Group Title: Circular Florida Cooperative Extension Service
Title: Calibration of fertilizer injectors for agricultural irrigation systems
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 Material Information
Title: Calibration of fertilizer injectors for agricultural irrigation systems
Series Title: Circular Florida Cooperative Extension Service
Physical Description: 4 p. : ; 28 cm.
Language: English
Creator: Smajstrla, A. G ( Allen George )
Haman, D. Z ( Dorota Z )
Zazueta, F. S ( Fedro S )
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1992
Subject: Fertilizers -- Application   ( lcsh )
Irrigation   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: A.G. Smajstrla, D.Z. Haman and F.S. Zazueta.
General Note: Caption title.
General Note: "January 1992."
 Record Information
Bibliographic ID: UF00014464
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6893
ltuf - AJC0393
oclc - 25526205
alephbibnum - 001708121


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c January 1992 Circular 1033

'........* INS T U T E OF FOOD A N D A G R IC U L T U R AL S.C IEN C E-S

Calibration of Fertilizer Injectors :

for Agricultural Irrigation Systems

A. G. Smajstrla, D. Z. Haman and F. S. Zazueta*

Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean

Fertilizer application through an irrigation sys-
tem is called "fertigation". Fertigation is practiced
for several reasons the primary reasons are
lower application costs and greater production re-
sponses. It is normally less expensive to apply fer-
tilizers with irrigation water than by other meth-
ods, especially if several applications are required
during the growing season. Also, for many Florida
crops, growth and yields have been increased when
fertigation was used to maintain optimum nutrient
levels at critical crop growth stages.

Other reasons for the popularity of fertigation
include: (1) the ability to make prescription applica-
tions of fertilizers; that is, to apply them precisely
when needed and only in the amounts needed, thus
maximizing their effectiveness, (2) a reduction in
environmental pollution because fertilizers can be
applied only in the amounts needed and thus large
quantities are not subject to leaching loss if heavy
rainfalls follow conventional applications, and (3) a
possible reduction in the total volume of fertilizers
applied because leaching losses are minimized.

Growers should only inject water soluble fertiliz-
ers or fertilizer suspensions that are compatible
with their irrigation system and crop production
system. Because they are potentially corrosive,
fertilizers should be flushed from the irrigation sys-
tem after each application. Fertilizer solutions
should always be injected before (upstream of) the
filters in microirrigation systems. The compatibil-
ity of fertilizer solutions with the irrigation water
and with any other chemicals being injected should
be tested to avoid the formation of chemical precipi-
tates in the irrigation system. IFAS production
guides should be consulted for the specific crop be-
ing produced to obtain recommendations for chemi-
cal formulations, fertigation rates and schedules.

*Professor, and associate professors, Agricultural Engineering
Department, University of Florida, FL 32611.

Calibration of fertilizer injection
Each method of fertilizer injection must be cali-
brated by the user. Calibration procedures vary
depending upon the injection method used and
the specific design of the injection equipment.
In all cases, however, the user must verify that the
manufacturer's calibration or the method being
used is correct by using a chemical flow meter
which is accurate in the flow range of gallons per
hour (or other rate being injected), or by measuring
the injection rate volumetrically.

Chemical flow meters
Flow meters are available which can be used to
directly measure the chemical flow rate while the
injection system is operating under field conditions.
Meters can often be mounted on the low pressure
(suction) side of injection pumps.

Caution: If a chemical flow meter is used on the
high pressure side of an injector, be certain that the
flow meter is rated for the pressure being used be-
fore installing it in that position. Failure to use a
properly installed, adequately pressure-rated meter
may cause it to be damaged, and it may be hazard-
ous to individuals working in the area.

Volumetric flow rate measurement
To measure flow rates volumetrically, a con-
tainer of known volume (such as a graduated cylin-
der) and an accurate timer (such as a stopwatch or
a watch with second hand) are needed. Measure
the time required to fill the container. Then calcu-
late the flow rate as the volume per time, typically
in units of gallons per hour (gal/hr or gph).

For example, assume that a 100 ml graduated
cylinder and stopwatch were used to measure injec-
tion rates. Assume that 92 ml of fertilizer solution

was collected in 4 minutes, 31 seconds. To convert
ml to gal, divide by 3,785 ml/gal. Then 92 ml = 92/
3,785 = 0.0243 gal. To convert seconds to hours,
divide by 3,600 sec/hr. Then 4 min, 31 sec = 271
sec, and 271/3,600 = 0.0753 hr. Finally, 0.0243 gal/
0.0753 hr = 0.32 gal/hr.

For many injection methods, the injection pump
calibration will change depending upon the pres-
sure in the irrigation system. The actual injection
rate will be less when injecting against pressure.
Therefore, the above procedure can be used only as
a first approximation of the final calibration set-
ting, and the calibration will need to be finalized in
the field during fertilizer injection into the irriga-
tion system.

