The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
/3 C- /
Slow Release Fertilizers Reduce N Leaching on Young Citrus
BRIAN J. BOMAN
Sixteen buried drainage lysimeters were constructed from 605 gal.
commercially available polyethylene containment basins.: headnh0'i~ 44t basins
were 82 inches in diameter by 30 inches deep. Excavated soil was used to fill
the tanks and was layered and compacted to closely match the original conditions
of the Oldsmar Fine Sand series soil. Drainage water was pumped from the
lysimeters and metered and rainfall was measured at the site. "Green" base
microsprinklers with young-tree downsprays which confined the discharge to about
a 4 ft. diameter were used to irrigate the trees. Irrigations were initiated
whenever soil tension reached 10-15 cbar. Irrigation durations ranged from 15-40
minutes, depending on the accumulated reference evapotranspriration (ETr) since
previous irrigation or rainfall.
Bareroot nursery-grown 'Midsweet' orange on Carrizo citrange rootstock
trees were planted in the lysimeters on May 5, 1991. Tree spacing was 10 ft in-
row by 25 ft across-rows, resulting in 174 trees ac-1. The tree canopies had been
pruned at the nursery prior to transplanting, so tree height and canopy width
were very uniform. On June 6, initial tree physical measurements were taken.
Trunk diameter was measured 4 inches above the bud union with a caliper. Trunk
diameter, tree height, and canopy widths were again measured on each tree on
February 21 and December 10, 1992. Canopy volume was calculated as: Vol = (Ht
X W2)/4; where Vol = canopy volume, Ht = height, and W = canopy width.
With the exception of fertilization, all cultural practices were identical
for all of the lysimeter trees. Four fertilization treatments were compared,
S each treatment replicated in four lysimeters. They included two slow release
materials, fertigation, and traditional broadcasting of soluble granular
fertilizer. The traditional and fertigation treatment trees each received 0.3
lb tree-1 of nitrogen (N), following University of Florida recommendations (Koo
et al., 1985). Trees receiving slow release materials each had 0.13 lb tree-
of N applied (12 oz. of material), following the manufacturer's recommendations.
The slow release materials included Osmocote 17-6-9 (N-P-K), designated SR-
1, and Meister 17-6-12, designated SR-2. The slow release materials were applied
in a single application on June 7 which was incorporated into the soil. A hoe
was used to make a shallow trench about 10 inches from the tree trunk, 12 oz. of
fertilizer (SR-1 or SR-2) was spread in the trench, and then the soil was raked
back over to cover the trench.
Lysimeters receiving the fertigation treatments (FERT) had 17 applications
of liquid fertilizer which were generally on a 7-10 day interval. The fertilizer
was injected into the irrigation line with an electric metering pump followed
by a brief flushing period. Total fertigation and flush cycle times were
typically about 30 minutes. The liquid fertilizer was an 8-2-8 analysis made
from NH4NO3, H3PO4, and KC1.
The fertilizations made using the traditional broadcast applications of
soluble granular materials (TRAD) were made about every 6 weeks (June 7, July 19,
Aug. 30, Oct. 11, and Nov. 22). The 8-4-8 analysis material was made from
NH4NO H3P04, and KC1. For each application, 0.75 lb of the fertilizer was
broadcast in a 1-meter diameter area around the tree by hand.
Water samples were taken weekly by applying a vacuum to the ceramic cups
or by sampling leachate water pumped from the lysimeters. Water samples were
frozen until processed by the University of Florida Analytical Research
Laboratory in Gainesville. The quantity of nutrients leached was determined by
multiplying the quantity of leachate by the concentration of the constituents
(NH4-N + NO -N) in the water sample to determine the total N leached. The
quantity of I leached and growth parameters were analyzed using standard analysis
* of variance procedures, with the experiment designed as a randomized complete
Ft. Pierce AREC 93-1
Total rainfall for the 9- 30o .m SR-
month study period was 40.5 2500 o- mS -.
inches. About 85Z of this z200- R--D
amount occurred during the *S-
normal summer wet season '-
beginning in mid-June and ro100-
continuing through mid-October E .
(Fig. 1). Weekly rainfall
amounts as high as 8 inches -4o
occurred during this wet
season. No irrigations were soo 3
required from the end of June
until late October (Table 1) -200
and due to very wet soil
conditions several fertigations 10o
were delayed and made at later L
dates. With the exception of a 2-
couple weeks, little or no &
rainfall was recorded during i.
most of the weekly periods from I
mid-October until mid-February. o h I on-'. ,n .I I
Therefore, several irrigations F
were required during this time.
