that IBDU-fertilized trees were larger in

trunk diameter than control and NH4NO3-fertilized trees

(Table 1).


Tree Growth Analysis

The young trees in both experiments generally showed

vigorous growth regardless of treatment. Trees fertilized

with NH4NO3 and IBDU had denser canopies than the unfer-

tilized control trees, and those fertilized with NH4NO3






















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had darker foliage than IBDU-fertilized trees. Leaves

from the control trees in Experiment 1 were dull green and

began to shed toward the end of the study, indicating that

N was becoming a limiting factor (22) and the trees were

responding to the cold weather. Leaves from the control

trees in Experiment 2 were also dull green, but they did

not shed.

Ammonium nitrate and IBDU appeared to increase the

dry matter content of nearly all tree component parts as

compared to the control in Experiment 1 (Table 2). A dif-

ference in dry matter content was particularly apparent in

the leaves collected from both of the fertilizer-N treat-

ments which were statistically greater than the control.

The NH4NO3-fertilized trees showed numerically greater dry

weight values compared to the IBDU treatment in the more

succulent components of the tree, but they were not statis-

tically different.

It is not plausible to compare the effect of N

sources on the growth of the tree parts in Experiment 2,

because of the initial tree size differences. Nevertheless,

there were differences (nonstatistical) particularly in

green twig dry weight indicating a greater response to

NH4NO3 fertilization as compared to the other treatments

(Table 2). Trees fertilized with NH4NO3 were the smallest

and yet had numerically greater green twig dry weight than

the rest. This trend was similar to the one in Experiment

1. Significant differences between treatments in fibrous











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and lateral roots, and the scion trunk in Experiment 2, but

not in Experiment 1, showed that these differences were a

reflection of the initial tree size differences in Experi-

ment 2 rather than treatment effects.

Irrigation level did not statistically influence the

distribution of dry matter or the N content of the various

tree fractions in Experiment 1 (Table 3). There were no

significant interactions between N source and irrigation

level. Data related to irrigation level effects, except

for those in Table 3, have been omitted from this study be-

cause irrigation level effects were not statistically dif-

ferent and there were no irrigation level x N source in-

teractions.

The dry weight of the total root system and the above-

ground parts of the tree in Experiment 1 were consistently

highest for the NH4NO3 treatment (Table 1), followed by the

IBDU and control treatments; however, the data did not show

any statistical differences. Nitrogen source also had no

statistically significant effect on total tree dry weight

or % trunk diameter increase. Nevertheless, N fertiliza-

tion resulted in larger % trunk diameter increase over the

control trees in both Experiments 1 and 2 (Table 1).

Trunk growth was greater in Experiment 2 than in Experi-

ment 1 presumably because of the beneficial effect of the

warm spring weather on tree growth. Total tree dry weight

in Experiment 2 did not appear to have been altered by


















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treatments presumably due to the short duration of the ex-

periment. It reflected the trend in the initial trunk di-

ameter measurements (Table 1).

Irrigation level had no statistically significant ef-

fect on the dry weight of the root or shoot system of the

trees in Experiment 1, or the dry weight and the % trunk

diameter increase of the tree. There was, however, a ten-

dency towards greater trunk growth with an increase in

irrigation level. Koo (55) reported a similar trend for

mature citrus trees in Florida.

Several factors may have contributed to the absence of

an irrigation effect in Experiment 1. These would have in-

cluded duration of the experiment, the quantity and fre-

quency of irrigation as well as the time of year. The

trees were irrigated only during the last 6 weeks of the

experiment. It is possible that irrigated trees did not

have enough time to respond to the irrigation treatments.

Furthermore, it has been shown in Florida that extra water

applied to mature citrus trees between June and December,

the approximate period of this study, did not contribute

substantially to tree growth (55).

The lack of significant N source effects on tree

growth in Experiment 1 can possibly be attributed to a di-

minished growth rate in winter (25). Also, the 14 weeks of

experimentation probably were not long enough for the

treatments to show their maximum effects on tree growth.









Patten and Domoto (78) and Miller (70) recently showed that

N treatments had no significant effect on apple tree growth

during the first growing season.

In Experiment 1, N source statistically altered the

pattern of dry matter distribution (Table 4). A greater

proportion of the total dry weight was located in the tap-

root of the NH4NO3-fertilized and control trees as com-

pared to the IBDU-treated trees. Both NH4NO3- and IBDU-

fertilized trees had statistically greater proportion of

their dry weight distributed in the crotch branches in com-

parison to control trees. The % dry weight of the scion

trunk of the NH4NO3-treated trees was less than that of

either the control or the IBDU-treated trees. The fact

that the dry weight distribution was similar to the actual

dry weight of certain tree parts (Table 2) makes it doubt-

ful that all of these were true treatment differences.

