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Chemigation - 1997

Low-flow irrigation systems are well suited to the injection of fertilizers and other materials because of the good water control allowed.  Chemigation both increases the efficiency of chemical application - placing the material in the root zone, resulting in decreased chemical use and cost - and reduces the hazard to those handling and applying the chemicals.  It is also less potentially harmful to the environment, compared, for instance, to air applications.  It also makes chemical distribution relatively uniform, allows flexibility in timing of applications, and reduces the labor required for applications.  However, chemigation can still cause environmental damage, particularly when the chemicals move readily with the irrigation water.  Over-irrigation resulting in deep percolation can contaminate groundwater when a mobile chemical is injected.

Contamination can occur if (1) the irrigation water pumping plant shuts down while the injection equipment continues to operate, causing contamination of the water source, or (2) the injection equipment stops while the irrigation system continues to operate, causing the irrigation water to flow into the chemical supply tank and overflow onto the ground.

Backflow prevention devices, including vacuum breakers (atmospheric and pressure types) and check valves (single and double) are available.  Local regulations should be followed in selecting and using these devices.

If the injection pump is electrically driven, an interlock should be installed so that the injection pump will stop if the irrigation system pump shuts down.  To keep water from flowing backward into the chemical tank, a check valve or a solenoid valve, normally kept closed, can be installed in the injection line following the injector.  If an electrical solenoid valve is used, it should be connected to the injector pump and interlocked with the irrigation pump.

Many different substances can be injected through irrigation systems, including chlorine, acid, fertilizers, herbicides, micronutrients, nematicides and fungicides.  Of these chlorine, acid and fertilizers are the substances most commonly injected.  Chlorine or acid injection is used in low-flow systems to prevent clogging caused by biological growths (algae and bacterial slimes) and chemical precipitation (particularly calcium carbonate).

The injection point should be located so that the injected material and the irrigation water can become thoroughly mixed well upstream of any branching of the flow.  Injecting at a point in front of the filter will reduce the chance of contaminants entering the system.  The system should be allowed to fill and come to full pressure before injection begins.  Following injection of fertilizers, the system should be operated to flush the fertilizer from the lines.  Leaving residual fertilizer in the line may encourage clogging from chemical precipitates or organic sources, such as bacterial slimes.  When injecting acids or chlorine for control of slimes, it is best to leave some residual material in the line.  When injecting any material make sure the irrigation components are compatible with the material, for example metal valves and injected acids can lead to some catastrophic consequences.

Irrigators wishing to inject chemicals have a variety of injection equipment from which to choose, including differential pressure or batch tanks, venturi devices and positive displacement pumps.

Differential pressure tanks are the simplest of the injection devices.  The tank of chemicals is connected at a point of higher pressure than the outlet connection.  This pressure differential causes irrigation water to flow through the tank containing the chemicals to be injected.  As the irrigation water flows through the tank, some of the chemical goes into solution, passes out of the tank and into the downstream system.  Because the tank is connected to the irrigation system, it must be capable of withstanding the operating pressure of the irrigation system.

Batch tanks are inexpensive and simple to use, but have the disadvantage that the chemical mixture in the tank becomes more and more dilute with time, decreasing the concentration in the irrigation water as injection continues.  If a set amount of chemical is to be injected and concentration during the injection is not critical, batch tanks may be appropriate to use.  If the chemical concentration must be kept relatively constant, such as with chlorine, batch tanks are not appropriate.  Also, if a pressure differential of about 5% cannot be obtained, the batch tank will not siphon.

Venturi devices consist of a constriction in a pipe's flow area, resulting in a suction at the throat of the constriction.  The venturi is frequently installed across a valve or other point where a 10 and 30 % pressure loss can be obtained.  Because of these significant pressure losses, the injector should be installed parallel to the pipeline so that flow through the injector can be turned off with a valve when injection is not occurring.  The injection rate of a venturi device is determined by the size of the venturi and the pressure differential between the inlet and outlet ports.  Venturi devices are inexpensive and relatively simple to operate, and equipped with a flow regulator can inject chemicals at a relatively constant rate.

When a constant and precise injection concentration is needed or when a pressure differential is difficult to achieve, positive displacement pumps are preferable.  These use a piston or diaphragm pump powered by electricity, gasoline or water.  These injectors are the most expensive of the injection devices, with costs for electric pumps running $750 or more.

Any materials injected into an irrigation line need to be soluble.  Any dry materials need to be mixed with water prior to injection.  Solubility of materials, such as fertilizers, vary widely, depending on the physical properties of the materials, as well as on water temperature and pH.

Chemical reactions with the salts in water are common, as well.  Waters high in bicarbonates could lead to precipitation of calcium carbonate if calcium nitrate is used.  If water calcium or magnesium levels are high along with a high pH, phosphate fertilizers, such as ammonium phosphate, can precipitate as calcium and magnesium phosphates. These precipitates can clog emitters.  Using a water conditioner, such as sulfuric acid or urea-sulfuric acid, to lower the water pH can minimize these precipitation problems.  Mixing injected fertilizers can pose problems, so ask your fertilizer dealer which ones are compatible.

Chemigation is cheap, efficient, effective and labor saving.  Any grower not using an injector is missing a valuable tool in improving the management not only of their trees, but of their irrigation system.