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  Chemigation Guidelines
by
David F. Zoldoske and Gregory S. Jorgensen

CATI Publication #900804
© Copyright August 1990, all rights reserved

Introduction
Chemigation, or the use of your irrigation system to apply chemicals and fertilizers to crops, has gained in popularity in recent years. Much of this growth can be attributed to the adoption of pressurized sprinkler and micro-irrigation systems. These systems have shown themselves to be a good vehicle for many applications.

The application of fertilizers and pesticides through an irrigation system can be a safe and effective method, provided the system is properly designed and operated, and that safety precautions are followed. Benefits of chemigation include economical application, the potential for precise application, a reduction (due to fewer trips with tractors and sprayers) in soil compaction and mechanical damage to the crop.

Backflow Prevention
With the growth of chemigation, a new concern has arisen-backflow prevention. Water sources must be protected from contamination from chemicals introduced downstream. With all the "bad" press growers have received in recent years concerning pesticide residues and groundwater pollution, it is not surprising that government has started legislating controls. Several states, such as Minnesota and Nebraska have taken the lead in requiring protection on all agricultural well heads. These "chemigation valves" as they are referred to, are designed to stop the mixture of water and chemical from draining or siphoning back into the irrigation water source.

States which require the use of chemigation valves call for them to be sized in accordance with the manufacturer's recommendations, have an inspection port (four-inch minimum diameter orifice) and low pressure drain valve, and be made of chemical-resistant parts.

For small growers who use the same well for both drinking water and irrigation purposes, a higher level of protection is usually required by the local health department.

The highest form of protection is provided by an "air-gap separation" (AG). This is defined as an air-gap separation at least double the diameter of the supply pipe, but in no case should the AG be less than one inch. A common example of this method is a pipe dumping water into the top of a standpipe. However, any pressure developed by the pump or head gained in elevation is lost due to the implementation of the air gap. The air-gap separation is particularly suited for surface irrigation systems and adequately protects the water source from contamination.

The next level of protection is provided by the "reduced pressure principle backflow prevention device" (RP). These devices are required on irrigation systems that are, or can be injected with fertilizers, herbicides, or pesticides which are connected to potable water supplies.

"Pressure vacuum breakers" (PVB) or "atmospheric vacuum breakers" (AVB) have some limited applications. These devices are much simpler and less expensive than RPs.

However, PVBs and AVBs must not be used in areas where they are subject to backpressure. They must be installed at an elevation above the highest water outlet.

Additional equipment requirements may include a functional, normally closed, solenoid valve located on the intake side of the injection pump and connected to the system interlock to prevent material from being withdrawn from the supply tank when the irrigation pump is shut down in the course of an irrigation or power failure.

System Considerations
Once the proper backflow prevention device is installed, it is time to look at your irrigation system. The application uniformity of your irrigation system is an important consideration. Although no irrigation system is able to apply water so that each location receives exactly the same amount, it is important that variations be kept within reasonable limits for proper water, fertilizer, and chemical distribution.

With proper design and operation, applying chemicals through an irrigation system can be economical. Caution must be used to prevent source water contamination .

For a system with a Distribution Uniformity (DU) of 70, for example, the typical application rate in the quarter of the area receiving the least water/chemical will be 30 percent less than the overall average application. For a symmetrical application pattern, (this will be the case for most sprinkler and drip systems) there will be another quarter of the irrigated area that typically receives 30 percent more than the average water/chemical application.

Given the cost of agricultural chemicals, over-application is expensive, and may lead to environmental or other problems. On the other hand, under-application will not give the desired pest control or fertility response. The best defense against these problems is to be sure that your system is designed and maintained so as to provide as uniform an application as possible.

New Regulations
PR Notice 87-1, released by the Environmental Protection Agency, now requires pesticide manufacturers to include specific statements on the label regarding application through irrigation systems. The statements are required on all pesticides shipped after April 30, 1988. If the material is not registered for chemigation, the following statement should be found on the label: "Do not apply this product through any type of irrigation system."

For materials that are registered for chemigation, the following generic label statements are required:

a) Apply this product only through one or more of the following types of irrigation systems: sprinkler (including center pivot) lateral move, end tow, side roll, traveler, big gun, solid set, or hand move; flood (basin); furrow; border or drip (trickle) irrigation systems(s). Do not apply this product through any other type of irrigation system.

b) Crop injury, lack of effectiveness, or illegal pesticide residues in the crop can result from nonuniform distribution of treated water.

c) If you have questions about calibration, you should contact state extension service specialists, equipment manufacturers or other experts.

d) Do not connect an irrigation system (including greenhouse systems) used for pesticide application to a public water system unless the label-prescribed safety devices for public water systems are in place.

e) A person knowledgeable of the chemigation system and responsible for its operation, or under the supervision of the responsible person, shall shut the system down and make necessary adjustments should the need arise.

In addition to the generic label statement, the label will also contain specific label statements depending on the type of irrigation system.

Further requirements for materials intended for chemigation include posting requirements for toxicity category I, and nonspecific label statements, which are recommendations for mixing, agitation, and guidelines for the application of the pesticide with regard to duration of the water application.

It is important to note that these are only guidelines and the requirements may vary by county, state or federal regulations. For further information, or to ensure that your system meets the EPA requirements, contact your county agricultural commissioner's office.