Moreover, treatment means in dry weight were not statis-

tically different except for the leaves. Some of the sig-

nificant treatment differences may very well be a reflec-

tion of initial differences in the trees. Nevertheless,

there was a trend for NH4NO3-treated trees to have a

greater dry matter in crotch branches, green twigs and

leaves in comparison to IBDU-fertilized and control trees.

As was the case with the dry weight of the tree fractions,

irrigation level did not significantly affect the distri-

bution of dry matter.













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The distribution of dry weight was similar in Experi-

ments 1 and 2 (Table 4). In Experiment 2, the control

trees had proportionally more of their dry weight in the

rootstock and scion trunks as compared to the fertilized

trees. The % dry weight in the green twigs of the control

trees was, however, significantly less than in the NH4NO3-

treated trees; but the greatest difference was in the leaf

fraction. The % dry matter in the leaves of the fertilized

trees was about twice as much as in the unfertilized con-

trol trees. Furthermore, the leaves of the fertilized

trees in Experiment 2 represented a higher % of the total

than in Experiment 1. This is a reasonable expectation be-

cause of the stronger growth flush in the spring than in

the fall-winter.


Tree N Content and Distribution

The main effects of the N sources in Experiment 1

showed that the N concentrations were highly significantly

different in all component parts of the tree, with NH4NO3

treated trees having the highest values followed by the

IBDU-treated and control trees (Table 5). Nitrogen con-

centration was generally highest in leaves followed by

fibrous and lateral roots, green twigs, taproot, crotch

branches, rootstock trunk, and scion trunk. This order is

similar to the data of Barnette et al. (5) for a mature

tree of a different cultivar. Furthermore, the N con-

centrations in the current study were higher than the







40


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values reported by Barnette et al. (5). Such differences

are not unusual when a young, vigorously growing tree is

compared to a mature, fruiting tree (99). Also, the rate

of N application in the current study was higher.

No significant differences due to irrigation level

were detected in the N concentration of the various tree

components- Table 3).

The results obtained in Experiment 2 were consistent

with those in Experiment 1 in that the order of N concen-

tration in all tree parts was NH4NO3- > IBDU-N > control

(Table 5). Except for rootstock and scion trunks, these

values were significantly different from one another at the

1% level. The N concentration for all tree parts was

higher in the NH4NO3-treated trees than in their counter-

parts in Experiment 1, particularly in the leaves, fibrous

and lateral roots, and in the taproot. It would not be un-

reasonable to assume that these relatively higher concen-

trations were partly a consequence of concentration ef-

fects, in view of the smaller trees assigned to the NH4NO3

treatment. Also, the higher soil temperature in the spring

could have been conducive to more N absorption by the

trees.

The total amount of N in a tree part, the product of

the dry weight of that part and the N concentration on a

dry weight basis, was not statistically affected by irri-

gation level in Experiment 1. There was, however, a

statistically significant relationship between N source and









total N content of tree parts (Table 6). Numerically, the

trend was similar to the N concentration in that NH4NO3-

fertilized trees had the highest values followed by IBDU-

fertilized and control trees. The data were, however, less

consistent statistically.

The N content of the tree parts was reflected in the

total amount of N in the tree (Fig. 2). Differences due

to N source were highly significant with NH4NO3-treated

trees containing about 3 and 1.5 times greater total N than

the control and IBDU-treated trees, respectively. No sta-

tistically significant differences were detected due to ir-

rigation level.

In Experiment 2, the total amount of N in each tree

part (Table 6) and consequently in the entire tree (Fig.

2) was lower than in Experiment 1 probably because of the

smaller trees used in Experiment 2. No meaningful impor-

tance can be attached to the statistical differences in

Experiment 2 in the total N content because of the inequal-

ity of tree sizes. Despite the fact that the trees selected

for the IBDU treatment were larger and had 1.6 times the dry

weight of the NH4NO3-treated trees, total N in the NH4NO3-

fertilized trees was 1.05 times that of the IBDU-fertilized

trees. Equivalent biomass, including the root system, would

absorb more N from NH4NO3 than it would from IBDU in a short

period as in Experiment 1.







43



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There was no significant relationship between the dis-

tribution of N in the various tree parts and irrigation

level in Experiment 1. Nitrogen source significantly af-

fected the distribution of N in the taproot and scion trunk

(Table 7). Regardless of N source, 30-34% of total tree N

was located in fibrous and lateral roots. Leaves repre-

sented only 10-13% of the total dry matter of the tree but

contained 30-33% of the total N in the tree. These figures

correspond to those reported in earlier work for mature

citrus trees (20, 63). Sixty, 57, and 55% of the total N

in the tree were found in the aerial parts of the control,

IBDU- and NH4NO3-fertilized trees, respectively. This may

suggest that the more readily available the N source, re-

sulting in luxury consumption, the greater the proportion

of N stored in the root system (86).

As in Experiment 1, N source significantly affected

the distribution of N in Experiment 2 (Table 7). Compared

to Experiment 1, however, the proportion of N was greater

in leaves and less in the fibrous and lateral roots of the

fertilized trees than the control. The proportion of N

in the taproot of NH4NO3-fertilized trees was higher than

for IBDU-fertilized and control trees in both experiments.

