University of Florida
AGRICULTURAL EXPERIMENT STATIONS
J. R. Beckenbach, Director, Gainesville
in cooperation with
U. S. DEPARTMENT OF AGRICULTURE
INTRODUCTION -- ...... .. ... 3
RECOMMENDATIONS FOR BEARING CITRUS TREES.--.- 4
Time of Fertilization -.- ...... .. .. 4
Amount of Fertilizer to Use .- -- 4
Elements to be Included in the Fertilizer 5
pH Control ------ .--.--.--.. 13
Calculation of Fertilizer Rates and Analyses 13
RECOMMENDATIONS FOR NON-BEARING AND
YOUNG BEARING CITRUS TREES 16
LEAF AND SOIL ANALYSES 18
Soil Analysis ......------------ 19
Leaf Analysis -- ........ 21
RECOMMENDED READING .. .. .. 23
A cooperative contribution from the Citrus Experiment Station, Lake
Alfred, and the U. S. Horticultural Station, U. S. Department of Agricul-
ture, Orlando, Florida.
H. J. Reitz, C. D. Leonard, Ivan Stewart
R. C. J. Koo, D. V. Calvert, and C. A. Anderson
Citrus Experiment Station
P. F. Smith and G. K. Rasmussen
U. S. Horticultural Station
Recommended Fertilizers and
Nutritional Sprays for Citrus
H. J. Reitz, C. D. Leonard, Ivan Stewart, R. C. J. Koo,
D. V. Calvert, C. A. Anderson. P. F. Smith, and G. K. Rasmussen
Most Florida citrus is grown on sandy soils which are ex-
tremely low in natural fertility. Such soils have so low an
exchange capacity that they can retain only small amounts of
applied exchangeable plant nutrients against the leaching action
of rainfall. Other applied plant nutrients may be held in varying
degrees of availability in the soil by other mechanisms. For
these reasons, citrus trees must be fertilized with some elements
abundantly and regularly and with others only infrequently to
obtain high production of good quality fruit.
Citrus will thrive under a wide range of nutrient levels, and
it is impossible to outline a single fertilizer program that is best
for all conditions. A wide variety of programs is now being used,
and many result in high yields of good fruit. The program rec-
ommended here recognizes the existence of differences in grove
conditions, including soils, rootstocks, scions, ages of trees,
previous programs of fertilization, insect and disease control,
and other factors.
The amount and quality of the fruit crop in any one year
are not determined by the ratio of elements or the rates used in
any single fertilizer application or even in a single year's pro-
gram. They are influenced by all cultural practices previously
carried out, and environmental conditions of the past several
years. These include insect and disease control, irrigation, soil
conditions, weather, and other factors in addition to fertilizer
Continued research and development in the field of citrus
nutrition make it desirable to issue new recommendations every
few years. Recommendations given in the following sections are
intended primarily for average sandy, well drained soils low in
organic matter, such as the Lakeland and Blanton sands, and
for normal, healthy trees. For lowland and calcareous soils,
Florida Agricultural Experiment Stations
modifications are presented according to available information.
For bearing groves under 10 years of age, see the section
"Recommendations for Non-Bearing and Young Bearing Citrus
BEARING CITRUS TREES
Time of Fertilization
Both research and grower experience have shown that
either two or three applications of fertilizer per year are satis-
factory under normal conditions in all citrus growing areas.
Applications may be made in fall (October-December), winter
(January-February), and late spring (May-June), or any two
of these times. Timing of fertilizer applications is less impor-
tant than the total amount used per year. The method chosen,
however, should be followed consistently, as frequent changes
may adversely affect the bearing habit of the tree.
Amount of Fertilizer to Use
The best single guide for determining the amount of ferti-
lizer to use on bearing trees is a carefully made estimate of the
grove's production capacity, based on tree size and yield records
of the past several years. Experimental results also indicate the
maximum amounts of some elements that can be profitably ap-
plied per acre. Leaf and soil analyses are also helpful guides
(see below), and can be used along with a knowledge of past
fertilizer practices, to improve precision over standard recom-
mendations. The quantities recommended in this bulletin are
based on production capacity in terms of field boxes (about 90
pounds of fruit) and are expressed in Table 1 on the basis of
both yield per tree and yield per acre. These rates allow ample
supplies of the nutrients required for new growth, possible yield
increases, variations due to rootstock, and variety differences.
Grapefruit and oranges require about the same amount of ferti-
lizer on a per-acre basis, but different amounts on a per-box
The rates recommended in Table 1 are applicable in a
majority of cases, but both young bearing and extremely high
yielding groves may require special consideration. The lowest
Fcrtiier.N (mid tifritioiial Sprai~i.s for Citrits
amounts recommended apply primarily to normal young bearing
groves still developing productive capacity through tree growth.
They also apply to older low-producing groves. Neither of these
kinds of groves should receive less than the minimum amounts
shown in Table 1, regardless of the yield. These minimum
amounts are ordinarily necessary to maintain the tree in good
condition. The intermediate ranges in Table 1 cover the normal
variation in productivity of groves. Except for a very few ex-
ceptional groves, it will not be advisable to exceed the maximum
amounts shown in Table 1. Extremely high rates under some
conditions, such as drought and heavy metal toxicity (primarily
excessive copper in the soil), may reduce yield. Trees that have
lost substantial proportions of their tops due to freezes will need
less fertilizer than before the damage. Trees only defoliated, by
freezes, hurricanes, or attacks by pests, may temporarily need
more fertilizer than before. Rates and timing may need to be
adjusted to moisture supply, including both rainfall and irri-
gation. No set of rules will ever eliminate the need for skill and
insight on the part of the grove operator.
Elements To Be Included In The Fertilizer1
Research has shown that citrus requires 15 different
chemical elements for healthy growth. Twelve of these must
come from the soil or from applied fertilizers, soil amendments,
or nutrient sprays. They are nitrogen (N), phosphorus (P),
potassium (K), magnesium (Mg), manganese (Mn), copper
(Cu), zinc (Zn), calcium (Ca), sulfur (S), boron (B), iron (Fe),
and molybdenum (Mo). All 12 of these essential elements are
applied to some Florida citrus groves. The other three, carbon,
hydrogen, and oxygen, are provided by air and water. Chlorine
(Cl) is generally believed to be required for plant growth, but
it has not been shown essential for citrus.
