Drip Chemigation: Injecting Fertilizer, Acid and Chlorine (B 1130) University of Georgia Extension Drip irrigation is an important component of vegetable production systems in Georgia. Drip irrigation is more desirable than other irrigation methods for several reasons. Two important advantages are (1) water conservation and (2) potentially significantly improving fertilizer management. Fertigation is the timely application of small amounts of fertilizer through drip tubes directly to the root zone. Compared to conventional ground application, fertigation improves fertilizer efficiency. 2017-01-30 13:47:24.607 2006-06-02 14:27:23.0 Drip Chemigation: Injecting Fertilizer, Acid and Chlorine | Publications | UGA Extension Skip to content

Drip Chemigation: Injecting Fertilizer, Acid and Chlorine (B 1130)

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Darbie M. Granberry, Extension Horticulturist
Kerry A. Harrison, Extension Engineer
William Terry Kelley, Extension Horticulturist

Drip irrigation is an important component of vegetable production systems in Georgia. In 1982, drip irrigation was used on fewer than 3,000 acres of Georgia vegetables. Ten years later, more than 17,000 acres of the state’s vegetables were drip-irrigated. This trend is not unique to Georgia. In a 1993 survey of the southeastern United States, 86 percent of respondents indicated acreage of drip irrigation was increasing in their states, and 97 percent of the drip-irrigated vegetables were grown on plastic (polyethylene) mulched beds. More vegetable growers are using plastic-mulched beds and drip irrigation to enhance yields and quality and, in some instances, to promote earlier maturity. Because drip irrigation has proved to be the best irrigation method for vegetables grown on plastic mulch, it has become an integral part of this production system.

Drip irrigation is more desirable than other irrigation methods for several reasons. Two important advantages are (1) water conservation (drip requires about half as much water over the growing season as sprinkler irrigation), and (2) the potential exists for significantly improving fertilizer management.

Fertigation is the timely application of small amounts of fertilizer through drip tubes directly to the root zone. Compared to conventional ground application, fertigation improves fertilizer efficiency. Subsequently, comparable or better yields and quality can be produced with 20 percent to 50 percent less fertilizer.

Mineral precipitates (often seen as scale deposits), algae and bacteria clog drip emitters. Clogged emitters cause variable water distribution during irrigation and uneven fertilizer application during fertigation. Variable water or fertilizer application hinders uniform crop development, reduces yields and jeopardizes quality. For growers to effectively use drip technology, they must prevent clogging drip emitters.

Chemigation

Chemigation Technology

Chemigation is an inclusive term referring to the application of a chemical into or through an irrigation system. It includes the application of fertilizers, acids, chlorine and pesticides.

Fertigation is specifically the application of fertilizer (plant nutrients) through an irrigation system.

Acidification is the introduction of an acid, such as phosphoric, sulfuric or hydrochloric (muriatic) acid into an irrigation system.

Chlorination is the introduction of chlorine, such as liquid sodium hypochlorite (household bleach) or chlorine gas into an irrigation system.

Because drip emitters are small, they clog easily. An adequate filtration system is necessary to prevent the introduction of soil particles (sand, silt and clay) and water-borne debris into drip tubes. Additional anti-clogging techniques include acidification, which prevents or removes mineral precipitates, and chlorination, which removes and prevents the growth of bacteria and algae.

To effectively fertigate crops, growers must properly maintain drip systems so they apply water and fertilizer uniformly. In addition, growers need to determine (1) which fertilizer formulations are most suitable for injection, (2) the most appropriate analysis for specific crops at specific stages of growth, (3) the amount to apply during a given fertigation event, and (4) the timing and frequency of applications.

Benefits of Chemigation

Uniform Application

Chemigation facilitates the uniform distribution and precision placement of fertilizers and other chemicals.

Timely Application

In most cases, materials can be applied regardless of weather or field conditions.

Reduced Application Costs

In general, cost of application by chemigation is about one-third the cost of conventional application methods.

Improved Management

Timely applications of small but precise amounts of fertilizer directly to the root zone allow growers to effectively manage fertilizer programs. This conserves fertilizer, saves money and optimizes yield and quality.

Reduced Soil Compaction

Chemigation reduces tractor and equipment traffic in fields. This reduces soil compaction.

Reduced Exposure to Chemicals

Chemigation minimizes operator handling, mixing and dispensing of potentially hazardous materials. Also, people and non-target crops are not exposed to inadvertent chemical drifts.

Reduced Environmental Contamination

When used with the recommended safety devices, properly-designed and accurately-calibrated chemigation systems help preserve quality of the environment.