Measurements taken one FIG. 1. Daily rainfall and leached N (NH4-N +
month after planting indicated NO3-N) by treatment expressed as weekly means
no differences in initial tree (top panel) and cumulative (center panel).
measurements with an average
trunk cross-sectional (X/C) area of 1.7 cm2 and average tree height of 90 cm. The
fertilization treatments resulted in no significant (P=.05) differences in tree
growth at the end of the study period (Table 2), although the means for the trunk
X/C area and canopy volumes were greater for the slow release materials. Overall,
trunk X/C area increased about 3 times and canopy volumes increased about 1.5
times during the 9-month period. The slow release fertilizers achieved
equivalent growth to the FERT and TRAD methods, even though they received only
44Z of the N.
The timing of leaching was directly related to rainfall events, with most
of the leaching occurring during the wet season. About 80Z of the total N
leached for the TRAD lysimeters occurred prior to mid-October and about 95Z of
the total for the two slow release materials also occurred during this time
period. A greater portion of the FERT treatment leaching occurred during the dry
season, when about 40Z of the total N was leached.
Table 1. Irrigations, rain, However, fertigation applications continued on
and fertigation applications these trees until early December, well into the dry
from June 1991 to Feb. 1992. season.
There was no significant difference (P=.05)
Month Rain Irrig. Fertig. in the quantity of N leached (NO3 + NO4) during the
(in) (No.) (No.) June-February period between the two slow release
materials (Table 3). About 60Z of the total N
June 5.4 7 0 leached from the slow release materials occurred
July 13.9 0 2 during the last week of July when almost 200 mm of
Aug. 8.1 0 rain fell. The soil remained extremely wet for the
Sep. 6.9 0 3
Oct. 5.7 1 3 whole week due to daily rains of 35-50 mm,
Nov. 0.9 0 3 resulting in soil conditions which were favorable
Dec. 1.3 5 2 for N leaching. Minimal N was leached from the
Jan. 0.7 2 0 slow release lysimeters after the heavy summer
Feb. 2.8 4 0 rains ceased in the first part of October. This
was probably due to the lack of heavy rainfall
events. However, another factor may have been the
decreased amount of N available for leaching in these lysimeters as the coatings
dissolved over time and encapsulated nutrients moved into the soil solution.
Leaching in the TRAD lysimeters typically peaked in the week following the
application of the fertilizer. The leaching decreased considerably during the
Ft. Pierce AREC 93-1
Table 2. Mean tree growth measurements for
subsequent weeks and February 1992 (n=4, no significant differences
subsequent weeks and peaked or any of the parameters at P=0.05).
* again following the next
application. Minor amounts of Total N Trunk X/C Tree canopy
leaching occurred from Material Applied area height voluPe
percolation below the root g cm m m
zones during dry periods (Fig.
1, October-November). However, SR-1 60 5.5 1.25 0.31
the largest portion of the FERT SR-2 60 5.4 1.44 0.33
leaching occurred from rainfall FERT 135 4.6 1.24 0.25
events following shortly after TRAD 135 4.5 1.29 0.28
4Si nfirant larhino in thp
Ft. Pierce AREC 93-1
FERT lysimeters occurred following saturating rains in early December and in the
FERT and TRAD lysimeters in early February.
When the weekly quantity of leached N was compared to rainfall during the
period, it was apparent that rainfall amount was directly related to leaching
(Fig. 2). Only small amounts of N were recovered from the TRAD lysimeters during
weeks when little (< 13 mm) or no rainfall occurred. This leaching, and a
slightly greater amount from the FERT lysimeters, resulted from irrigations which
carried the soluble fertilizers below the root zones of the young trees. The
slow release materials had almost no leaching from irrigations, and leaching was
insignificant until the rainfall exceeded 25 mm per week.
The boundary conditions of
the lysimeters correlate closely
with actual 'field conditions for
citrus on sandy soils in the 250-
South Florida Flatwoods Land
Resource Area. Following heavy
rains, a perched water table 200-
forms above the hardpan, creating.
forms above the hardpan, creating A / .............................................I
conditions similar to the E
lysimeters. The water table 15
drains laterally into the grassed
water furrows and on into 5
drainage ditches, allowing 100 /
nutrient removal by vegetation z
and volatilization to the TRAC
atmosphere. 50. .. FERT
It appeared that the SR-2
irrigation shcedules that were SR-
used were very satisfactory. 0 0.1-0.5 0.5-1 1-2 2-3 >3
Irrigations alone resulted in Weekly Rainfall (inches)
almost no leaching with the slow
release materials, and only minor FIG. 2. Mean N leached (NO -N + NO -N) by
amounts in the traditional treatments for various weekly rainfall
fertilization treatments. amounts (n=6 for no rain, n=10 for 0.1-0.5,
However, N leaching was found to n=6 for 0.5-1.0, n=5 for 1-2, n=3 for 2-3,
be closely correlated with the and n=4 for >3 inches).
amount of rainfall received each
week (Fig. 2). When rainfall exceeded 13 mm per week for the TRAD and FERT
treatments and 25 mm per week for the slow release materials, noticeable
increases in the leaching rates were apparent.
