Ted W. Tyson, Extension Agricultural Engineer

Larry M. Curtis, Extension Agricultural Engineer

There are sound economic reasons for chemigation, or the injection of fertilizers or farm chemicals through an irrigation system. There are even better reasons to take the steps necessary to protect water sources from possible backflow of the chemical during the injection process. This hazard exists regardless of the type of water source (reservoir, stream, or ground water) or the type of chemical (fertilizer, pesticide, or water treatment).

This publication discusses reasonable and available steps to take for protecting water resources while receiving the economic benefits of applying chemicals and plant food through irrigation systems.

The Irrigation System

Consider injecting chemicals only through drip/trickle, solid set, center pivot, or linear move irrigation systems. The uniformity and efficiency of application through other methods of irrigation is generally too low to be cost-effective.

Reasons To Chemigate

Existing water quality, particularly with drip/trickle irrigation systems, may require the addition of water treatment chemicals. Primarily, chlorine and acids are used to clean out bacteria or chemical deposits that clog the system and hinder its operation.

Uniform application of fertilizers or plant protection chemicals is much easier to get with a properly designed and operating irrigation system than with conventional application methods.

Smaller amounts of fertilizer can be applied by chemigation. throughout the growing season as the crop needs it. With conventional methods, large amounts of fertilizer are applied two or three times a season, which increases the chances that the fertilizer will be washed away or carried below the plants' root zone by heavy rains.

Fertilizing while irrigating frees the operator to do other things at the same time.

With sprinklers, herbicides can be incorporated to a uniform depth across the field, depending on the depth of water applied.

The danger to the operator from the chemicals may be reduced. Instead of several small tanks of chemical mix, only one or two large tanks are required for chemigation. In addition, the operator can avoid constant contact with the chemicals. Overall, the operator's contact with the chemicals is reduced, and consequently the risk of accidents is reduced as well.

The cost of chemigation, compared to that of aerial or ground application, is generally very competitive, particularly when two or more applications per year are made. Chemigation. costs usually average from one-third to one-half of the cost of either aircraft or tractor applications.

Because the irrigation system is already in place, tractors and other equipment do not have to make repeated trips across the field. This method not only saves money, time, and energy; it also reduces soil compaction, which helps maintain a good environment for root growth.

Protection Of The Water Source

The most important safety measure when injecting any chemical is the installation of a backflow prevention or anti-siphon device. This includes an operating anti-siphon check valve, an atmospheric vacuum breaker, and a low-pressure drain between the water source and the chemical injection point. The low-pressure drain should empty at least 20 feet from the well or water source.

Safety interlocks between the injection device and the water source and a check valve in the injection fine are also important to prevent the contamination of the water source by the injected chemical. The safety interlocks shut off the injection pump when the irrigation pump stops. The check valve will prevent backflow toward the injection pump.

For automatic operation, particularly with pivot or linear move systems, a pressure switch is necessary in the discharge line of the injection device. The pressure switch will shut off the irrigation pump when injection pressure decreases or vanishes because of problems with the injection pump.

Figure 1 shows the arrangement of this safety equipment with an engine-powered inigation system.

EPA Pesticide Requirements

Effective April 30, 1988, the Environmental Protection Agency's (EPA) Federal Label Improvement Program for Chernigation enacted the following additional requirements for the application of pesticides through irrigation systems.

A functional, reduced-pressure zone, backflow preventer (RPZ), or the functional equivalent, must be used between the water source and the pesticide injection point instead of the backflow/antisiphon device mentioned earlier when the chemigation system is connected to a "public water system." EPA defines a public water system as "... a system for provision to the public of piped water for human consumption if such system has at least 15 service connections or regularly serves an average of at least 25 individuals daily at least 60 days out of the year. "

The pesticide injection pipeline must contain a functional, normally closed, solenoid-operated valve located on the intake side of the injection purnp. This valve must be connected to the system interlock to prevent fluid being withdrawn from the supply tank when the irrigation system is either automatically or manually shut down.

The system must contain functional interlocking controls to automatically shut off the irngation pump when the water pressure decreases below design levels. The injection pump must be interlocked to automatically stop when the irrigation pump stops.

Pesticide injection systems must have metering pumps or venturi injection equipment made of materials compatible with pesticides and capable of being fitted with a system interlock.

