Historic note

Group Title: Research Report - University of Florida Agricultural Research and Education Center ; GC1981-15
Title: Factors to consider when using osmocote for poinsettia production in Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00056133/00001
 Material Information
Title: Factors to consider when using osmocote for poinsettia production in Florida
Series Title: Research Report - University of Florida Agricultural Research and Education Center ; GC1981-15
Physical Description: Book
Language: English
Creator: Harbaugh, B. K.
Wilfret, G. J.
Publisher: Agricultural Research & Education Center, IFAS, University of Florida
Publication Date: 1981
 Record Information
Bibliographic ID: UF00056133
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 62394262

Table of Contents
    Historic note
        Historic note
        Page 1
        Page 2
        Page 3
        Page 4
Full Text


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
of Florida

Po P

IFAS, University of Florida
Bradenton, Florida
Bradenton AREC Research Report GC1981-15 October 1981


B. K. Harbauch and G. J. Wilfret
Fertilization practices for poinsettia production often include Osmocote(R) to
supply part or all of the nitrogen, potassium, and phosphorous required for good plant
growth. However, many growers in Florida have experienced salt injury when using
this product and confusion over proper rates and formulations is common. The
following experiment was to illustrate the effects of temperature on the release
rate of two formulations commonly used for poinsettia production and to relate
these observations to the dangers, confusion, and proper use of this controlled
release fertilizer for poinsettia production in Florida, and especially South Florida.

Osmocote -i-14-1.. (3-4 month release) at 2.8 grams or Osmocote 18-6-12 (9-12
month release) at 2.2 grams (equivalent N to the Osmocote 14-14-14) was placed
in 400 ml of water in 500 ml jars and incubated for 96 days at 16, 230 or 300C
(600, 730, 86F). There were 3 replicates of each Osmocote formulation for each
temperature. There were an additional 3 replicates in which water was decanted
every 3 days to eliminate the effect of increasing salts in the solution on the
release rate. Total SS, NO, NH4, K and P in each solution were analyzed at 3, 6,
12, 24, 36, 48, 72, and 96 days.

The results for each element were consistent with the formulations and repre-
sented the proper percentage of the total soluble salts released. For ease of
presentation, only the release curves for total soluble salts are presented (Fig.
1 and 2). Since there were no significant differences in release rates between
solutions decanted every three days and those with no water change, these values
were combined. Thus, each point on the figures represents the mean of six obser-

The release curves illustrate some very significant relationships which can
be extrapolated to poinsettia production problems observed in grower operations.
For example, even though the Osmocote label states that the 3-4 or 8-9 month
release was calculated at 70F, these temperatures generally are exceeded during
the forcing period and always are exceeded during stock plant production phase
in Florida. Available data show that daily soil temperatures in pots average 86F
or higher during poinsettia production. Therefore, the top lines on the figures
represent the release rates expected if Osmocote is used for poinsettia production
in Florida. Osmocote 14-14-14 released much too quickly at 860F, with a functional
life of about 1 months (80% of the encapsulated nutrients were released by 48 days).
By "functional life", we mean that the release of nutrients is adequate for rapid
plant growth. Even though some nutrients continue to be released, the amount and
rate of release is adequate only for plant maintenance. Osmocote 18-6-12 released
about 70% of the encapsulated nutrients by 96 days, which is consistent with the
estimated functional life of 3-4 months observed in other experiments and in grower

A second example illustrates the problem of initial severe salt injury due to
excessive soluble salts that many growers have experienced when using Osmocote
14-14-14 during the summer. It in not unusual to use a 16# yd rate or 1 table-
spoon Osmocote per 6" pot (9 kg/ri ) which is about 18 gms per pot (6.4 times the
rate used in this test). Again using the 860F release curve, one can predict

approximately 17,000 ppm total soluble salts (TSS) released by 12 days. Allowing
for leaching, plant utilization, absorption of nutrients into the soil matrix and
concentration of nutrients into a soil solution less than used in the experiment,
it is understandable why 7,000 to 10,000 ppm TSS readings from saturated paste
extracts of grower samples periodically have been reported. Too many nutrients
are released too fast at 860F with nearly 1 times more soluble salts released
at 12 days at 860F than at 700F.