Finalizing calibrations in the field
To finalize the calibration of an injector, measure
the rate of fertilizer depletion from the storage
tank. Install a small graduated supply tank if the
bulk storage tank is not graduated for accurate
measurement of volumes injected.

It is a good idea to inject fertilizers from a small
graduated supply tank rather than to pump di-
rectly from a large bulk storage tank. The small
tank should be sized to contain the fertilizer solu-
tion needed for one application, and only the
required amount of fertilizer solution should be
placed in the small tank before the irrigation dur-
ing which it will be injected. This procedure can
improve the effectiveness of fertilizer injection be-
cause (1) only the amount of fertilizer in the small
supply tank can be injected during irrigation, thus
preventing excess applications from accidentally
being made, (2) the amount of fertilizer injected can
easily and accurately be read if the supply tank is
relatively small and has graduations permanently
marked on it, and (3) only the fertilizer in the small
tank will be diluted if backflow from the irrigation
system occurs because of failure of the injection
pump and backflow prevention system.

For injection methods which use a suction tubing
between the injection pump and the supply tank,
the injection rate can be measured with a chemical
flow meter or by connecting the tubing to a gradu-
ated cylinder. Measurements should be made
while the injector is operating under normal condi-
tions, including normal injection rates and normal
irrigation system operating pressures. Then ad-
justments in the injection rate can be made as the
injection system operates.

Calculating fertilizer injection
For all methods of injection, the required fertil-
izer injection rate must be known. The required
injection rate can be calculated from the following
equations for solid set (including sprinkler and mi-
cro systems), center pivot and traveling gun irriga-
tion systems.

Solid set irrigation systems
Solid set systems include sprinkler and micro
(drip and microsprinkler) irrigation systems. For
solid set irrigation systems, the fertilizer injection
rate in gallons per hour (gph) is calculated from:

Rate=[ 100 A F ]/[ P H W] (1)
where Rate = fertilizer injection rate (gph),
A = area to be irrigated (ac),
F = fertilizer amount to be applied per acre (lb/ac),
P = fertilizer fraction, percent of fertilizer per gal-
lon of fluid injected (%),
H = fertilizer application time (hr), and
W= weight of fertilizer solution (lb/gal).

As an example of the use of Equation (1), assume
that 20 lb of nitrogen (N) must be applied per acre
on a 5-acre citrus nursery using sprinkler irriga-
tion. The fertilizer to be used is a liquid solution of
Ammonium Nitrate (21% N; 10.73 lb/gal)1. The
normal irrigation cycle is 2 hr, and fertilizer injec-
tion begins immediately after the system has
reached normal operating pressure. Fertilizer will
be injected for 1.5 hr, leaving almost 0.5 hr to flush
the fertilizer from the irrigation system and off of
the plants. Using equation (1):

Rate = [100 5 ac 20 lb/ac]
[21% 1.5 hr 10.73 lb/gall
Rate = 29.6 gph

Thus, the required 20 lb of N can be applied per
acre by injecting 29.6 gal of Ammonium Nitrate per
hour for the 1.5 hr injection time. The total volume
to be injected would be 29.6 gal/hr times 1.5 hr =
44.4 gal.

'Because liquid fertilizer solutions are often custom mixed, the
nutrient concentrations and weight of the fertilizer solution
must be obtained from the fertilizer supplier. Common values
are used in this publication.

If the above 20 lb of N per acre must be applied
to a 5-acre citrus grove that is irrigated with a
microsprinkler system, Equation (1) can still be
used. Assume that the micro system is operated for
6 hr at each irrigation and that fertilizer is injected
for 4 hr. Fertilizer injection begins immediately
after the system has reached normal operating
pressure, leaving almost 2 hr to flush the fertilizer
from the irrigation system. Then:

Rate = [100 5 ac 20 lb/ac]
[21% 4 hr 10.73 lb/gal]
Rate = 11.1 gph

Thus, the required 20 lb of N can be applied per
acre by injecting 11.1 gal of Ammonium Nitrate per
hour for the 4 hr injection time. The total volume
to be injected is 11.1 gal/hr times 4 hr = 44.4 gal,
which is the same volume as calculated in the pre-
vious example. Note that because the fertilizer in-
jection time increased, the injection rate required to
apply the same 20 lb of N per acre decreased.

It is important to note that because citrus
microsprinkler irrigation systems do not irrigate
the entire soil surface, the fertilizers applied using
these systems will be placed only on the irrigated
portion of the soil surface. For example, if only 50
percent of the soil surface is irrigated with the
spray system in the above example, then the N ap-
plication rate in the irrigated zone will be 40 lb/
acre, and that in the non-irrigated zone will be 0 lb/
acre, to obtain the average rate of 20 lb/acre. Like-
wise, if only 20 percent of the soil surface is irri-
gated, the application rate in the irrigated area
would be 5 times the average on a gross acre basis.

