Walter C. Bausch
Colorado State University Cooperative Extension
- Chemigation
is an agricultural practice that uses irrigation
water as a transport mechanism for the application
of chemicals to soils and crops.
- Uniform
application of the irrigation water is essential
to ensure uniform chemical distribution over the
field.
- Steep
slopes and hilly terrain may require pressure regulators
on sprinkler heads and higher operating pressures
to get satisfactory uniformity of water application.
- The
greatest concern associated with chemigation is
the potential pollution hazard; however, specific
hazards can be avoided through the installation
and maintenance of various safety devices.
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Chemigation
is an agricultural practice that uses irrigation water as
a transport mechanism for the application of chemicals to
soils and crops. Considerable interest in this practice has
evolved during recent years due to increased energy costs
and advancements in irrigation system design. Chemigation
is not a new concept. Rather, the basic idea originated more
than 25 years ago with the application of fertilizers through
a sprinkler irrigation system (Bryan and Thomas, 1958). Recently,
other chemicals such as herbicides, fungicides, insecticides
and nematocides have been effectively applied with irrigation
water.
Since
the current emphasis is on total chemigation, i.e., both soil
and foliar applied chemicals, sprinkler irrigation is the
only irrigation method that lends itself to this practice.
Other irrigation methods (surface and trickle/drip) can accommodate
only soil applied chemicals.
Uniform
application of the irrigation water is essential to ensure
uniform chemical distribution over the field. Most sprinkler
irrigation systems can be designed and operated to achieve
good uniformity of water application. Solid set, periodic
move (portable laterals and side rolls), and traveling big
gun sprinkler systems generally have a coefficient of uniformity
(CU) of 70 to 75 percent. The CU of continuous move systems
(center pivots and linears) normally ranges from 85 to 90
percent (P ir, 1975).
Of the
above-mentioned sprinkler irrigation systems, center pivots
and linears are probably best adapted to chemigation because
their coefficient of uniformity compares favorably with other
application methods. However, as with aircraft and ground
sprayers, the CU of these sprinkler irrigation systems decreases
with increased wind speed. Also, steep slopes and hilly terrain
may require pressure regulators on the sprinkler heads and
higher operating pressures to get satisfactory uniformity
of water application.
Many
advantages accrue to chemigation. This practice has been demonstrated
to:
- provide
good uniformity of chemical application,
- allow
timely application of chemicals,
- allow
easy chemical incorporation and activation,
- reduce
soil compaction,
- reduce
mechanical damage to the crop,
- reduce
operator hazards,
- possibly
reduce chemical requirements, and
- be
an economical method for chemical application.
There
also are disadvantages associated with chemigation.
- Chemical
applications may have to take place when additional soil
water is not required.
- Ground
water pollution is a possibility.
- Chemical
applications during windy conditions can result in serious
non-uniformity, especially with solid set and periodic move
sprinkler systems.
- Sprinkler
drift losses may become excessive when wind speeds exceed
15 mph.
- Wind
effects also can cause poor chemical deposition on the leeward
side of bed or hill planted crops.
Of greatest
concern is the potential pollution hazard associated with
chemigation. Two specific hazards are: (1) the irrigation
pumping plant may shut down from mechanical or electrical
failure while the injection pump continues to operate, thus
causing a mixture of water and chemical to backflow into the
irrigation well or other water source; and (2) the chemical
injection pump may stop while the irrigation pump continues
to operate, thereby causing water to backflow through the
chemical supply tank and overflow onto the ground. These hazards
can be avoided through installation and maintenance of various
safety devices.
Interlock
the irrigation pumping plant and the chemical injection pump
to provide shutoff of the chemical pump if the irrigation
pump stops. Interlocking the two pumps will prevent the possibility
of the irrigation pipeline being filled with the chemical
mixture from the supply tank. For internal combustion engine
powered irrigation pumps, the chemical injection pump can
be belt driven (Figure 1-A) from the drive shaft or from an
accessory pulley on the engine. For an electrical motor-driven
irrigation pump, a separate electric motor (Figure 2-A) is
required to power the chemical injection pump. The electric
controls for the two motors would be interlocked as shown
in Figure 2-B to ensure that the injection pump motor stops
when the irrigation pump motor stops.