Nitrogen.-The percentage of nitrogen is the first figure of
a fertilizer formula.
Nitrogen is the key fertilizer element in yield and has a
pronounced effect also on the growth and appearance of the tree.
It leaches readily from the soil, so very little reserve can be
built up, and regular applications are required to maintain an
adequate supply to the tree. Many sources of nitrogen are satis-
1For general information on fertilizer materials and mixtures, see Fla. Agr.
Exten. Bul. 177, "Know Your Fertilizers".
Table 1. Pounds of fertilizer mixture or material to be applied to furnish
the annual nitrogen requirement of oranges under average
grove conditions (3/4 of these amounts is enough for grape-
Fertilizer for Fertilizer for
Nitro- each application with each application with
gen in Yield 2 applica- 3 applica- Yield 2 applica- 3 applica-
fertilizer tions/year tions/year tions/year tions/year
(Percent) (Boxes) (Pounds) (Pounds) (Boxes) (Pounds) (Pounds)
8 4 or less 10.0
8 5 12.5
8 6 15.0
8 7 17.5
8 8 20.0
8 10 or more 22.3
10 4 or less 8.0
10 5 10.0
10 6 12.0
10 7 14.0
10 8 16.0
10 10 or more 17.9
12 4 or less 6.7
12 5 8.3
12 6 10.0
12 7 11.7
12 8 13.3
12 10 or more 14.9
16 4 or less 5.0
16 5 6.3
16 6 7.5
16 7 8.8
16 8 10.0
16 10 or more 11.2
20.5 4 or less 3.9
20.5 5 4.9
20.5 6 5.9
20.5 7 6.8
20.5 8 7.8
20.5 10 or more 8.7
33.5 4 or less 2.4
33.5 5 3.0
33.5 6 3.6
33.5 7 4.2
33.5 8 4.8
33.5 10 or more 5.3
6.7 288 or less
14.9 720 or more
5.3 288 or less
11.9 720 or more
4.4 288 or less
9.9 720 or more
3.3 288 or less
7.4 720 or more
2.6 288 or less
5.8 720 or more
1.6 288 or less
3.6 720 or more
Rates are expressed on both a per-tree and per-acre basis for either two or three ap-
plications per year based on 0.4 pound of nitrogen per box average yield or estimated
bearing capacity. To use the table, select the appropriate section in the left hand column
representing the nitrogen content of your fertilizer and read down to the line representing
the yield of your trees. Use the amount shown per application.
**In most cases do not use less than 115 pounds or more than 250 pounds of nitrogen
per acre per year.
Fertilizeers aicd Nutritional Sprays for Citrius
factory for citrus, provided pH control is practiced and no
harmful contaminants are carried in the nitrogen material.
On average normal bearing orange trees, apply 0.4 pound
of total N per year per box of fruit. On grapefruit, apply 0.3
pound per box per year. For example, on the per-box basis, a
six-box orange tree would require 2.4 pounds of N per year or
24 pounds of a fertilizer containing 10 percent nitrogen. On the
per-acre basis, a grove producing 350 boxes of oranges per acre
would require 140 pounds of N or 1400 pounds of 10 percent
nitrogen fertilizer per year. Most Florida groves need no more
than 200 pounds of N per acre per year, and it seems fairly safe
to conclude that few Florida groves are apt to show benefit
from more than 250 pounds N per acre per year, no matter how
large the yield. Exceptional groves may respond to more than
this, but in many cases yield may be reduced.
On finer textured soils, such as in the Indian River area,
use the same rates as above, but only up to a maximum of 150
pounds of total nitrogen per acre per year. Proportionate in-
crease in rate of fertilizer application should be made to com-
pensate for any fertilizer thrown into the water furrow in
bedded groves. Application of 0.5 to 0.6 pound of N per box of
fruit to oranges may increase yield significantly, but use of
these higher rates is frequently associated with smaller, coarse,
green fruit. Where good color is of special importance, as in
fresh fruit trade, use of less than 0.4 pound of N per box may
Phosphorus.-Phosphorus is expressed as P2O. equivalent
(called available phosphoric acid) on the fertilizer tag. It is
applied as ammoniated, ordinary, or triple superphosphate.
Phosphorus accumulates in acid soils in an available form. The
average crop of fruit removes about 20 pounds P2O.- (one ferti-
lizer unit) per acre. Many older groves contain adequate
amounts of available phosphorus in the soil to supply this de-
mand for many years. Thus, mature citrus trees (over 20 years
of age) growing on soil which has been fertilized with phos-
phorus for several years will produce high yields of good quality
fruit without further phosphorus additions.
Younger groves that are just coming into full bearing (11
to 20 years of age) should occasionally receive some phosphorus
to replace that removed by the crop and for the benefit of the
cover crop. This need may be met by using 0.2 pound of P2O0
Florida Agricultural Experiment Stations
per box of fruit every fourth year. For example, using 8-4-8
type of mixture every fourth year and 8-0-8 three years out of
four will satisfy this need.
Potassium.-Potassium is expressed as K2O equivalent
(called potash) on the fertilizer tag. It is usually supplied as
muriate of potash or sulfate of potash. On early and midseason
orange trees, apply 0.4 pound, and on late season oranges and
grapefruit, 0.3 pound of potash per year per box of fruit, up to
a maximum of about 250 pounds per acre.
On calcareous or marl soils, apply about one-fourth more
potash than is recommended above.
On acid soils, excessive potash fertilization of the Valencia
orange may produce large, coarse, and poorly colored fruit, and
irn grapefruit particularly, high acid levels in the juice. Lack of
potash, regardless of soil type or citrus variety, usually results
in small-size fruit. Too much potash tends to lower the uptake
of magnesium by the tree.
Magnesium.-Magnesium is expressed as MgO (magnesium
oxide) equivalent on the fertilizer tag. The form applied in the
fertilizer should be water soluble and is usually the sulfate. In
addition to water soluble magnesium applied in fertilizers, citrus
trees obtain magnesium from dolomite (a combination of mag-
nesium and calcium carbonates) commonly used for pH control.