A third factor to consider is the confusion over the rate and formulation to be
used. Some growers ignore the release rate given on the label because of preference
for a balanced N-P-K ratio (Osmocote 14-14-14) or a high N ratio (Osmocote 18-6-12).
This indiscriminate use of one formulation over the other without regard to release
rate adds to the confusion already created by high soil temperatures. Osmocote
18-6-12 released about 4000 ppm TSS by 96 days at 860F. It would only take Osmocote
14-14-14 24 days to release an equivalent amount at 860F. If the grower had developed
the Osmocote 14-14-14 rate during winter months (or used rates from production guides
from Northern states with low soil temperatures averaging 600F for example) and then
switched to Osmocote 18-6-12 in the summer, he actually would end up with similar
release patterns (Osmocote 14-14-14 at 600F releasing nearly 4000 ppm TSS by 96 days
versus Osmocote 18-6-12 at 860F releasing 4000 ppm TSS by 96 days). On the other
hand, a rate of Osmocote 18-6-12 developed in the winter or in Northern growing
areas would probably be 2 times that needed in the summer, (Osmocote 18-6-12
only released about 1700 ppm TSS by 96 days at 600F; to have the equivalent 4000 ppm
TSS released at 60F would require approximately 2 times more Osmocote 18-6-12
than required at 860F). The additive effect of these examples of course would
develop the salt injury problem at its worst. That is, if a grower used Osmocote
18-6-12 in the winter he would have had to use the 2 times higher rates to get the
necessary nutrients released. If he then decided to use Osmocote 14-14-14 at the
same rates in the summer for a balanced N-P-K ratio, he would have an extremely
high fertilizer rate which was released about twice as fast as desired. We have
seen examples of this with grower samples reading 10,000-15,000 ppm TSS using the
saturated paste extract. The opposite of this example would of course be insufficient
nutrients supplied by Osmocote 18-6-12 used in the winter at rates found satis-
factory for Osmocote 14-14-14 in the summer.

In summary, the authors are not endorsing nor discouraging the use of Osmocote,
but rather trying to illustrate the necessity of understanding the principles behind
the release of nutrients. Growers must be aware of soil temperatures and use the
proper formulation and rates to compensate for deviations from the 700F release
rates stated on the label. At 860F, the release rate of both formulations tested
was about twice as fast as labeled at 700F, and thus the functional life of Osmocote
14-14-14 is 1-2 months instead of 3-4 months, while Osmocote 18-6-12 is 3-4 months
instead of 8-9 months. The results also illustrate why it is extremely risky to use
Osmocote 14-14-14 during the summer in Florida. If Osmocote 14-14-14 is used, rates
normally recommended with soil temperatures at 70"F should be reduced 1/3-1/2
the 700F rate and repeated in 2 months. If you are using production guides from
Northern production areas, the rates may be 1-2 times that needed in Florida, and
may also be released twice as fast, leading to serious salt injury. The Osmocote
18-6-12 formulation appears to be the safest to use in Florida for summer stock
plant production and for the first application when forcing begins in August. The
Osmocote 14-14-14 would be satisfactory for the second half of a split application
made in October which should last through poinsettia sales in December.

Fig. 1. Release rate of Osmocote 14-14-14 at 60, 73, or 86 F in water.


5000 73


3000 -(
y> / /60 C D

o 20
3000 -

S2000 30




3 6 12 24 36 48 72 96

Fig. 2. Release

5000 -

rate of Osmocote


60, 73, or 86

OF in water.





' 60 o

"50 .

40 a

-q Q
-30 0



36 12







;1. --q-

.,. ~

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