Because water and fertilizers are not applied to
the entire soil surface when microirrigation sys-
tems are used, fertilizer applications to micro-irri-
gated crops are often made on the basis of indi-
vidual plants rather than on a gross acre basis. In
this case, Equation (2) applies:

Rate = [ 100 A Fp NP ] /[ P H W ] (2)
where F = amount of fertilizer to be applied per
plant (Ib/plant),
NP = number of plants per acre, and the other
terms are as previously defined.

For example, assume that 0.1 lb of N is to be ap-
plied to each tree in a 12-acre drip-irrigated grove

of young citrus trees. There are 200 trees per acre
(tree/ac). The drip irrigation system is operated for
3 hr per irrigation. After startup of the irrigation
system, fertilizer is injected for 2 hr, followed by
almost 1 hour of irrigation to flush the fertilizer
from the system. Then, for Ammonium Nitrate:

Rate = [100 12 ac 0.1 lb/tree 200 tree/ac]
[21% 2hr 10.73 lb/gal]
Rate = 53.3 gph

Thus, the required 0.1 lb of N per tree can be ap-
plied to 12 acres by injecting 53.3 gph for the 2 hr
of fertilizer injection time. The total volume to be
injected would be 53.3 gal/hr times 2 hr = 106.6 gal.

Center pivot irrigation systems
For center pivot irrigation systems, the required
fertilizer injection rate is calculated from:

Rate = [ 100 A F ] / [ P H W ] (3)
where H = fertilizer application time, normally the
time required to make 1 complete revolution of the
irrigation system, and all other terms are as previ-
ously defined.

Assume for example, that 30 lb of N will be ap-
plied per acre through a standard 160-acre (138
acres actually irrigated) center pivot irrigation sys-
tem. The fertilizer used is Urea-Ammonium Ni-
trate (28% N; 10.65 lb/gal). The irrigation time re-
quired to make 1 complete revolution is 48 hr. The
rate of injection can then be calculated as:

Rate = [ 100 138 ac 30 Ib/ac ]
[ 28% 48 hr 10.65 lb/gal ]
Rate = 28.9 gph

Thus, 28.9 gal of Urea-Ammonium Nitrate with
28 percent N must be injected per hour to apply 30
lbs of N per acre to the 138 acre area irrigated. A
total of 28.9 gal/hr times 48 hr = 1,388 gal must be

Traveling gun irrigation systems
For traveling gun irrigation systems, the injec-
tion rate is calculated from:

Rate = [ 100 S L F ] / [ 43,560 P W] (4)
where S = rate (speed) of traveler movement (ft/hr),
L = distance between travel lanes (ft), and all other
terms are as previously defined.

Assume for example, that 20 lb of N are to be
applied per acre to improved pasture irrigated with
a traveling gun system. Travel lane spacings are
200 ft and the traveler speed is 7 ft/min (420 ft/hr).
The fertilizer applied is Ammonium Nitrate (21%
N; 10.73 lb/gal). The required injection rate is cal-
culated from:

Rate = [100 420 ft/hr 200ft 20 lb/ac ]
[ 43,560 21% 10.73 lb/gal]
Rate = 17.1 gph

Thus the required 20 lb of N per acre would be
applied by injecting 17.1 gal of Ammonium Nitrate
per hour of traveling gun operation.

Make sure that the irrigation system compo-
nents will not be deteriorated by the fertilizer com-
pounds used. Handle fertilizers carefully. Wear
the appropriate protective clothing and protective

Florida law requires that irrigation systems into
which fertilizers are injected be equipped with
backflow prevention devices to protect the water

supply against contamination. Follow these regula-
tions by installing backflow prevention devices be-
fore beginning to inject fertilizers and by properly
maintaining and periodically testing your backflow
prevention system.

Fertigation is an appropriate production practice
for many Florida crops because of reduced nutrient
application costs and increased yields as compared
to conventional fertilizer application methods. Pro-
cedures for the calibration of fertilizer injection sys-
tems using chemical flow meters and volumetric
flow rate measurements were presented. Equa-
tions were given to calculate the required fertilizer
injection rates for solid set (sprinkler and micro),
center piv;t, and traveling gun irrigation systems.
Example problems were given for each of these
vpes of irrigation systems.

Fertilizers can be safely injected if proper safety
precautions are taken. Protect yourself by wearing
protective clothing and selecting and using injec-
tion equipment properly. Protect the water supply
by applying the right amount of fertilizers and by
using the proper backflow prevention equipment.

Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June
30, 1914 Acts of Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that
function without regard to race, color, sex, age, handicap or national origin. Single copies of extension publications (excluding 4-H and youth
publications) are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers
s availablefrom C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida32611. Before publicizing
this publication, editors should contact this address to determine availability. Printed 2/92.

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