Where dolomite is used to control the soil pH at 5.5 to 6.5, appli-
cation of 0.1 to 0.2 pound MgO per box per year is usually suffi-
cient. If high calcium limestone is used for pH control, or if the
trees are on calcareous soil, apply 0.2 to 0.3 pound MgO. If leaf
symptoms of magnesium deficiency appear, the amount of sol-
uble magnesium applied in the fertilizer should be increased up
to twice the standard rate until the symptoms are no longer
present in mature leaves of subsequent flushes.
Manganese.-Manganese is expressed as MnO (manganese
oxide) equivalent on the fertilizer tag. Manganese may accumu-
late in available form in acid soils as a result of repeated appli-
cations when the pH is maintained in the recommended range,
so that the application of manganese is not necessary in many
groves. Manganese applications may be omitted where manga-
nese deficiency symptoms are absent. Also, groves showing mild,
transitory symptoms of manganese deficiency on young foliage
are common in Florida, but studies have shown that these symp-
Fertilizers and Nutritional Sprays for Citrus
toms are not related to fruit yield or quality. In groves on acid
soils where symptoms persist, apply 0.03 pound of MnO (pre-
ferably in the form of MnSO4) per year per box of fruit, or use
a manganese foliage spray (see below).
On calcareous soils, manganese should not be used in the
fertilizer, but applied as a foliage spray. In the spray use 3
pounds of manganese sulfate and 0.1 pound of hydrated lime
per 100 gallons or its equivalent in neutral manganese materials.
For further details on application of manganese in foliage
sprays, see Table 2, and consult the current "Better Fruit Pro-
gram, Spray and Dust Schedule''.
Table 2.-Pounds of copper, zinc, and manganese compounds to equal the
standard dosage* per 100 gal. of water*.
Metallic content shown on label ( )
24-27 34-36 48 52-56 75 80 85-90
Copper 3.0' 2.2 1.7 1.4 1.0 0.9
Zinc 3.0 2.0 1.4 -
Manganese .0 1.7 1.3 0.9
The standard dosage based on the metal content per 100 gal. is 0.75 lb. for copper,
1.0 lb. for zinc, and 0.75 lb. for manganese.
** For concentrate sprays, multiply the pounds required by 6.
t In making up sprays containing these soluble sulfates of zinc, copper, and/or man-
ganese, the fi.llo ing D rocedutl' s sh( uld he followed: Dissolve the sulfates first by dusting
them into the tank slowly. After these are dissolved, the hydrated lime is added. Use 1.2
pounds of hydrated lime for each 3.0 pounds of copper sulfate, 1.0 pound hydrated lime
for each 3.0 pounds of zinc sulfate, and 0.1 pound hydrated lime for each 3.0 pounds of
manganese sulfate. No lime is required if neutral zinc or manganese compounds are used.
Copper.-Copper is usually applied as copper sulfate or cop-
per oxide and is expressed as CuO (copper oxide) equivalent on
the fertilizer tag. Most of the applied copper accumulates in the
topsoil. Excess copper depresses growth and induces iron chlo-
rosis. Soils that contain approximately 50 pounds of total copper
per acre six inches need no copper in the fertilizer. Sufficient
copper will usually be applied in the first 10 years on a young
grove if the fertilizer contains about 0.25 unit of CuO.
Excessive amounts of copper have accumulated in many
grove soils. Some groves have been found to contain over 600
pounds of copper per acre in the top six inches of soil. Acid
sandy soils which contain more than 100 pounds of copper
(total amount, not an extractable portion) per acre six inches
2These programs may be obtained from the Citrus Experiment Station,
Lake Alfred; the Florida Citrus Commission, Lakeland; county agricultural
agents; or the U. S. Horticultural Station, Orlando.
Florida Agricultural Experimnent Stations
of soil are at a potentially dangerous copper level. The pH of
such soils should be maintained above 6.5, and no further copper
applied in the fertilizer. Fungicidal copper sprays should be
avoided as much as possible. Somewhat higher levels of copper
in calcareous and organic soils can be tolerated without injury
to the trees.
Apply no copper in the fertilizer to groves over 10 years
old, unless symptoms of copper deficiency occur. Omit copper
from the fertilizer on any grove which receives a copper spray
for melanose or scab control. Do not use copper in fertilizer for
young trees replanted on old grove sites.
Copper sprays are very effective in correcting copper de-
ficiency, and sprays produce quick results. In addition, they
provide very valuable fungicidal control of greasy spot, scab,
and melanose. (See Table 2 for instructions on mixing copper
Zinc.-Zinc should be applied in a nutritional spray, in
either dormant or preferably the post-bloom period, using 3.0
pounds of zinc sulfate plus 1.0 pound of hydrated lime per 100
gallons, or its equivalent in neutral zinc material. Spraying with
zinc every year is not necessary, unless zinc deficiency symp-
toms appear on the foliage. For other details on spraying, in-
cluding compatibility with other materials, see Table 2 and spray
schedules of the current "Better Fruit Program." Zinc applica-
tions in the fertilizer do not give satisfactory results for rapid
correction of deficiencies on most soils.
Boron.-Boron is usually applied as borax and is expressed
us B,O: equivalent on the fertilizer tag. Care should be exercised
in the use of boron since there is a relatively narrow range
between deficient and toxic levels. Boron may be applied as
borax, either in the fertilizer or in a nutritional spray, but
should not be applied in both in the same year. As a boron
maintenance program, apply B,O:, in the fertilizer at an annual
rate equivalent to approximately 1/100 of the nitrogen rate.
This amount may be applied by including borax in either one
or all fertilizer applications made during the year. For example,
if boron is to be applied once each year in 1/3 of the annual
fertilizer in an 8 to 10 percent nitrogen material, the B20. con-
tent should be 0.25 or 0.3 percent. If applied in all fertilizer,
the B:O, content of an 8 to 10 percent nitrogen mixture should
Fertilizers and Nutritional Sprnays for Citr.s
be 0.1 percent. Other mixtures and methods of application could
be used to apply equivalent annual amounts of B.O:,. For main-
tenance applications in sprays, use 1 3 pound borax containing
46 percent B.,O or equivalent per 100 gallons. Where deficiency
symptoms are present, double the amounts suggested above.
Boron use on arsenated grapefruit trees has a tendency to offset
peel gumming and fruit malformation induced by the arsenic.