Whether it is a characteristic of citrus trees to accumu-

late NH4NO3-N in the taproot or some factor related to the

time of sampling needs further investigation. In contrast

to the data in Experiment 1, 57, 67 and 66% of the total N


1













>
O
0) I







0) I



E


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U 1
.) U
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U I
A I







(a)




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0






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. 01





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in the tree were found in the aerial parts of control, IBDU-

and NH4NO3-treated trees, respectively. The greater pro-

portion of total N in the aerial parts of the fertilized

trees in the spring than in the fall-winter may reflect the

more vigorous growth activity in the spring. Also, trans-

location of N from roots may be greater in the spring. The

difference probably resulted from a lower activity of the

nitrate reducing enzyme system during periods of low soil

temperature in the winter (38).


Soil Profile Content and Distribution of N

Soil samples in Experiment 1 were taken to the 45-cm

depth after 1.42 and 2.26 cm of cumulative rainfall on

October 3 and October 19, 1978, respectively. Analysis of

the first set of samples indicated that NO3-N from NH4NO3

had moved to this zone (Fig. 3-A). As a result the sam-

pling depth was extended to 60 cm. A third set of samples

was collected after 4.87 cm of cumulative rainfall on

October 31, 1978. After the initiation of the irrigation

treatments, a fourth set of soil samples was obtained on

January 9, 1979.

At the first sampling (Fig. 3-A), there was evidence

that some N03-N from NH4NO3 had moved down to the 45-cm

depth while NH4-N from the same source had only moved to

the 30-cm depth. A greater proportion of the NH4-N was

still retained in the 0- to 15-cm zone. Only a small

amount of NH4-N from IBDU had moved to the 30 cm depth




























Fig. 3. Effect of N source on the exchangeable NH and
NO3 soil content. A, B, and C correspond to the
1st, 3rd and final soil sampling dates on October
3, October 31, 1978, and January 9, 1979, respec-
tively. (Expt. 1).























\0-



\0-


.,,,,----0


15-30


CONTROL NO3-N
CONTROL NH4'N
IBDU NO3'N
1BOU NH4 N
NH4 NO3NO3"N
NH4NO3NH4*N


3045


a8





"-S
--0


'-S


------


;- -- --' -5-,,,
-'------5
--,r-
C--~____._~_;----- ---~--~t--


15-30


30-45


45-60


C






'----------------



0-15 15-30 30-45 45-60

DEPTH (cm)


0-15


0-15


afi


30-45









presumably because of the sparingly soluble nature of IBDU

which remained in the surface soil. Eighty-five percent of

the applied N from NH4NO3 was accounted for in the 0- to

45-cm depth at the first sampling. The difficulty of ex-

tracting IBDU with 1 N KC1 precluded the determination of

NH4 and NO3 in the 0- to 15-cm depth. The total N proce-

dure, however, indicated that 94% of the applied N from

IBDU remained in the 0- to 45-cm depth at the first sampling.

These trends in inorganic N concentrations were also

revealed in the soil total N content in that values for

the fertilized plots were higher than those for the control

plots (Fig. 4-A). Soil samples were not sufficient for the

determination of exchangeable NH4 and NO at the second

sampling. Nevertheless, there was an indication that N

from NH4NO3 had probably moved past the 45-cm depth (Fig.

4-A).

At the third sampling it was apparent that NH4-N and

NO3-N from NH4NO3 had leached down to the 45- to 60-cm

depth (Fig. 3-B). This suggests that at least a part of

the downward movement involved the un-ionized NH4NO3

fertilizer. The well-drained nature of Astatula fine sand,

and its low organic matter content could have been factors

contributing to this movement. Some NH4-N and NO3-N from

IBDU had also moved down to the 60-cm zone, but on a smal-

ler scale relative to NH4NO3. Ninety-one percent of


:
































Fig. 4. Effect of N source on soil total N at the 1st (A)
and 2nd (B) sampling dates on October 3 and Octo-
ber 19, 1978, respectively. Mean separation with-
in depth by Duncan's multiple range test, 1%.
(Expt. 1).









cumulative


A0 A
C 0*
-\e-
8--


* -
S------
S---


CONTROL
IBDU-N
NH4NO3-N


2.26 cm of


cumulative

B


rainfall


0


.0 2H


30-45


15-30
DEPTH, cm


.0 2



0


-.0
C,


.0 4


0-15


--


.0 6


1.42 cm


-


rainfall









the applied N from IBDU remained in the 0- to 60-cm depth

at the third sampling; 82% of the NH4NO3-N could be ac-

counted for in the same zone. Data for soil total N (Fig.

5-A), although less consistent in the 15- to 45-cm depth

than those for exchangeable NH4 and NO3, supported the

trend in the downward movement of N from both fertilizer

sources.