Figure 2 shows the an-angement of EPA-required chemigation safety equipment for the injection of pesticides into irrigation water. State agencies and other political subdivisions with regulatory authority may have additional requirements.

The EPA also requires labeling of all pesticides approved for application through irrigation systems. Labels on any pesticide that is not intended to be applied through chemigation will state: Do not apply this product through any type of irrigation system. Labels on pesticides that are intended to be applied by chemigation will fist the types of irrigation systems intended to be used. Apply only those pesticides labeled for use in irrigation systems. Labels also have information on approved procedures and other restrictions.

Injection Pumps For Irrigation Systems

Pump Types. There are several types of injection pumps available. Positive displacement pumps can inject chemicals precisely and are operated by electricity, internal combustion engines, or water engines.

Electrical- or engine-powered pumps are usually the piston or diaphragm type. Generally, diaphragm pumps have less bearing surface exposed to the injected material and, as a result, have less wear and may last longer than a piston pump. Diaphragm pumps are also adjustable during operation, while piston pumps are not. For this reason, diaphragm pumps are becoming more popular.

Water-powered proportional pumps are also becoming more popular, particularly for drip/trickle systems. Some can be adjusted during operation and some cannot. Their real advantage is that they do not need any outside power source to operate. This feature can become a disadvantage, however, on highflow systems when the irrigation system operating pressure must be increased to provide water power to drive this type of pump.

Venturi suction devices are simple and inexpensive, operating by a pressure difference (generally 20 percent) from one side of the venturi to the other. The injection rate depends on this pressure drop. In systems with large irrigation flow rates, energy costs may prevent installation of the venturi in the main line. In this case, a venturi can be installed in series with a small centrifugal pump parallel to the main line. Even with the centrifugal pump, the pressuredependent venturi may not provide the accuracy necessary for pesticide injection without additional pressure regulation measures.

Differential pressure tanks or batch tanks are recommended for fertilizer injection only. Even then, they may not be entirely satisfactory. They have their inlet and outlet connected to the main line at two points having different water pressures. The pressure difference causes the water to flow through the tank and displace the fertilizer it contains. Although no power is necessary, the concentration of the fertilizer varies a great deal. When using batch injection, all of a batch should be emptied in each valve zone of the irrigation system to maintain distribution uniformity.

Pump capacity. Injection pumps are sized based on their injection capacity in gallons per hour (gph). The required injection rate for fertilizers and pesticides depends on the desired application rate of the injected chemical (pounds of active ingredient per acre), the percent strength and pounds per gallon of the chemical solution, and the number of acres irrigated per hour. Table 1 shows the solution strength and pounds per gallon of some popular nitrogen fertilizers. The injection rate can be calculated as follows:

Equation 1.

Gal

per       = 100 x pounds per acre x acre per hour

hour        percent solution x pounds per gallon of

                    Strength                     solution

Table 1. Solution Strengths And Specific Weights Of Some Liquid Nitrogen Fertilizers.

                                                                                             Solution Weight

Nitrogen Fertilizer                             %N                         (lb./gal. at 60*F.)  

Urea-Ammonium Nitrate                       28                                    10.65

Urea-Ammonium Nitrate                       32                                    11.06

Ammonium Nitrate                                21                                    10.73

Equation 2.

Gal.

per          =0.006 x parts per million x system gpm

 hour                   percent strength of source

Generally, the injection pump capacity for water treatment or pesticide application is so much smaller than that needed for fertilizer injection that two pumps are required.

Injection Point Location

With drip/trickle systems it is important to remember that the injection point should be between the anti-siphon device and the system filters. Give plenty of time for the fertilizer or chemical solution to mix uniformly in the irrigation water. It is a good idea to allow at least 25 to 30 feet and two 90-degree turns in the line between the injection point and the filter station. The turns create flow turbulence and allow any chemical precipitation to form in time for the filter to catch it, preventing it from clogging the system.

Fertilizer Application Management

While water treatment injection can begin immediately, for good uniformity, fertilizer injection should begin after the system has reached a steady operating condition in each zone fertilized. This may take 10 or 15 minutes, depending on the size of the system. Generally, only drip/trickle and solid set systems are divided into zones, depending on the available pumping capacity.

After the fertilizer injection in each irrigation. zone is completed, the system zone should be operated for at least 30 to 45 minutes with irrigation water only. This step flushes the system to make sure all of the fertilizer is applied and none stays in the system, where it might encourage growth of algae or create other clogging problems.