Molybdenum.-Molybdenum foliage sprays are recommend-
ed to correct molybdenum deficiency, which is commonly known
as "yellow spot." If yellow spot symptoms occur, they usually
appear in the summer and are occasionally severe in trees on
grapefruit rootstock. Spray trees showing mild yellow spot leaf
symptoms with 1.0 ounce of sodium molybdate per 100 gallons,
and in severe cases with 2.0 ounces. Application of molybdenum
sprays should be made in spring or summer since sprays ap-
plied in October or later may not cause regreening of the yellow
spots. However, a late fall spray or a spring dormant or post-
bloom spray will prevent the occurrence of yellow spot during
the following summer. It is recommended that molybdenum
sprays be applied only to groves having yellow spot.
The occurrence of yellow spot is usually associated with
acid soil. In correcting yellow spot it is especially important that
the soil reaction be adjusted to pH 5.5 to 6.5 with liming ma-
terials and maintained at that level. For compatibility of sodium
molybdate with other spray materials, see the current "Better
Fruit Program, Spray and Dust Schedule". Soil applications of
molybdenum salts have not been effective in controlling yellow
Iron.-On acid soils, application of the iron chelate of eth-
ylenediamine tetraacetic acid (FeEDTA) to trees showing iron
chlorosis is recommended. This should be applied at about 20
grams (2/3 ounce) of actual iron per tree. For example, applica-
tion of this amount of iron requires about 14 ounces per tree of
a chelate containing 5 percent iron or about 6 ounces per tree
of a chelate containing 12 percent iron. Half of this amount, or
10 grams of iron per tree, is sufficient to correct mild cases of
iron chlorosis. The iron chelate may be applied alone to the soil
around the chlorotic trees or may be mixed with the fertilizer.
Care must be taken in applying iron chelate while fruit is
on the trees. Otherwise, chelate dust may severely burn any
fruit on which it settles. A dustless form of the chelate is rec-
Floida Agricaltiural Expe'r'i)et Statiovs
commended if it is applied with a fertilizer distributor.
For iron deficiency on calcareous soils (pH above 7.0), it
is recommended that a soil application of the iron chelate of
hydroxyethyl ethylenediamine triacetic acid (Fe-EDTA-OH)
be used at the rate of 50 grams of actual iron per tree. This is
1-1/4 pounds of the FeEDTA-OH concentrate containing 9 per-
cent iron or about 2-1/4 pounds per tree of the chelate containing
5 percent iron. This chelate is not effective on all calcareous
soils. Where it is effective, greening of iron chlorotic leaves
may require four to six months. For calcareous soils where
FeEDTA-OH does not correct iron deficiency, good results can
be obtained by applying the iron chelate of ethylene diamine
di(o-hydroxyphenyl) acetic acid (FeEDDHA). As little as 10
grams actual Fe in this chelate per tree usually is effective with-
in two months in correcting iron deficiency in citrus growing on
calcareous soils in Florida. The iron content of the chelate varies,
but if it contains 6 percent, the minimum treatment would re-
quire 0.37 pound of the chelate per tree. This chelate is expen-
sive, so it is recommended that it be applied separately by hand
to only those trees showing iron deficiency symptoms on cal-
careous soils. It is not effective on acid soils. FeEDTA is not
recommended for calcareous (high lime) soils because large
amounts are required to correct the chlorosis on such soils.
Iron sulfate has not been found satisfactory for correcting
iron chlorosis when applied to either acid or calcareous soils,
and is not recommended. No form of iron, including chelates,
is recommended as a foliage spray.
Calcium.-Calcium is usually supplied insufficient amounts
for nutritional purposes in either dolomite or high calcium
limestone. It is abundant also in calcium nitrate and super-
phosphate. Usually no special consideration need be given to
applying calcium as a nutritional element, if sufficient lime or
dolomite is applied for pH control as discussed below. (See Soil
Analysis Soil calcium.)
Sulfur.-Sulfur in excess of the nutritional needs of citrus
trees is applied in sprays and dusts for mite control and in the
various sulfates used in fertilizer mixtures. Sulfur used in rust
mite control increases soil acidity and makes control of soil pH
more expensive. Thus, the amounts of sulfur used need not and
should not be larger than recommended in the "Better Fruit
Fertilizers and S utritionil Sprays for Citrus
It is recommended that the pH of acid soils be maintained
between 5.5 and 7.0. (See section on soil testing.) In young
non-bearing groves on previously unplanted soil, sufficient lim-
ing material should be applied whenever necessary to prevent
the pH from dropping below 5.5. As groves become older, there
has commonly been an accumulation of several elements in the
soil. Copper is the most important of these, as it has accumu-
lated to toxic amounts in the soil of many groves. Therefore
with increasing age of the grove, some increase in soil pH level
is desirable. The soil in average bearing groves should be main-
tained between 6.0 and 6.5. If the soil is known to contain 100
pounds of copper or more per acre, the pH should be maintained
between 6.5 and 7.0, regardless of the age of the trees. Using
high-calcium limestone is often advantageous in adjusting the
pH in this range. Some increase in minor element deficiencies
may occur if the pH is raised above 7.0.
Soil acidity arises from leaching and crop removal of basic
nutrients, using fertilizer materials leaving acid residues, and
using sulfur for pest control. Sulfur can be an important factor,
since 3-1/8 pounds of dolomite are required to neutralize the
acidity from the application of 1 pound of sulfur. The amount
of liming materials to apply depends upon the soil texture, the
amount of organic matter in the soil, the amount of acid- or
base-forming fertilizer applied, the amount of dolomite applied
as conditioner or filler in fertilizer, and the amount of sulfur
used in pest control. Therefore no specific recommendations for
rates of application of liming materials can be made to meet
the needs of all groves. However, 800 to 2,000 pounds of dolo-
mite or lime per acre per year is usually required to maintain
the soil pH at the desired level.
Calculation of Fertilizer Rates and Analyses
To illustrate the method of converting pounds of nutrients
per box to pounds of fertilizer per tree and to determine the
fertilizer analysis, the following sample calculations are pre-
Assume the following conditions: (1) 0.4 pound N, 0.4 pound
K_.0, and 0.15 pound MgO will be applied per box per year. (2)
Production is six boxes of fruit per tree. (3) The fertilizer will
Florida Agricultural Experiment Stations
contain 10 percent nitrogen. (4) One-third the year's supply
of fertilizer will be applied at this time.
6 boxes per tree x 0.4 pound N per box per year =
2.4 pounds N per tree per year.