Chemigation can save time, reduce labor requirements, and conserve energy and materials. However, chemigation is beneficial only to the extent that the drip irrigation system is adequately designed, fully functional and properly managed.

In many situations, chemigation is as good or better than conventional application methods. However, conventional application is still preferred or required for some materials. Never inject any material that is not labeled and recommended for the crop and for injection through the system. Always follow label directives.

General Principles of Chemigation: Safety Considerations

The irrigation pumping plant and the chemical injection pump should be interlocked so, if the irrigation pumping plant were to stop, the chemical injection pump will also stop. This will prevent chemicals from the supply tank from filling irrigation lines should the irrigation pump stop. With internal combustion engines, the chemical injection pump can be belted to the drive shaft or an accessory engine pulley. Injection pumps driven by electric motors require a separate one-third or one-half horsepower electric motor for the chemical injection pump. Controls for the motors should be electrically interlocked to stop the injection pump motor whenever the irrigation pump stops. This is shown in Figure 1 for electric motors.

Figure 1. A typical electrically driven chemigation system. Figure 1. A typical electrically driven chemigation system.

Check and vacuum relief valves (anti-siphon devices) are necessary safety devices. They prevent water or mixtures of water and chemicals from draining or siphoning back into the water source. Both valves must be located between the irrigation pump discharge and the point where chemicals are injected into the irrigation pipeline (Figure 1).

A check valve should be installed in the chemical injection line to prevent the back flow of water from the irrigation system into the chemical supply tank. If the injection pump stops and there is no check valve, irrigation water can flow through the injection line into the chemical supply tank. Subsequently, the tank may overflow and cause a chemical spill around the water source. Chemicals from such spills can contaminate ground and surface water.

An additional safety item is a small, normally closed solenoid valve to be electrically interlocked with the engine or motor that drives the injection pump. This solenoid valve provides a positive shutoff in the chemical injection line, which stops chemical or water flow in either direction if the injector pump stops.

For automated control, a pressure switch should be electrically interlocked with the safety panel on the irrigation system. This switch will automatically shut down the irrigation system and the injection pump if pressure is lost in the injection discharge line. Usually, loss of pressure in injection lines occurs when the chemical tank is pumped dry.

The American Society of Agricultural Engineers (ASAE) standard EP 409.1 can be used as a general guideline for backflow prevention devices. However, if you are chemigating in Georgia, keep in mind that the Georgia Department of Agriculture enforces backflow prevention regulations in Georgia. Please contact the Georgia Department of Agriculture for current guidelines.

Injection Pumps

Two basic types of injection pumps — the Venturi (Figure 2) and the metering pump (Figure 3) — are commonly used for injecting fertilizer and other chemicals into drip-irrigation systems. Field setups for both types should have an adjustable injection rate. Any components that will be in contact with fertilizer, chlorine or acid should be resistant to corrosion.

Figure 2. Venturi chemical injector. Figure 2. Venturi chemical injector.
Figure 3. Chemical metering pump. Figure 3. Chemical metering pump.

Venturi

The Venturi system creates a pressure differential that forms a vacuum. As water flows through the tapered Venturi orifice, a rapid change in velocity occurs. This velocity change creates a reduced pressure (vacuum), which draws (pulls) the liquid to be injected into the system. Since the injection rate will vary with the pressure differential across the Venturi, a precise regulating valve and a flow meter are recommended for calibrating the system.

Metering Pump

Positive displacement metering pumps are often used to inject chemical solutions into drip irrigation systems. Portable positive displacement pumps can be moved from field to field. Metering pumps may be powered by small electric motors or by hydraulic drive systems. Hydraulic drive systems use the water pressure in the system to power the pump. In the past, injection rates of positive displacement pumps were adjusted by changing the length of the piston stroke. However, injection rates of some of the more recent models can be adjusted with a variable frequency drive. This drive varies the speed of the injection pump with the flow rate of the irrigation system.

Diluting Chemical to Be Injected

Injection pumps must be accurately calibrated by properly adjusting the injection rate. Ideally, an injection pump should be capable of being adjusted to the desired injection rate. However, it is not always possible to obtain an injector pump that accurately injects at low chemical injection rates (commonly encountered with small drip systems). If the injector is to also inject fertilizer, it will need sufficient capacity for injecting fertilizer. Injection rates for fertilizers are usually much higher than injection rates for chemicals such as liquid chlorine or acid.

In some situations, it may not be possible to lower the injection rate enough to inject concentrated solutions at the desired rate. This problem can usually be overcome by adding a precise amount of water to dilute the concentration of the active ingredient in the solution (see example under “Chlorination to Control Algae and Bacteria”).