The final soil sampling was completed 14 weeks after

the trees were fertilized. Much of the N from NH4NO3 had

been depleted from the soil profile (Fig. 3-C). Only 30%

of the applied N from this source remained in the 0- to

60-cm zone. A part of the depletion could be attributed to

root absorption. Soil water contents (Table 9) were gener-

ally below the maximum water storage capacity; however,

rainfall events of 5.0, 1.3 and 2.6 cm on December 28,

December 29, 1978, and January 3, 1979, respectively,suggest

that leaching of some NH4NO3-N may have occurred beyond the

60-cm depth. Furthermore, provisions were not made in

this study for the determination of gaseous losses of N

which may have also occurred. As in the previous samplings

the concentrations of exchangeable NH4 and NO3 from IBDU

in the 15- to 60-cm depth were lower than those from NH4NO3

(Fig. 3-C). There was also no indication of IBDU-N having

been leached beyond the 60-cm depth since 66% of the applied

N could be accounted for within this depth. In comparison

to the other N sources, a greater proportion of the IBDU-N

was retained in the surface soil (Fig. 5-B).



























r r


(O 0 4
SH 4-1


4000
C Hl rl


4-J (I q)



0 f4 0 4-)
d 0 Q) P *
4J -r


j0 G
.I G.-
0 $ >i ( 4-J
4J 0 r-4 4 4J
O ) 4)


mdo-HM
S> U) r-

0 ) -H H



S 4 0r-4 U0


04 0

0 ( a t
U Q)
O fl
Od H
r-i -
4-4 -< a ra





0 >1 zri
Srd 1 -l



:3 34J E
(4-4 ZrdU)U








S- Q4
4-1 *~' r '
oE >i a









C n A(
















.^M










-_ Z 1 = # -(f>




O -


SO* -












'0 0*9


o 0
o 0




0*













Table 8. Quantitative estimates of
transpiration from mature


evaporation and evapo-
tree plots.


Period Rainfall & Class "A" pan Evapotrans- Potential
irrigation, evaporation piration net leach-
cm cm/day cm/day ing, cm
H ZOZ
H20z

Oct. 25- 4.87 0.39 0.22 +1.35
Nov. 9,
1978

Nov. 9- 2.63 0.32 0.23 -4.96
Dec. 12,
1978

Dec. 12, 18.40 0.27 0.13 +9.69
1978-
Feb. 17,
1979


ZExcess water (+) and deficit (-) to effect percolation.












Table 9.


Soil water content of young tree plots 24 hr.
after a rainfall or irrigation event.


Sampling Cumulative rainfall Soil water Field capacity
date and irrigation, cm (0-60cm), (0-60cm),cm
cm


Oct. 3, 1.42 3.91 4.18
1978

Oct. 19, 2.26 3.47 4.18
1978

Oct. 31, 4.87 3.77 4.18
1978

Jan. 9, 25.12 4.00 4.18
1979










In Experiment 2, sets of soil samples were taken 3

times on April 2, April 27 and May 4, 1979, following 2.28,

9.14 and 17.87 cm, respectively, of cumulative rainfall

and irrigation. These samples were not sufficient for the

determination of exchangeable NH4 and NO Soil total N

concentrations in this experiment were higher than those

from Experiment 1 (Figs. 6-8). Contamination from the

regular fall (1978) and spring (1979) fertilization of

adjacent mature trees may have contributed to the higher

soil values in Experiment 2 as compared to Experiment 1.

As in Experiment 1, however, there was a trend towards

decreased soil N content with increasing cumulative rain-

fall and irrigation (Figs. 6-8).

At the first sampling there was an indication of

some NH4NO3-N having moved to the 60-cm depth (Fig. 6),

while only a small amount of IBDU-N may have moved to the

30-cm depth. Some N from IBDU appeared to have reached

to the 45-cm depth as indicated by the bulge at the second

sampling (Fig. 7), but the data from Experiment 1 (Fig. 3B-

C) suggest that this could be a sampling error or error

inherent in the soil total N procedure. As in Experiment

1, a higher proportion of the N from IBDU, relative to the

other sources, was still retained at the surface soil at

the last sampling (Fig. 8).






60



C x
o0 0



04
me>







CD
CA M










SI I I- >






--J 0
0 u





0-
4E
0 --,













0 0 0 0
C /0 0 N O
I w

1'00





0 a%
co 4-4 r-
0 0 I 0
C-) o 0 oa2











4-U-





iM A8Q % 'N I- lS
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-' / <" "






61


0

oz Q



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It oo





e 0 2 "g





o
M CM





d,

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00 o-




4n

/ (Q0


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rO





S0 3















.LM A80 % 'N lb0S




































































0 0


1M AdO % 'N IDOS


E
C-



I-
Q-
LU


o X
O M


0 n











, -4
a,4




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OU





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4





ow
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i-q
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u -r-


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Recovery of Fertilizer-N in the Soil-Plant System

The recovery of fertilizer-N in the soil profile and

in the tree was calculated by subtracting the N content of

the control from the fertilized plots and expressing it as

a percent of the amount of N applied. For the NH4NO3 treat-

ment in Experiment 1, both soil total N and inorganic N were

used in the calculations. Data for the inorganic N procedure

indicated that, after 14 weeks, 30% of N apparently remained

in the 0- to 60-cm soil profile where NH4NO3 had been applied

(Table 10). Sixty-six and 41% of N from the IBDU and NH4NO3

sources, respectively, remained in this zone when the cal-

culations were based on soil total N values. The discrep-

ancy in the NH4NO3-N figures could have resulted from the

larger errors inherent in the total N procedure as compared

to the inorganic N procedure (12, 13).