2.4 pounds N per tree per year 3 (1/3 of annual application) =
0.80 pound N per tree in this application.
0.80 pound N 8 pounds fertilizer per tree in
0.10 pound N per pound of fertilizer this application.
6 boxes x 0.4 pound K20 per box =2.4 pounds 1K20 per tree per year.
2.4 3 (1/3 of annual application) = 0.80 pound K20 in this application.
0.80 x 100
0.80 x 100 10 K.,0 in fertilizer.
8 pounds fertilizer to be applied this application
Magnesium Oxide (MgO)
6 boxes x 0.15 pound MgO per box 0.9 pound MgO per tree per year.
0.9 3 (1/3 of annual application) =0.30 pound MgO in this application.
0.30 x 100=3.75% MgO in the fertilizer. This should be rounded out to 4.0%
Combining the percentages obtained by these calculations,
the complete fertilizer analysis would be 10-0-10-4-0-0", which
if used at the rate of 8 pounds per tree would provide 1/3 of
the tree's annual requirement.
If it is desired to use a higher analysis fertilizer, analyses
such as 13-0-13-5-0-0 would be satisfactory if used at 6.2 pounds
per tree. Where the use of phosphorus and manganese is ad-
visable, analyses such as 8-4-8-3-0.6-0, 10-4-10-4-0.75-0, or others
similar would be suitable. Application rates may also be calcu-
lated on a per-acre basis. Using the same assumptions as for
the example given above, except that the yield will be 350 boxes
per acre, the following calculations result:
350 boxes per acre x 0.4 pound N per box per year =
140.0 pounds N per acre per year.
"N-P>0 R -KO20-MgO-MnO-Cu0.
Fertili.ers, a(id N'tritio(nal Sprays for Citrus
140 pounds + 3 (1/3 of annual application) = 46.7 pounds N per
acre in this application.
46.7 pounds N 4_ 67 pounds fertilizer per
0.10 pound N per pound of fertilizer acre in this application.
The balance of the calculations proceed in parallel to the first
Many growers use nitrogen alone instead of one of the
regular applications of mixed fertilizer. Applications providing
nitrogen only should be included in the calculations for the
yearly total to arrive at the true amount of fertilizer elements
applied. For example, the situation used in the calculation above
can be met over an annual period by one application of a nitro-
gen material and two applications of mixed fertilizer, provided
the mixed fertilizer contains 50 percent more of the elements
other than nitrogen. One-third of the annual requirement for
nitrogen could be met by applying enough nitrogen material
to furnish 0.8 pound N per tree. Each of the two applications
of mixed fertilizer should then contain 1 3 of the annual nitro-
gen requirement and 1 2 of the potash, magnesium, and man-
ganese requirements. This could be furnished by 10 pounds per
tree of 8-0-12-5-0.9-0 mixed fertilizer.
To assist in the calculations involving use of fertilizers of
different nitrogen contents, Table 1 has been compiled.
Both the amount of fertilizer and its analysis must be con-
sidered in selecting a fertilizer program. For example, 1 ton of
16-0-16 mixture contains the same plant food quantities as 2
tons of 8-0-8. Higher analysis mixtures have the advantage that
less bulk need be handled and spread, thereby reducing the labor
and operational cost. On the other hand, it is not always advis-
able to use the highest analysis that it is possible to make, as
the physical condition of the mixture may be poor, and spread-
ing may be difficult or unsatisfactory. Generally it will be ad-
vantageous to use as high an analysis fertilizer as the fertilizer
manufacturer can deliver to the field in good physical condition
for satisfactory spreading.
Liquid fertilizers for soil application (such as solutions
containing 20 percent or more nitrogen) have the same value
to the trees as dry fertilizer of the same guaranteed analysis
and containing the same fertilizer ingredients. The choice be-
tween liquid and dry fertilizers should be made on the basis of
relative over-all cost of the fertilizer program-
Florida Agricultural Experiment Stations
RECOMMENDATIONS FOR NON-BEARING AND
YOUNG BEARING CITRUS TREES
This section concerns the fertilization of groves up to the
age of 10 years, planted in previously uncultivated acid, sandy
soils. The soil conditions found in such cases are somewhat
different from those found in heavily fertilized old-grove soils.
Uncultivated soils are generally very infertile with respect to
all the essential elements except phosphorus in some cases.
Therefore young trees in previously unfertilized soils should
receive regular applications of nearly all essential elements.
Trees in replanted areas or occasional replants in present groves
should receive the fertilizer mixtures recommended for bearing
trees, but in reduced amounts sufficient to give about the same
nitrogen and potassium levels as recommended for trees planted
on new land (Table 5).
Many fertilizer formulas may be satisfactory; however, the
most desirable formula should contain only adequate amounts of
all the elements essential to good tree growth and not excessive
amounts of any element. Nitrogen and potassium are, by far, the
most soluble nutrients and are rapidly leached from the limited
root zone of newly planted trees. Consequently, they are required
in larger amounts than the less soluble nutrients which tend to
accumulate and remain available in the root zone. The following
ratio of elements will satisfy the requirements of young, non-
bearing citrus trees under most conditions: N-l, PO ,-1 4, K0O-
1, MgO-1 4, MnO-1/16, CuO-1/32, B,0:-1/100. The elements in
this ratio and order can be made into a fertilizer mixture such
as 8-2-8-2-0.5-0.25-0.1. Higher analysis mixtures are generally
more economical and may be used, but the difficulty of obtain-
ing uniform applications around the root zone is increased.
Extreme care should be taken to avoid root damage due to
excess salt concentrations in localized areas brought about by
A suggested schedule of fertilization for a grove planted
during the dormant season is given in Table 3. The rate and
number of applications of fertilizer should be based primarily
on the age of the tree and its condition. Trees should be fertilized
about every seven to eight weeks for the first year. The fre-
quency of application may be reduced in succeeding years. Rate
of application the first year should be relatively low because of
the limited root system of the tree, but should be increased
FPrtilizer-PS mid Nitritionol SpoiYs for Citrus
sharply the next few years. Spread fertilizer in a 30-inch circle
the first year, usually in the bottom of the water ring. Avoid
putting fertilizer against the trunk. After the first year, the
fertilized area should be steadily enlarged each application. A
good rule to follow is to cover an area twice the diameter of the
tree canopy. Thus, for a tree with a three-foot canopy, apply
the fertilizer uniformly over a circle six feet in diameter. Fer-
tilizer applications should be omitted between October 1 and
February 15 for the first year or two to reduce the possibility
of inducing untimely growth flushes in the winter.