Chemigation Calibration

The calibration of the injection process is as important as the calibration of any other method of fertilizer or pesticide application. The injection calibration process, though, involves calibrating both the irrigation system and the chemigation equipment.

With pivot or linear move systems, knowing how long it takes to cover the irrigated field while applying the desired amount of water is critical. It should be actually measured. Manufacturer's application tables are not accurate enough for chemigation.

Measure, in hours, minutes, and seconds, the actual time it takes to apply the desired amount of water on the field. Measure also, if you are not sure, the actual acreage covered in this time. This actual irrigation time and actual acres covered will be used in the injection capacity formula to calibrate the chemigation process.

For injection of fertilizer into drip/trickle systems, an injection time equal to design zone irrigation set time less one hour is recommended. This allows adequate system-flush time for the zone. It also spreads fertilizer injection over a sufficiently long time that fertilizer/water concentrations generally do not approach saturation levels. This helps to prevent system clogging or less-than-expected crop response.

For water treatment in drip/trickle systems, knowing the irrigation system flow rate is an absolute must. The system gpm can be estimated by adding up the gph capacities of all emitters operating at one time and dividing the total by 60 minutes per hour, if the actual emitter operating pressure is known. It general ly is not known, however.

The system gpm may also be deten-nined by measuring the system pump lift distance, the pump rpm (revolutions per minute), and the outlet pressure, and then reading from the appropriate pump output curve. The pump curve method win work if distance measurements are accurate and the pump impeller has no significant wear.

The simplest and most accurate method of determining the system gpm is to install a system flowmeter. Propeller flowmeters range in price from $100 to $600 depending on size. These measure instaneous; system flow (gpm) and record total flow in gallons. Pitot or bobbin type flowmeters, ranging in cost from $40 to $100, measure only instantaneous flow. While not as accurate as propeller meters, pitot-type meters are generally sufficient for water treatment or fertilizer injection. Proper meter installation is critical to accurate readings. For best results, allow 6 to 10 diameters of straight pipe on either side of the meter.

The Calculation

To figure the amount of solution to inject per hour to apply the amount of chemical desired, use the injection PUMP capacity formulas presented earlier.

For example, using Equation 1, to apply 20 pounds of nitrogen per acre to a 15-acre drip/trickle zone in 7 hours using a 32-percent urea-ammonium nitrate solution (1 06 pounds per gallon), the injection rate would be:

Gal.

per       = 100 x 20 lb./ A. x 15 A./7 hr.      = 12.1

hour                 32 x 11.06 lb./gal.

You would need 12.1 gph of this 32-percent nitrogen solution to apply the nitrogen at the rate described above. A slightly higher volume of nitrogen/water solution could be injected per hour, as long as it contained at least 12.1 gallons of this fertilizer solution.

To apply the same 20 pounds per acre on 150 acres in 70 hours with a pivot system would also require 12.1 gallons per hour of this solution. The gallons per hour needed will change as any of the formula factors change.

To calculate the injection rate for a 5.25-percent household bleach solution to obtain an irrigation solution of 5 ppm when pumping 100 gpm, use Equation 2 as follows:

Gal

per             = .006 x 5 x 100     =  0.57

hour                       5.25

You must inject 0.57 gallons per hour of this 5.25-percent household bleach solution to obtain the desired 5 ppm solution strength in the irrigation water. A slightly higher volume of bleach/water solution could be injected per hour, as long as it contained at least 0.57 gallons of 5.25-percent bleach.

References

Boswell, Michael J. 1985. Micro-Irrigation Design Manual. James Hardie Irrigation. El Cajon, California.

Curtis, Larry M. 1982. Chemigation : Chemical Application Through Irrigation Equipment. SW82-2. Alabama Cooperative Extension Service, Auburn University.

Pitts, Donald J., and PH L. Tacker. 1987. Trickle Irrigation: Causes and Prevention of Emitter Plugging. MP 271. Arkansas Cooperative Extension Service, University of Arkansas.

Tinsworth, Edward F. 1987. PR Notice 87-1, Label Improvement Program For Pesticides Applied Through Irrigation Systems (Chemigation). Registration Division (TS-767C), Environmental Protection Agency, Washington, D.C.

Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.