In Experiment 2, where the initial soil total N con-

tents were higher as compared to Experiment 1, the total

amount of N in the 0- to 60-cm depth was also greater

(Table 10). On the basis of total N calculations, 70 and

62% of the applied N were present in the soil from IBDU and

NH4NO3 sources, respectively. Higher soil N recoveries in

this experiment in comparison to those in Experiment 1

probably represented the difference in duration of each

experiment, 6 and 14 weeks, respectively.

The relatively high N recoveries from soil where IBDU

had been applied in both experiments suggested very little,













00

i n
[^


I z

0 -1





O1 Z
0
E-4


r~-
Ln

l l 0


0

I L
(D


'.o



l r


N





0




a)
>




04

H


a)
i-l



*d

r-1
a)

a
ni


\ 6
!m 0
m co
(N
n>CO
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r"-

1 0






I 0


oI I




(N 'D Ln













~O

.0 N N
0 0



O H H
0 0






H 1


I I I



en *03
O r6
I I






I I oI
I I I


rl
N
H


0
r-i
I*


1
l i
H-


I I I
I II


00
0
in


00






00


rl






z



z


\ g

mco

'.0(N
* oo
y? <-


m
0,

a)
N3


0,


I l
3'>


I I I


I

lo N

I 0 (N



Ill


I I I
I I I


I to


r~>
In

I 0
| |


a3

| L





I 0
W


i-l
1 0


Ln

11 \o

Sr-i.
O H

Ln LA
LO 0


r--
(N

in

00






N
(N
%r'
rl



0
(N


LO









in



Z





0
z
c^


U
0c


0






H-
a
0









0
i-I


)
N

*4
ro


*,1



to




N
H,

4J
(1)


4-1





ro










00
c0r


















0





O'
row
rl




















ri )
i-(,
kr








+) 4-
o


1-1
0 Z
-4 I








"O
C
o a
u H



\ U



00
(N 0\


r-
r1 (










if any, leaching loss of N from this source. Some leaching

of N from NH4NO3 may have occurred. Other complex processes

involved in N reactions, including denitrification and other

gaseous losses, may have contributed to lower soil recover-

ies of N from NH NO3 relative to IBDU.

The recovery of N by the tree was calculated for the

total area fertilized (90,000 cm2); but as the root sys-

tem occupied a circular area of less than 60 cm diameter

(2828 cm2), N recovery was also calculated for the latter

area. In Experiment 1, on the basis of the smaller area,

25 and 45% of the N added were recovered in those trees

fertilized with IBDU and NH4NO3, respectively. It was only

0.79 and 1.42% for the same respective sources when the

larger area was the basis of calculation. The differences

in N recoveries from the 2 areas imply a higher efficiency

of N use if the fertilizer is applied on a smaller area

around the tree rather than on a larger one where the po-

tential for N loss may be great.

It has been suggested that plant recovery of N by

the difference method generally yielded higher values com-

pared to the tracer procedure, presumably because ferti-

lization may stimulate additional N uptake from soil

sources (109). This might have contributed to the high N

recoveries, particularly where NH4NO3 was the source. The

higher tree recovery of NH4NO3-N appeared to be related to









the ready availability of this fertilizer in comparison to

IBDU, a greater proportion of which was retained in the

surface soil.

On the basis of the smaller area calculations, trees

fertilized with IBDU and NH4NO3 in Experiment 2 recovered

only 15 and 16%,respectively of the applied N (Table 10).

Recoveries from the respective sources were 0.47 and 0.52%

for the large area. The lower tree recovery of applied N

in Experiment 2 vs. Experiment 1 was partly related to the

smaller trees used in the former experiment and partly be-

cause the experiment was run only for 6 weeks. The trees

did not therefore have enough time to absorb N to the same

extent as in Experiment 1. Also, the narrow range in the

recovery of applied N seemed to be due to the fact that

IBDU-fertilized trees were larger than NH4NO3-fertilized

trees. If NH4NO3-fertilized trees had been of equal size,

the difference in the recovery between the 2 fertilizer

sources could have been larger.