Point of Injection

Chemicals should be injected into the system at a point before the filters. Filters help prevent particulate
matter which may be in the chemical solution from entering the irrigation system and causing clogging problems.

Determining Injection Rate

Before calibrating the injection pump, determine the desired injection rate. Use the following steps as a guide.

  1. Determine the area (acres) to be chemigated.
  2. Determine the volume of chemical solution (gallons) to be applied per acre.
  3. Determine the total number of gallons needed to treat the area (step 1 x step 2).
  4. Determine how long (hours) the system will be run during this chemigation event.
  5. Calculate the desired injection rate in gal/hr (step 3 divided by step 4).
  6. Use the following equations to convert gallons per hour (gal/hr) to milliliters per minute (ml/min) or ounces per minute (oz/min). Equation 1: 63.09 x gal/hr = ml/min. Equation 2: 2.13 x gal/hr = oz/min.

Example: 10 acres are to be chemigated with 1.3 gal of solution per acre and the chemical is to be injected for 1 hour. (A) How many gallons of chemical solution will be required? (B) What is the desired injection rate? (C) How many ml of solution would a correctly calibrated pump inject each minute?

  1. Determine how many gallons of chemical solution are required (step 1 x step 2). 10 acres x 1.3 gal per acre = 13 gal. Therefore, 13 gallons of chemical solution are required for 10 acres.
  2. Determine the injection rate (step 3 divided by step 4). Thirteen gallons divided by 1 hour = 13 gal/hr. Therefore, the desired injection rate is 13 gallons per hour.
  3. Determine how many ml of solution are to be injected each minute (equation 1). 63.09 x 13 gal/hr = 820.17 ml/min. Therefore, a correctly calibrated injection pump will inject 820 ml (rounded to nearest whole number) of chemical solution in 1 minute.

Calibrating the Injection Pump

Chemigation should never be attempted without accurate calibration. Manufacturers’ suggested settings are helpful guides. However, to ensure that recommended amounts are being applied at the desired concentrations, calibrate the injection pump on-site.

The objective of calibrating the injection pump is to adjust the pump injection rate to the desired injection rate. The pump injection rate is determined by measuring the volume of solution pumped through the injection pump (injected volume) during a specific duration of time (usually 60 to 120 seconds).

The injected volume can be determined by any of the following methods:

Method 1 — Using a graduated cylinder, measure a selected volume of the solution to be injected. The selected volume should be of sufficient quantity to allow injection for several minutes. Place this known volume into a container connected to the intake line of the injection pump. With the system operating and fully charged, activate the injection pump and determine the number of seconds required for this known volume to be injected.

Method 2 — This method is similar to the above method. The primary difference is, in method 2, only a portion of the measured chemical solution is injected. Using a graduated cylinder, measure a selected volume of the solution to be injected. This selected volume should be of sufficient quantity to allow injection for several minutes. Place this known volume into a container connected to the intake line of the injection pump. With the system operating and fully charged, activate the injection pump for a specific duration of time. This injection period should be for several minutes. However, it should be short enough so that only a portion of the solution is injected. At the completion of the injection period, measure the volume of solution left in the container. The volume of injected solution is determined by subtracting the amount remaining after injection from the original volume.

Method 3 — In this method, the solution pumped through the injection pump during a given period of time is collected and measured. With the system operating and fully charged, activate the injection pump for a specific time (2 to 5 minutes). Divert the output line from the injection pump into a container. A pressure regulating device should be installed on the output line to simulate system back pressure. Measure the output with a graduated cylinder to determine the volume of chemical injected.

Since operating pressures and flow characteristics of irrigation systems may influence injection rates, it is necessary to perform calibration procedures with the irrigation system operating and fully charged. Before beginning calibration, make sure the system is primed, that it is operating at the same pressure it will be during injection, and that suction and discharge lines do not contain air bubbles. Also, during calibration, keep the point of injection at the same height that it will be during actual chemigation. Once the pump’s injection rate has been determined, this rate can be adjusted until the desired injection rate is achieved.

Example — Using the calculations from the previous example and following manufacturer’s operating instructions, set the injector pump to 13 gallons per hour (from step 5 of the previous section, “Determining Injection Rate”). With the system running, fully charged and the point of injection at the same height that it will be during actual injection, collect and measure the solution pumped through the injector pump in 60 seconds. The volume should be 820 ml (from step 6). If any amount other than 820 ml is pumped through the injector pump, re-adjust the injector pump setting and repeat this procedure until 820 ml are collected in 60 seconds. When 820 ml are collected in 60 seconds, the injector pump is calibrated to inject at the desired rate of 13 gallons per hour.