Table 3. Suggested fertilization guide for young citrus trees
up to 10 years of age.
applicaPounds per application per tree:
Year in grove each year Range Average
First V 0.25 -0.40 0.33
Second 4 0.70 1.25 1.0
Third 4 1.5 2.5 2.0
Fourth 3 3.5 4.5 4.0
Fifth 3 4.0 5.0 4.5
Sixth 3 4.5 5.5 5.0
Seventh 3 5.0 6.0 5.5
Eighth : 5.5 -6.5 6.0
Ninth 3 6.0 -7.0 6.5
Tenth 3 6.5 7.5 7.0
For trees planted after February 15. the number of applications should he correspond-
** Use S-2-,-2-0.5-0.2.5-i .1 mixture or equivalent.
On acid soils an application of dolomite or high calcium lime-
stone should be made before or shortly after planting, either
uniformly at the rate of 1 ton per acre, or at about 10 pounds
in a circle of 10- or 12-foot radius around each tree. One appli-
cation by either of these methods will give sufficient pH control
for at least the first year, after which the pH should be tested
and adjusted annually to attain the levels recommended in the
section on pH control for bearing trees.
The emphasis for the first five years should be on making
good tree growth, and the quality of the crop should be second-
ary. From about the sixth to tenth years the trees come into
appreciable commercial bearing but still benefit from a complete
fertilizer. This period is the time to begin to shift to a program
for bearing trees. This would include changing from hand to
machine application of fertilizer. When a mechanical spreader
Florida Agricultural Experiment Stations
is first used, it is desirable to drive close to the trees, using a
one-side spread such that only a swath of 10 to 12 feet down
the tree row is fertilized. After one or two years of this, the
fertilizer can be spread uniformly over the whole ground area
except for the water furrows of bedded groves. Use three appli-
cations per year at the rates indicated in Table 3. From the
eleventh year on, follow the program for full bearing trees given
in the first part of this bulletin.
In most cases it is advisable to supplement the fertilizer
applications with nutritional sprays containing zinc. Make one
application each year using 3 pounds of zinc sulfate plus 1 pound
of hydrated lime per 100 gallons or its equivalent in neutral
zinc material. On calcareous soils, or wherever symptoms of
manganese and copper deficiencies appear, the nutritional spray
should also contain manganese and copper. These may be sup-
plied by adding 3 pounds of copper sulfate, 3 pounds of man-
ganese sulfate, and 1.3 pounds of lime or equivalent amounts of
neutral materials to each 100 gallons of spray. (See Table 2.)
If iron deficiency symptoms appear, treatments similar
to those suggested in the section on bearing trees should be
used. Use caution in applying iron chelate to young trees, as
there is danger of severe damage by over-dosage. One- and two-
year-old trees seldom need iron chelate applications. For older
trees, apply iron chelate at the rate of 5 to 10 grams of actual
iron per tree, the amount depending on the size of tree and
severity of symptoms.
Deficiency symptoms of molybdenum (yellow spot) may
be eliminated by treatments as for bearing trees.
SOIL AND LEAF ANALYSES
Aside from the determination of soil pH, no soil or leaf
analysis is absolutely necessary for citrus production in Florida.
Adherance to the general recommendations given above, to-
gether with close observation of the leaves, twigs, and fruits
for appearance of visible symptoms of deficiencies or toxicities,
is adequate for determining the fertilizer requirements of the
Although not absolutely essential, soil and leaf analyses do
provide an opportunity for more precise control of nutritional
level, permit the detection of non-visible detrimental conditions
in the soil or tree, and may permit economies in the fertilizer
Fertilizers and Nutritional Sprays for Citrus
If soil and leaf analyses are to be of value, they must be
carried out precisely, according to a sampling procedure adopted
in advance. Otherwise, the data cannot be interpreted according
to the analytical standards provided. These procedures are
somewhat difficult, but the importance of adhering to the
recommended procedures cannot be overemphasized. If precise
work cannot be guaranteed, soil and leaf analyses may prove
expensive, misleading, and confusing rather than helpful.
When properly used, soil analysis can serve as a useful
guide in improving the lime and fertilizer programs for a par-
ticular grove. Soil tests are especially informative when con-
tinued over a period of years so that trends can be followed.
Soil tests for pH, calcium, phosphorus, and copper are useful.
Soil tests for the more readily leached elements are of little
value, since there is a lack of experimental data to correlate
yield responses with levels of the fertilizer element in the soil.
The reliability of soil analyses depends upon a careful,
proper method of collecting the soil samples. Each soil sample
should consist of a composite of about 16 soil cores taken to a
depth of six inches, from an area at the dripline of the tree (the
outer edge of the tree canopy). One core should be collected
from the dripline of 16 different trees scattered throughout
the area of the grove represented by that particular sample.
Separate samples should be taken for each part of a block that
is visibly different in tree or soil characteristics. Sample areas
should correspond to areas used for leaf samples. (See Table 5,
first footnote.) Regardless of the apparent uniformity of a
block, no one sample should represent an area larger than about
20 acres. Thus, an 80-acre grove would require at least four
soil samples as a minimum number.
Samples should be collected according to a regular schedule.
If samples are to be collected once a year, the best time
would be in late summer after the rains have rinsed the soluble
salts from the topsoil. Each sample should be air-dried, screened,
and thoroughly mixed before analysis.
Soil pH.-The soil pH of every citrus grove should be de-
termined once each year, using the soil sampling procedure
given above. A 1:1 volume of soil and distilled water should
be used and the pH read after standing 15 to 30 minutes and
Florida Agricultural Experiiment Stations
carefully stirred. Color tests are not satisfactory for pH. The
results obtained should be interpreted as advised in the section
"pH Control" given above.
Soil Calcium.-Experimental data are not yet adequate to
indicate the optimum level for calcium in the soil under all
conditions. On the basis of pH considerations primarily, an
exchangeable soil calcium level of about 500 pounds of Ca (700
pounds CaO) per acre on soils similar to Lakeland fine sand
seems reasonable and desirable. Soils having a higher exchange
capacity would have a higher minimum requirement for calcium.