Check
and vacuum relief valves (anti-siphon device) are required
in the irrigation pipeline. This check valve may sometimes
be referred to as an anti-back-flow valve. These are shown
in Figure 1-B and Figure 2-C. They keep water or a mixture
of water and chemical from draining or siphoning back into
the irrigation well. Both valves are located between the irrigation
pump discharge and the point of chemical injection into t
e irrigation pipeline. The check valve should provide a tight,
positive seal and be installed to permit easy removal for
maintenance and replacement of worn and non-functional components.
A suggested
method of installation is to use bolt flanges as shown in
Figure 3. Valves D and I (Figure 3) are recommended. Valve
D provides a fresh water supply for clean-up; valve I is a
manual drain to determine whether or not the check valve is
functioning properly. An observation port (Figure 3-A) also
could be installed as an additional method to determine the
functional status of t e check valve. A third valve (Figure
3-H) is a manual low-point drain valve that would provide
the capability to drain the water-chemical mixture from the
irrigation pipeline. Valve I (Figure 3) could be replaced
with an automatic drain valve that would automatically drain
seepage (if any) through the check valve thereby ensuring
that a water-chemical mixture could not backflow into the
well when the irrigation pump stops. If the pipeline is drained
(automatically or manually), this mixture should be contained,
that is, not allowed to drain onto the ground.
A check
valve in the chemical injection discharge line (Figure 1-C
and Figure 2-D) also is required to stop waterflow into the
chemical supply tank from the irrigation system. This valve
sometimes referred to as an in-line check valve should be
installed at the injection port (Figure 3-G) on the irrigation
pipeline. Its function is to prevent a possible chemical spill
at or near the irrigation well. In addition to this check
valve, a small, normally closed solenoid valve could be installed
in the chemical injection suction line (Figure 1-D and Figure
2-E.) This feature would provide a positive shut-off so that
neither the chemical nor the water could flow in either direction
if the chemical pump stops. An electrical interlock with the
motor driving the injection pump would be required.
Automatic
shut-down of the irrigation system, irrigation pump, and the
injection pump if pressure is lost in the injection discharge
line would be possible by installing a pressure switch in
the injection discharge line (Figure 1-E and Figure 2-F).
Primary reasons for pressure loss would be due to depletion
of chemical in the supply tank, plugging of the filter/strainer,
or injection pump failure. The pressure switch should be electrically
interlocked with the irrigation system control panel.
Maintenance
of the installed safety devices is essential to ensure that
all equipment is functional. This will require frequent inspections
and possible replacement of worn and non-functional components.
As a result, chemigation will be a safe practice and not a
pollution hazard.
- ASAE
Engineering Practice: ASAE EP409. Safety devices for applying
liquid chemicals through irrigation systems. 1983-1984 Agricultural
Engineers Yearbook of Standards, American Society of Agricultural
Engineers.
- Bryan,
B.B. and E.L. Thomas, Jr. 1958. Distribution of fertilizer
materials applied through sprinkler irrigation systems.
University of Arkansas Agricultural Experiment Station,
Fayetteville, AR, Bulletin 598, 12 pp.
- Fischbach,
Paul E. 1982. Applying chemical through irrigation systems:
safety and environmental considerations. In: Young, J.R.
and D.R. Sumner (eds.), Proceedings 2nd National Symposium
on Chemigation, Tifton, GA. August.
- Pair,
C.H. (ed.) 1975. Sprinkler irrigation. Fourth edition. Sprinkler
Irrigation Association, Silver Spring, Maryland.
Disclaimer
and Reproduction Information: Information in NASD does not represent
NIOSH policy. Information included in NASD appears by permission
of the author and/or copyright holder. More
NASD Review: 04/2002
Service
in Action
2.801
,
Cooperative Extension, Colorado State University. Published
January 1985. Reviewed October 1992. Copyright 1992. For more
information, contact your county Cooperative Extension office.
Walter
C. Bausch, former Colorado State University Cooperative Extension
irrigation specialist, agricultural and chemical engineering.
Reviewed by Stan Pilcher, Cooperative Extension entomology
agent, Golden Plains area.
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