A common approach for comparing the relationship of

fertilizer sources with mineral nutrient absorption by

plants in the vegetative growth phase is to relate the to-

tal dry matter of the plant to its total nutrient content,

i.e., dry weight/total nutrient ratio. The lower this ratio,

the greater the nutrient absorption from that source. As-

suming the ratio is linear with time, the N status of the

trees was projected by plotting the ratio data for
































0






0

4-4
CO














v-, -I
0
M





















0







H


4-)


0O


*0

-a




a4
(U*

*Hr
+i
(0 -l-
r- cl
a) ;


















*r




68





\\



\i










0 0 0i



0 c0 0 o
N / IM A80 OIVd









Experiments 1 and 2 (Fig. 9). Theoretically, up to 26

weeks after fertilizer-N application NH4NO3-N would con-

tinue to be absorbed more than IBDU-N, but after 28 weeks

the reverse would be expected. There is evidence that the

period of extended availability from soluble N sources,

after the initial flush of growth of vegetable crops,

approaches that of the more slowly available N sources

(115). In view of the extremely soluble nature of NH4NO3,

however, most of the N will have probably been depleted

from the soil after 26 weeks.

Total apparent N recovery in the soil-plant system in

Experiment 1 amounted to 76 and 91 for NH4NO3- and IBDU-N,

respectively (Table 10). The total N procedure indicated

a higher recovery of N from the NH4NO3 plots in comparison

to the soluble N procedure. The apparently variable total

N recovery was probably due to the larger errors associated

with the use of the total N procedure in soil analysis vs.

the soluble N approach (12, 13).

In Experiment 2, where only soil total N was analyzed,

85 and 78% of the added N from IEDU and NH4NO3, respec-

tively, were apparently recovered in the soil-plant system

when the small area was the basis of calculation. It was

not possible to account for the deficit in N balance in

both experiments. Nevertheless, soil N data in Experiment

1 (Fig. 3A-C) and rainfall events of 5.0, 1.3 and 2.6 cm with-

in a 6-day period imply that some leaching of N, at least from









NH4NO3, may have contributed to the deficit. Processes

involved in the N cycle, including denitrification and

other gaseous losses of N may also have been reflected in

the unaccounted for part of the balance.



Experiment 3


Nitrogen Distribution in Bearing Orange Trees

Fruit yield and estimate of leaf number

Each of the control, IBDU- and NH4NO3-fertilized trees

showed fresh fruit yields of 0.7, 3.7 and 3.7 field boxes

of 40.8 kg, respectively (Table 11). These data indicated

that yields can be severely limited by a lack of N. Ni-

trogen-fertilized trees had about 4 times as much total

fruit dry weight as the control tree. The unreplicated

nature of the experiment, however, precluded the establish-

ment of definite conclusions pertaining to the yield data.

Nevertheless, it is generally accepted that increased N

supply results in increased citrus fruit yield. Data from

a Florida experiment (87) with 'Pineapple' orange showed

that as the rate of N was increased, up to 202 kg/ha/year,

yield increased.

The estimated leaf number of the fertilized trees was

in reasonable agreement with the 70,200 leaves estimated

by Barnette et al. (5) for a grapefruit tree in Florida,

but lower than the 92,708 leaves sampled from a 12-year-old

'Valencia' orange tree in California (110). Trees in the













E >1

Z 0


2 0





r--
4- t07
4J (1
0 (1
E-4 -4 Z


41


0
O
0 dP


0


4Ja
0 3
41 a)
1q


I -H U
a) 4J





> -





0-


3 .P
ro yo





4o) 1 ,


N
0} 4-)
a) *d

0 4
S4-4


0

Ln

r-4



0
0


CN

















CO
r-




In


0
In




in
o







o


a)
N
*-l

4J


4-1)
0
S -1Q


a)
N
*-4
-,-
I --I

OQQ


0

I -4

0 -1
Z 4-










California study were, however, larger as indicated by

their height and width. The control tree in the current

study had a lower leaf number than either of the fertilized

trees by virtue of a lesser leaf density which can be at-

tributed to a lack of N.

Fruit and leaf N content

Several approaches have been used to determine citrus

fertilizer needs. One school of thought advocates return-

ing to the soil the amount of nutrients removed in the crop

of fruit. Embleton et al. (33) considered this approach

unsound, particularly with respect to the less mobile nu-

trients. Nevertheless, because they represent part of the

N balance sheet, data related to crop removal of N when

combined with leaf analytical data have a significant po-

tential for guidance in the N management of citrus groves.

The concentration of N in the fruit from the ferti-

lized trees was higher than that of the unfertilized trees

(Table 11). As a result, when both total fruit dry weight

and N concentration are considered, the amount of N re-

moved in fruit was in the order of NH4NO3- > IBDU-fertilized

> control trees. Five times more N was removed in fruit

from fertilized trees as compared to the control tree.

Reitz (89) reported that in Florida 36.3 kg of N are re-

moved in every 20,400 kg of fresh fruit. Recalculation

of the data in Table 11 indicates that 32.2 and 29.9 kg

of N would be removed in 20,400 kg of fruit from the NH4NO3-

and IBDU-fertilized trees, respectively.