Fertigations on young trees in an actual grove setting can present a
challenge to minimize N leaching. During the first year or two, growers
fertigating often prefer microsprinklers which confine the discharge to a small
area about 1 to 1.5 m in diameter around the tree. This concentrates the water
and nutrients to the small root zone of the young tree, and prevents wasting
water and fertilizer on grasses and weeds beyond the wetted area. In this study,
a complete fertigation and flush cycle could be completed in 30 minutes. Even
so, leaching occurred on many occasions due to percolation below the root zone
on the sandy soils. In many groves, materials injected at the pump station may
Table 3. Mean quantity of total N (mg of NH -N + NO3-N) leached
for 4-5 week periods from June 18, 1991 to February 25, 1992 (n=4).
mm/dd 7/24 8/21 9/18 10/16 11/19 12/16 1/23 2/25 Total
Rain-in 7.6 12.1 7.2 7.5 0.6 1.1 2.0 2.4 40.5
SR-1 21ba 747a 143b 215b 24a 18b 2b 22b 1192b
SR-2 58b 762a 163b 256ab la 21b 2b 54b 1317b
FERT 23b 219b 451ab 696a 70a 377a 39a 454a 2327ab
TRAD 237a 460ab 772a 730a 5a 69b 15ab 470a 2759a
aMeans in the same column followed by the same letter are not
significantly different by Duncan's Muptiple Range Test (P=0.05).
take 30 minutes or more to reach the most distant tree. A complete fertigation
and flush cycle in these groves may take over 1 hour, thus increasing the
possibility of leaching soluble nutrients.
The slow release fertilizers were effective at reducing N leaching compared
to fertigation and traditional fertilization methods. The total N leached by the
fertigated.trees was about 2 times more than for trees fertilized with slow
release materials and the traditional applications resulted in about 2.3 times
more than the slow release treatments. However, for all treatments, the quantity
of N leached was less than 2.5Z of the total N applied.
Dasberg et al. (1987) reported 15N leaching losses of 15-16Z of that
applied for sprinkle irrigated bearing citrus trees and that up to 40Z could be
leached from flood irrigated trees. Another study with 6-year-old citrus trees
fertigated with a drip system resulted in 2-8% leaching of the total N applied
(Syvertsen et al., 1992). Calvert (1971) reported losses ranging from 8-32Z of
the applied'N on young trees irrigated with sprinklers. The low percentage of
applied N that was leached from this study was probably due to a combination of
factors. Only minimal leaching resulted from irrigations, which were carefully
scheduled to minimize leaching. More importantly, the low levels of N recovered
* in the drainage water can probably explained by denitrification of significant
NO3-N upon reaching the water table (Meek et al., 1970). Additionally, uplfux
from the water tables as upper soil levels dried may have transported some of the
NO3 back into the unsaturated zone, and thus made it available for extraction by
Trade names are used in this publication to provide specific information
and their mention does not constitute an endorsement by the University of
Florida. Funding and equipment for this project was provided by Diamond R
Fertilizer Company, Helena Chemical Company, Grace-Sierra Company, the St. Lucie
County (Florida) Soil and Water Conservation District, Maxijet, Inc., and Rain
Bird Sales, Inc..
Calvert, D. V. (1975). "Nitrate, Phosphate, and Potassium Movement into Drainage
Lines Under Three Soil Management Systems." J. Environ. Qual., 31(2), 229-
Dashberg, S. (1887). "Nitrogen Fertilization in Citrus Orchards." Plant and
Soil., 100, 1-9.
Koo, R. C. J., Anderson, C. A., Stewart, I., Tucker, D. P. H., Calvert, D. V.,
and Wutscher, H. K. (1984). "Recommended Fertilizers and Nutritional
Sprays for Citrus." Inst. Food Agr. Sci., Univ. Fla., Bul. 536D. 30 pp.
Meek, B. D., L. B. Grass, L. S. Willardson, and A. J. Mackenzie. (1970).
"Nitrate Transformations in a Column with a Controlled Water Table." Soil
Soc. Soc. Amer. Proc., 34, 235-239.
Syvertsen, J. P., M. L. Smith, and B. J. Boman. (1992). "Growth, Mineral
Nutrition, and Nutrient Leaching Losses from /soil of Salinized Citrus
I Trees." J. Agric., Ecosystems, and Environment. (in press).
Ft. Pierce AREC 93-1