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
GULF COAST RESEARCH AND EDUCATION CENTER
g WATER MANAGEMENT SERIES
. ... ,. / / I ..
Institute of Food and Agricultural Sciences, University of Florida
GCREC Bradenton Research Report
Principles of Chemigation
Gary A. Clark
PRINCIPLES OF CHEMIGATION
Gary A. Clark, P.E.
Assistant Professor of Agricultural Engineering
University of Florida
Gulf Coast Research and Education Center
Chemigation is the application of chemicals to a crop through an
irrigation system. Chemicals are any substances injected into the
irrigation water which alter the physical or chemical makeup of the water
and include materials such as acid, chlorine, fertilizer, and pesticides.
The intent of chemigation is to provide a benefit to the crop production
system by using the irrigation system to convey a chemical to the crop,
or by altering the chemical makeup of the water supply to benefit the
crop, alter the soil characteristics, or to clean the components of the
irrigation system. Chemigation should not be used if a net benefit is not
realized such as reduced labor to apply chemicals or improved efficiency
of chemical application. In addition, the injected chemicals must not have
an adverse reaction with any component of the irrigation system or with
the chemical makeup of the supply.
Chemigation can be used to provide a net benefit to the production
system by following some basic guidelines and operational principles.
Elements of chemigation include irrigation system design, sizing and
placement of injectors, chemical mixture and injection calculations,
maintenance, and safety including proper backflow prevention.
Irrigation System Design
Proper irrigation system design is critical to the successful
operation of chemigation. Water application must be uniform, within
design standards, and to the cropped area. Non-uniform water
application results in non-uniform chemical application. Chemical
application uniformity cannot exceed water application uniformity. Thus,
non-uniform application results in either over-application of chemicals
to one area, under-application of chemicals to another area, or a
combination of both. This in turn results in economic losses either
through reduced crop quality or low efficiency of chemical application.
Furthermore, excessive chemical application in order to compensate for
non-uniformity, may result in a violation of the application label.
New irrigation systems should be designed with the intent to use
the system for chemigation purposes. Existing irrigation systems should
be evaluated in terms of uniformity of application, efficiency of water
delivery, and compatibility of existing parts to assess the
appropriateness of chemigation. These tasks should be performed by
individuals properly trained in irrigation design and chemical application.
Injection System Design
Design of the injection system includes sizing of the injector,
placement of the injector, and providing power and control to the
injector system. Injectors must be sized to inject the intended chemical
within the operational time constraints of the irrigation system.
Sometimes, multiple or portable injection systems will be necessary for
large and/or diverse crop production systems. Time factors associated
with chemical injection include the time to pressurize the irrigation
system, the time to inject all of the intended chemical, and time to flush
the chemical from the irrigation lines. The amount of time required to
flush the chemical out of the irrigation system depends on irrigation
system design and the velocity or speed of water travel in the pipelines.
Higher velocities result in shorter flush times. However, proper
irrigation design limits flow velocities to be no greater than 5 feet per
second. Shorter flush times can also be achieved by placing the injector
as close as possible to the application zone.
Never mix chemicals with the injector and always flush injectors
with water between chemigation cycles which use different chemicals. In
general, fertilizers may be injected as a bulk solution without regard to
the concentration of the fertilizer in the water as long as the fertilizer
remains in solution and is not at crop damaging concentrations. Venturi
or bulk tank injection systems may be used for this task. When specific
fertilizer concentrations are desired or pesticides are injected,
positive displacement type pumps will be required.
Chemical Injection Calculations
Chemical Mixtures and Injections
Chemicals may be applied as a precisely managed level of injected
concentration or as a bulk mass of chemical with possibly varying
concentration levels. Concentration injection requires a precise
injection system and is more tedious and involved than bulk injection.
The injection system must be specifically calibrated for the irrigation
system it is to be operated on and under the operating conditions that
will exist when chemicals are to be injected. Variations in operating
pressure, system flow rate, and at times even temperature can influence
the calibration of the system. Bulk injection simply involves the
injection of a desired volume or amount of chemical into the system. The
injection rate does not need to be precisely controlled.
Concentrations are generally expressed in parts per million (ppm),
which is not a convenient term for mixing purposes. Chemicals may be
supplied in dry form or liquid form. The following equation can be used
to determine the mass of chemical mixture (such as fertilizer mixture)
required to achieve a particular ppm level of a certain chemical element.
Ib of chemical mixture
to add to 100 gallons
For example, a
100 desired ppm
(----------- con. X) 1205
(% conc. X) 1205
200 ppm concentration level of nitrogen as a fertilizer
a 20-20-20 dry fertilizer mixture will require:
100 200 ppm
(2--------) -- 1205
0.83 lb. of fertilizer mixture, per
100 gallons of water
When chemicals are supplied in liquid form, it is more convenient to
measure volumes rather than masses or weights. This requires the
specific density or specific weight (Sx) of the liquid mixture such as
pounds of solution per gallon of liquid. This property should be provided
by the manufacturer or chemical supplier and can be used to simply
convert from required Ibs. to required gallons. For example a liquid
fertilizer provided as a 4-0-8 solution has 4% nitrogen by weight.