Leaf N concentration followed the same trend as in the

fruit. Nitrogen concentration for the 3 single-tree plots

was in the order: NH4NO3- > IBDU-fertilized > unfertilized

trees. This clearly reflected the more rapid absorption of

NH4NO3-N over IBDU-N and the control. The total amount of

N in the leaves was, however, slightly larger for the IBDU-

treated tree because of its greater leaf number. It is

doubtful if the greater leaf number is related to IBDU

fertilization since the duration of the experiment was

short. The total amount of N in the leaves of the fertilized

trees was in agreement with the 200 g of N reported by Cam-

eron and Appleman (20) for a 10-year-old 'Valencia' orange

tree. This close agreement lends added credence to the pro-

cedure used in this study for estimating the leaf number.


Soil Total N Content and Distribution

Fertilized fallow plots received 33% (235.7 g/6 m dia-

meter area) of the total annual amount of fertilizer-N ap-

plied to the fertilized tree plots (707.2 g/tree). Three

sets of soil samples were collected: on November 9, December

11, 1978, and February 16, 1979. These sampling dates corres-

pond to cumulative rainfall and irrigation of 4.87, 7.50 and

25.90 cm, respectively, since October 25, 1978. Samples were

not sufficient for the determination of exchangeable NH4 and

NO3; hence, only the total N procedure was used.

At the first sampling, some N from both IBDU and NH4NO3

appeared to have moved at least down to the 45-cm depth in









the fallow plots (Fig. 10). The amount of NH4NO3-N

leached, as reflected by the lower concentration in the 0-

to 15-cm depth and higher concentration in the 15- to 45-

cm zone, was greater than that of IBDU-N. In the ferti-

lized tree plots, NH4NO3-N appeared to be depleted rapidly

from the 0- to 15-cm zone, while a considerable amount of

IBDU still remained in this zone (Fig. 11). The apparent

rapid depletion of NH4NO3-N from this zone could be attri-

buted to root absorption in view of the readily available

nature of NH4NO3 fertilizer. Higher N concentrations, par-

ticularly for the IBDU treatment, in the 75- to 105-cm

depth, as compared to the control, could have been due to

the residual N from previous applications.

Data for the second sampling date (Fig. 12) suggest

that some N from both fertilizer sources has leached to the

120-cm depth in the fertilized plots. Soil water contents

(Table 12) and evapotranspiration data (Table 8) also tend

to support this. Since the movement of N, particularly

NO3-N, in uncropped soils is closely related to water move-

ment in the profile (106), it is likely that some N leached

past the 120-cm depth. In the mature tree plots, there was

evidence that some N had moved at least to the 75-cm

depth when NH4NO3 was the fertilizer applied (Fig. 13). It

is difficult to trace the movement of N in the soil pro-

file in the presence of a tree, however, because of root

absorption.-






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As in Experiments 1 and 2, soil total N content gener-

ally decreased with time in the tree plots partly because

of tree absorption of N. At the last sampling, almost all

of the applied NH4NO3-N in the fallow and tree plots had

dissipated from the 0- to 15-cm depth, but considerable

IBDU-N remained (Figs. 14 and 15).


Recovery of Fertilizer-N in the Soil, Fruit and Leaf

The recovery of applied N in the soil, fruit and leaf

was calculated by the difference procedure used in both

Experiments 1 and 2. Compared to the young tree experi-

ments, however, less confidence can be expected in the N

recovery data for Experiment 3. The absence of replica-

tions in the mature tree experiment precluded the estab-

lishment of plausible conclusions. Furthermore, since the

trees had been fertilized for several years it is diffi-

cult to separate the effect of previous fertilization on

N contents of tree parts and the soil. The use of an

empirical procedure in the estimate of leaf total N and

the large sampling and analytical errors associated with

the soil total N procedure are additional factors which

reduce the confidence in the N recovery data.

Nevertheless, the N recovery data (Table 13) may serve

a useful purpose in stimulating additional research. The

proportion of applied N recovered in the 0- to 120-cm soil

depth of the IBDU- and NH4NO3-fertilized mature tree plots

was 44 and 30%, respectively. The fruits accounted for

27 and 30% of applied N, and the leaves, 11 and 10%












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Table 13.


Apparent applied N recovery (%) in mature tree
plots.z


Treatment Fruits Leaves Soil Total


IBDU-fertilized 27.60 10.77 44.00 82.77
tree

NH4NO3-fertilized 30.08 9.85 29.55 69.48
tree


ZCalculated from the difference in N content, for each N
balance component, between the control and fertilized tree
plots and expressed as the % of the amount of N applied.










for the same respective sources. The apparent total re-

covery of applied N when fruits, leaves and the soil are

considered amounted to 83% for the IBDU treatment, and 69%

for the NH4NO3 treatment.