If a lower level is found, application of dolomite is recommended
even if the pH is found to be in the recommended range. The
use of gypsum to supply calcium is not a proven practice and
is not recommended. Determinations should be made as regu-
larly as for soil pH.
Soil Phosphorus.-For acid sandy soils, there are sufficient
data correlating tree performance with soil test values for phos-
phorus to make such soil tests useful supplements to general
recommendations for phosphate fertilization.
Soil samples may be analyzed by any of the three methods
named in Table 4. Soils that test higher than the levels given
probably do not need immediate phosphate additions. Soils that
test lower than these amounts should receive some phosphate
in the fertilizer, in the ratios recommended for young bearing
Table 4. Adequate phosphorus test values for soil samples taken in
citrus groves on acid sandy soils.
Minimum adequate level -
Extraction method* pounds per acre 6 inches
Acid ammonium acetate 22 50
Bray P| 80 185
Bray P. 130 300
The acid ammonium acetate extracting solution is approximately 1 normal ammonium
acetate adjusted to pH 4.8 with acetic acid. The Bray Pi extracting solution is 0.03 normal
ammonium fluoride and 0.025 normal hydrochloric acid. The Bray P_ extracting solution is
0.03 normal ammonium fluoride and 0.1 normal hydrochloric acid. For more details about
procedures, see Fla. Agr. Expt. Sta. l. u. 653, "Phosphorus Fertilization of Citrus."
Since phosphorus is fairly stable in the soil, soil phosphorus
determinations need only be made every few years. If very high
values are found, the test need not be repeated for many years.
Fcrtilizers and Nutritional Sprays for Citrus
Soil Copper.-Acid sandy soils may be tested for excessive
copper levels by the method described in Florida Agricultural
Experiment Station Bulletin 544, "A Rapid Test for Possible
Excesses of Copper in Sandy Soils." Any value over 100 pounds
per acre indicates potential toxicity.
Leaf analysis is not essential to the planning of a fertilizer
program. However, leaf analyses have been conducted in many
fertilizer rate and source experiments, and surveys have been
conducted in many commercial citrus groves. A considerable
amount of information has been accumulated in this way. This
makes possible a judgment as to the acceptable levels of the
various elements in leaves. These levels are given in Table 5.
Leaf analyses below the lower limit of the range in Table 5
may be associated with visible symptoms of deficiency, or re-
duced yield. Leaf analyses above the upper limit of the range
usually are associated with wastefully high rates of application
of fertilizer, and may reduce yield. Performance of the trees
should not be limited by nutrition at any level of leaf analysis
within the range shown for each element. Effects on fruit
quality are different with different elements. High quality fruit
sometimes accompanies high and sometimes low leaf composi-
tion, depending on the effect of the individual elements. These
effects are discussed below.
The main usefulness of leaf analysis lies in establishing
over a period of years trends in the nutritional status of the
trees resulting from a certain fertilizer practice. These trends
can best be detected by a regular annual leaf analysis program.
If the values found by leaf analysis for an element are near the
extremes of the ranges given in Table 5, the fertilizer program
should be adjusted accordingly.
Leaf Nitrogen.-Nitrogen content near the lower value given
in Table 5 generally is associated with some yellowing of the
foliage. When good fruit color is of special importance, as in the
fresh fruit trade, the lower portion of the satisfactory range
for leaf nitrogen given in Table 5 is desirable.
Leaf Phosphorus.-Tree performance will be satisfactory
over the entire range given in Table 5. Deficiency need not be
suspected until the leaf content reaches the low point of the
Florida Agricultural Experiment Stations
Table 5.-Satisfactory ranges of nutrient elements in standard samples* of
leaves from commercial citrus groves in Florida.
Element Chemical symbol Satisfactory range
Nitrogen N 2.3 to 2.9%
Phosphorus P 0.09 to 0.15%
Potassium K 1.2 to 1.7%
Magnesium Mg 0.30 to 0.45%
Manganese** Mn 25 to 75 ppm
Copper** Cu 5 to 10 ppm
Zinc Zn 25 to 80 ppm
Boron** B 40 to 150 ppm
Iron** Fe 40 to 60 ppm
60 to 90 ppm
Calcium Ca 2.5 to 5.0%
The standard leaf sample consists of not less than 100 spring-flush leaves from at least
20 trees, taken when four to five months old from non-fruiting twigs. Thus, leaves should
be collected in July or August. The area represented by one sample should be of uniform
general tree appearance, of a single variety and rootstock, under one fertilizer program,
and not in any case larger than 20 acres. Leaf samples should be delivered to the analytical
laboratory promptly in fresh condition if they require washing (see footnote **). If the
leaves must be mailed, ship them in a plastic bag promptly after picking. Leaves that do nut
require washing may be air-dried promptly after picking and then sent in.
**Values for iron are valid only if the leaf samples are washed individually and thorough-
ly with a detergent solution, and rinsed in deionized or distilled water before drying. Leaves
that have been sprayed with copper, zinc, or manganese should not be analyzed for these
elements even if washed, as it is impossible to eliminate interference from contamination.
Leaf Potassium.-Production may be satisfactory over the
entire range given. If fruit size tends to be too large or juice
acidity too high, it may prove advantageous to maintain leaf
potassium near the lower limits shown in Table 5. Likewise, if
sizes run too small or acidity too low year after year, leaf
potassium should be maintained near the upper limit. Quite
often a compromise is necessary, in which case the mid-range
Leaf Calcium.-There is a wide range in satisfactory leaf
calcium levels. No particular significance need be attached to
values within this range.
Leaf Manganese, Zinc, and Iron.-The leaf symptoms of
deficiency of these elements are readily identified, and their
occurrence on a few twigs or branches gives adequate warning
of approaching deficiency levels. Uneconomically high levels
may be detected by leaf analysis.
Leaf Copper.-Deficiency symptoms probably will not ap-
pear at the lower limit given in Table 5. Leaf analysis for copper
is of little value for detecting toxic levels of copper in the soil.
Soil analysis is indicated.
Fertilizers and Nutritional Sprays for Citrus 23
Leaf Boron.-Boron deficiency leaf symptoms are poorly
defined, but toxicity is quite characteristic. Leaf analysis may
be useful in identifying groves near deficient boron levels.