However, the amount of nitrogen is not known unless the specific weight
of the chemical (fertilizer) solution is known, such as 10 lb. per gallon.
As was previously mentioned, this value varies among chemical mixtures
and should be on the chemical label, but may be obtained from the
Concentration Injection Rates
Concentration injection rates work with the flowing water in
irrigation systems and a requirement to maintain a desired concentration
of a chemical in that system. This requires injecting a chemical supply
mixture at the proper rate to maintain the desired concentration level.
If the specific weight of the chemical is close to that of water, the
following equation, can be used to estimate the injection rate necessary
to maintain the desired concentration of a chemical X.
Qi = ---------------
Injection rate in gallons per minute (gpm),
Water supply flow rate (gpm),
Desired ppm level of chemical X, and
Percentage of chemical X in the stock solution.
As an example, consider a chlorine injection cycle where the irrigation
system flow rate is 550 gpm, a 5.25 % chlorine solution is used, and a 10
ppm free chlorine concentration is intended.
Qw = 550 gpm; ppmx = 10; and %X = 5.25; and
Qi =-------- =0.10 gpm; the necessary injection rate of
(5.25)(10000) chlorine solution
Injection rates of gallons per minute (gpm) can be converted to gallons
per hour (gph) by multiplying the gpm value by 60.
To determine the required injection volume for bulk applications, the
mass of the desired chemical, the percent concentration of the chemical
in the mixture, and the specific weight of the mixture solution must be
100 (Mass of required chemical, Ib.)
V = -------------------------------------- (3)
(% cone X) (Specific Wt. of the mixture, lb./gal)
V = Required mixture volume (gal), and
% cone X = The percent concentration of the desired
chemical in the mixture.
For example, a grower wishes to apply 4 lb. of N per 1000 feet of
plant row each week in one injection cycle; 20 acres are to be irrigated
per set; and the system has 4500 feet of plant row and drip tube per
acre. What size feeder tank is necessary for injecting a 4-0-4
fertilizer solution which has a specific weight of 10.0 lb per gallon?
The weekly production requirement of total N is:
(20 acres)(4500 ft/acre)(4 lb of N/1000 feet) =
= 360 lb of N per week.
The injection volume per application is (Equation 3):
100 360 lb.
V = --------------------- = 900 gallons of fertilizer solution
4 % 10.0 lb./gal per weekly injection cycle
Therefore, the feeder tank must be at least 900 gallons to provide the
needed volume for each weekly fertilizer application.
Sometimes chemicals are injected on a periodic basis to maintain a
certain injected concentration of that chemical during that period. In
this case, the required injection volume of stock solution depends on the
length of the injection period, and the injection rate. The required
stock solution volume can be determined from Equation (4) as follows:
V = (Qi) (Ti) (8)
where V = Required mixture injection volume (gal),
Qi = Injector flow rate (gpm), and
Ti = Injection period (minutes).
For example, a micro irrigation system manager desires to inject 10
ppm of free chlorine into his irrigation system for a period of 40
minutes. The irrigation system delivers water at a rate of 550 gpm, the
chlorine stock solution contains 5.25% of sodium hypochlorite (free
chlorine). From a previous example, the required injector flow rate was
0.10 gpm. Therefore, the required injection volume is:
V = (0.10 gpm)(40 minutes)
= 4 gallons of stock solution.
Therefore, only a small feeder tank is required for this application.
Injection Periods and Calibration
The length of the injection cycle is important from an irrigation
management viewpoint. With respect to the injection period, several
criteria may need to be addressed, such as the frequency of chemical
application (daily, semi-weekly, weekly, etc.) and the maximum time
allocated per irrigation zone. For example, for daily chemical
applications the number of irrigation zones multiplied by the injection
period per zone cannot exceed 24 hours. Furthermore, if the injection
period exceeds the maximum irrigation period which results in over-
irrigation and leaching, then split chemical applications are necessary.
The injection volume was discussed in the previous section. Injection
rate may be provided by the supplier of the injection system. However,
whether the injection rate is already available or not, calibration is
required. Calibration should be performed on the irrigation system which
is to be used with the injection system. Also, because irrigation system
operating pressures and flow characteristics may influence injection
rates, calibration should be performed while the irrigation system is
A simple calibration procedure involves physical measurement of the
injected volume during the measurement period. This procedure can be
performed by either measuring the time to inject a known volume of
liquid, or by measuring the volume of liquid injected during a pre-set
specified injection period. In each method a container is filled with a
known volume of the chemical to be injected. Water or colored water may
be substituted for the chemical but may not provide accurate results
with some injectors if the viscosity is very different from that of the
Information pertaining to the injection of chemicals into irrigation
systems was discussed in terms of concentration injections, bulk
injections, and quantity of chemicals to be injected. This information
was provided to assist irrigation system designers and operators with
the chemigation aspects of irrigation system design, scheduling and
management. Additional information and detail may be found in University
of Florida Cooperative Extension Service Bulletin 250 by G. A. Clark, A. G.
Smajstrla, D. Z. Haman, and F. S. Zazueta (1990).