It is suggested that the deficits in the N balance

could be partly accounted for in the permanent structures

of the tree. Smith (100) estimated that 31% of the total

N was found in the leaves of 15-year-old 'Valencia' orange

trees in Florida. On this basis, control, IBDU- and NH4NO3-

treated trees in this experiment would contain 402, 648

and 627 g of N, respectively. It was also indicated that

42% of the total N of the tree was found in parts other

than leaves and fruit (100). On the basis of this assump-

tion 13 and 12% of the applied N would be expected to be in

tree parts other than fruits and leaves for IBDU- and NH4NO3

treated trees, respectively. When these figures are added

to the measured recovery of applied N, 95 and 81% of ap-

plied N from IBDU and NH4NO3 sources, respectively, could

be accounted for in the soil-plant system. No account

could be made for the remaining 5-19%.

These apparently high recoveries of applied N may in-

dicate that the trees had absorbed some N already present

in the soil in addition to that applied in this study.

Also, substantial leaching of N may not have been signifi-

cant. Although the potential for the leaching of N exis-

ted (Table 8), the large root systems of the mature trees









may have absorbed a greater portion of the applied N as

it moved in the soil profile.

In view of the low nutrient retaining capacity of

Florida soils planted to citrus, it has often been sugges-

ted that the efficiency of N utilization by citrus is very

low (25-30%). In the current study, however, higher re-

coveries for both young (45%) and bearing (40%) trees are

reported when NH4NO3 was the N source. These high recover-

ies imply that citrus trees may be absorbing a greater pro-

portion of fertilizer-N than previously suspected. The

unreplicated nature and short duration of the mature tree

experiment, however, call for additional adequately repli-

cated experiments. Moreover, it would be beneficial to in-

vestigate the N balance of young citrus trees over a period

longer than the ones used in this study.














CONCLUSIONS


1. Growth parameters of young citrus trees in short-term

experiments, trunk diameter increase, and dry weights

of tree component parts and the entire tree, showed no

statistical differences due to N sources. There was

a trend, however, for N from IBDU and NH4NO3 sources to

result in greater dry matter, particularly in the more

succulent aerial parts, when compared to the control.

Leaves from fertilized trees in both fall-winter and

spring studies were statistically greater in total dry

weight than those from the unfertilized leaves. The

apparent lack of statistical differences in growth

parameters due to N source was probably related to the

short duration of the studies. The trees did not have

sufficient time to respond to the treatments.


2. The dry weights of certain tree components as a percent of

the total plant were statistically different due to N

source, but it is doubtful if they were true differ-

ences because the pattern of dry matter distribution

paralleled that of the dry matter. These differences

were probably the result of selective use of different

size trees for each N source.









3. There was a highly significant relationship between N

source and N concentration and total N of component

tree parts, and total N content of the entire tree.

In every case, the order was NH4NO3- > IBDU-fertilized

> control trees. This confirms that a soluble N source

such as NH4NO3 is more readily absorbed by the trees

in the short-term than a controlled release form of N.

Other investigations, however, suggested that the ab-

sorption of N from both fertilizer sources might be

different in longer periods.


4. Regardless of N source, 30-34% of the total tree N was

distributed in fibrous and lateral roots, and 30-33% in

the leaves. This illustrates the importance of recover-

ing the entire root system as much as practical in

studies of this nature.


5. A greater proportion of the total N in the fertilized

tree was distributed in the aerial parts in the spring

study than in the fall-winter study, indicating that

N absorption and translocation to the aerial parts was

greater in spring.


6. Data related to tree growth and N content parameters,

and soil total N content did not reveal any statistical

differences due to irrigation level in the fall-winter

study. The narrow range and short duration of irriga-

tion treatments were probably not sufficient to elicit

responses to the treatments.









7. Nitrogen balance data, based on the 2828 cm2 plot area,

showed that total apparent N recovery in the soil-

plant system was 91 and 76% for IBDU- and NH4NO3-N,

respectively. Sixty-six and 30% of applied N from

IBDU and NH4NO3 sources, respectively, were retained

in the 0- to 60-cm soil profile. Twenty-five and 45%

from the same respective sources could be accounted for

in the young trees. The remaining 9-24% was presumed

to have been leached or lost to the atmosphere.


8. In the bearing tree study, no difference was detected

in fruit yield between IBDU and NH4NO3 sources. Fer-

tilized trees, however, had about 4 times as much

yield as the unfertilized tree.


9. The relatively high recovery of applied N in young and

bearing trees suggest that substantial leaching of N

may not be a major route for N loss from citrus groves

established on deep-well drained soils when the ferti-

lizer is applied over the root zone at reasonable

rates.


10. A major portion of IBDU-N was retained in the 0- to

15-cm depth, yet the growth response of the IBDU-

fertilized young trees was similar to that of NH4NO3-

fertilized trees. There is, therefore, a potential for

a single application of IBDU to nonbearing young citrus

trees which are normally fertilized several times a


I






90


order to establish if a single application of IBDU is

feasible from the standpoint of economy and horticul-

tural considerations.














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