Recent Publications on Fertilizing Florida Citrus
Calvert, D. V., R. R. Hunziker, and H. J. Reitz. A Nitrogen Source Experi-
ment with Valencia Oranges on Two Soil Types in the Indian River Area.
Proc. Fla. State Hort. Soc. 75: 77-82. JS-1531. 1962.
Calvert, D. V., and H. J. Reitz. A Fertilizer Rate Study with Valencia
Oranges in the Indian River Area. Proc. Fla. State Hort. Soc. 76: 13-17.
Davis, P. L., and P. L. Harding. Keeping Quality of Marsh Grapefruit
after Nitrogen and Potash Fertilization. Proc. Fla. State Hort. Soc. 72:
Fiskell, J. G. A., and W. F. Spencer. Forms of Phosphate in Lakeland
Fine Sand after Six Years of Heavy Phosphate and Lime Applications.
Soil Sci. 97: 320-327. JS-1632. 1964.
Florida Citrus Commission, Publisher. Better Fruit Program, Spray and
Dust Schedule for Citrus. Revised annually.
Hunziker, R. R. The Relationship of Soil Potassium and Leaf Potassium
Status to Yield of Citrus in the Indian River Area. Proc. Fla. State
Hort. Soc. 73: 36-39. JS-1185. 1960.
Koo, R. C. J. The Use of Leaf, Fruit, and Soil Analysis in Estimating
Potassium Status of Orange Trees. Proc. Fla. State Hort. Soc. 75: 67-72.
Leonard, C. D., and I. Stewart. Soil Application of Manganese for Citrus.
Proc. Fla. State Hort. Soc. 72: 38-45. JS-980. 1959.
Yellow-vein in Citrus. Proc. Fla. State
Hort. Soc. 73: 69-79. JS-1154. 1960.
Leonard, C. D., I. Stewart, and I. W. Wander. A Comparison of Ten
Nitrogen Sources for Valencia Oranges. Proc. Fla. State Hort. Soc.
74: 79-86. JS-1357. 1961.
Mathias, A. F. Changes in Fertilizer Program and Yields in Citrus Since
1931. Proc. Fla. State Hort. Soc. 73: 9-12. 1960.
Rasmussen, G. K., and P. F. Smith. Effects of H-ion Concentration on
Growth of Pineapple Orange Seedlings in an Alternate Solution and
Water Cultures. Proc. Amer. Soc. for Hort. Sci. 73: 242-247. 1959.
Evaluation of Fertilizer Practices for
Young Trees. Proc. Fla. State Hort. Soc. 74: 90-95. 1961.
Reitz, H. J., and R. R. Hunziker. A Nitrogen Rate and Arsenic Spray
Experiment on Marsh Grapefruit in the Indian River Area. Proc. Fla.
State Hort. Soc. 74: 62-67. JS-1350. 1961.
Reitz, H. J., and R. C. J. Koo. Effect of Nitrogen and Potassium Fertiliza-
tion on Yield and Fruit Quality of Valencia Orange on Calcareous Soil.
Proc. Fla. State Hort. Soc. 72: 12-16. 1959.
Effect of Nitrogen and Potassium Fertiliza-
tion on Yield, Fruit Quality, and Leaf Analysis of Valencia Orange.
Proc. Amer. Soc. for Hort. Sci. 75: 244-252. JS-926. 1960.
Sites, J. W., I. W. Wander, and E. J. Deszyck. The Rate and Timing of
Nitrogen for Grapefruit on Lakeland Fine Sand. Proc. Fla. State Hort.
Soc. 74: 53-57. JS-1335. 1961.
Smith, P. F. A Case of Sodium Toxicity in Citrus. Proc. Fla. State Hort.
Soc. 75: 120-124. 1962.
SQuality Measurements on Selected Sizes of Marsh Grape-
fruit from Trees Differentially Fertilized with Nitrogen and Potash.
Proc. Amer. Soc. for Hort. Sci. 83: 316-321. 1963.
Twenty Years of Differential Phosphate Fertilization on
Pineapple Oranges. Proc. Fla. State Hort. Soc. 76: 7-12. 1963.
Smith, P. F., and G. K. Rasmussen. Field Trials on the Long-Term Effect
of Single Applications of Copper, Zinc, and Manganese on Florida Sandy
Citrus Soil. Proc. Fla. State Hort. Soc. 72: 87-92. 1959.
SRelation of Potassium Nutrition to
Size and Quality of Valencia Oranges. Proc. Amer. Soc. for Hort. Sci.
74: 261-265. 1959.
Relationship of Fruit Size, Yield and
Quality of Marsh Grapefruit to Potash Fertilization. Proc. Fla. State
Hort. Soc. 73: 42-49. 1960.
Potassium-Deficiency Symptoms in
Grapefruit under Field Conditions in Florida. Proc. Amer. Soc. for Hort.
Sci. 78: 169-173. 1961.
Effect of Potash Rate on Growth and
Production of Marsh Grapefruit in Florida. Proc. Amer. Soc. for Hort.
Sci. 77: 180-187. 1961.
Effect of Nitrogen Source, Rate, and
pH on the Production and Quality of Marsh Grapefruit. Proc. Fla. State
Hort. Soc. 74: 32-38. 1961.
Spencer, W. F. A Rapid Test for Possible Excesses of Copper in Sandy
Soils. Fla. Agr. Expt. Sta. Bul. 544. 1954.
Some Considerations Pertaining to the Use of -:,
Analyses in Citrus Production. Proc. Soil & Crop Sci. Soc. of Fla. 2-'
Phosphorus Fertilization of Citrus. Fla. Agr. Expt
Sta. Bul. 653. 1963.
Spencer, W. F., and R. C. J. Koo. Calcium Deficiency in Field-Grown Citrus
Trees. Proc. Amer. Soc. for Hort. Sci. 81: 202-208. JS-1439. 1962.
Spencer, W. F., and I. W. Wander. A Comparison of Magnesium Sources
for Young Orange Trees. Proc. Fla. State Hort. Soc. 73: 28-35. JS-1176.
Stewart, I., C. D. Leonard, and I. W. Wander. Comparison of Nitrogen
Rates and Sources for Pineapple Oranges. Proc. Fla. State Hort. Soc.
74: 75-79. JS-1356. 1961.