Fertigation

Fertilizers are the chemicals most often injected int drip irrigation systems. The potential for fertigation is one of the primary reasons many growers install drip irrigation. Properly managed applications of plant nutrients through drip systems significantly enhance fertilizer efficiency while maintaining or increasing yield. On the other hand, poorly-managed fertigation may result in substantial yield losses.

Fertilizers are available in different forms and concentrations. Formulations usually contain two or more nutrients and the solubilities of various formulations vary significantly. Fertigation involves deciding which nutrients (and how much) to apply, selected the most effective formulations, properly preparing solutions for injection and scheduling injections (Table 1) to ensure that essential nutrients are available as needed.

Table 1. Injection Schedules for Selected Mulched Vegetables.
Crop Estab.
Methodw
Typical Bed
Spacing (ft)
Total
N
Lb/A*
K2O
Crop Development Injection Rate
(lb/A/day) N and K2O
Stage Weeksyz
Cantaloupe
(Muskmelon)
TP 5 120 120 1 2 1.0
        2 3 1.5
        3 3 2.0
        4 2 1.5
        5 2 1.0
-
Collard TP 6 120 120 1 3 1.5
        2 6 2.0
-
Cucumber S 5 120 120 1 1 1.0
        2 2 1.5
        3 6 2.0
        4 1 1.5
-
Eggplant TP 6 120 120 1 2 1.0
        2 2 1.5
        3 6 2.0
        4 3 1.5
-
Pepper TP 6 160 160 1 2 1.0
        2 3 1.5
        3 7 2.0
        4 1 1.5
        5 1 1.0
-
Pumpkin S 8 120 120 1 2 1.0
        2 2 1.5
        3 4 2.0
        4 2 1.5
        5 1 1.0
-
Tomato TP 6 160 160 1 2 1.0
        2 3 1.5
        3 7 2.0
        4 1 1.5
        5 1 1.0
-
Summer Squash S 5 120 120 1 2 1.0
        2 2 1.5
        3 2 2.0
        4 5 1.5
        5 1 1.0
-
Watermelon S 8 120 120 1 4 1.0
        2 2 1.5
        3 2 2.0
        4 3 1.5
        5 2 1.0
-
Winter Squash S 8 120 120 1 3 1.0
        2 3 1.5
        3 2 2.0
        4 4 1.5
        5 1 1.0
-
z Includes any starter fertilizer.
y Where 20% of N and K2O have been applied as starter, injections can be omitted for 1 or 2 weeks.
x For extended-season crops, N maintenance applications can proceed at 1.0 to 1.5 lbs/A/day. Use tissue testing to fine-tune amounts.
w Establishment method (seed or transplant) affects the schedule. Transplanting shortens growth cycle and injection schedule by 1-2 weeks.
NOTE: This table is adapted from "Fertilizer Application and Management for Micro (or Drip) Irrigated Vegetables in Florida." Florida Cooperative Extension Special Series Report SS-VEC 45, April 1991, co-authored by George J. Hochmuth and Gary A. Clark. Used with permission.

This section specifically addresses types of fertilizer formulations, fertigation strategy, and fertilizer injection rates. For a more complete discussion on effectively managing fertigation, see University of Georgia Extension Bulletin 1108, Plasticulture for Commercial Vegetable Production, available from your county Extension office.

Many sources of nitrogen and potassium are suitable for injection through drip irrigation systems. They include various nitrogen solutions, ammonium nitrate, calcium nitrate, potassium nitrate and potassium chloride. Granular fertilizer, liquid fertilizer or a combination of the two may be used when fertigating.

Solubility of Fertilizer Formulations

Solubility indicates the relative degree to which a substance dissolves in water. Solubility of fertilizer is a critical factor when preparing stock solutions for fertigation, especially when preparing fertilizer solutions from dry fertilizers. As indicated in Table 2, fertilizer formulations vary considerably in their ability to dissolve in water.

Hot water increases solubility and makes dissolving fertilizer easier and quicker. Hot water may be especially helpful when dissolving a fertilizer such as potassium nitrate, which actually cools the solution as it dissolves.

Because solubility is reduced when water cools, it is not a good practice to heat water in order to dissolve “extra” fertilizer (more than is soluble at normal temperatures). As the solution cools, this extra fertilizer will come out of solution (precipitate or “salt out”) and possibly clog drip emitters.

Growers routinely make large quantities of fertilizer stock solutions for injection during several fertigations over a period of time. When making stock solutions that will not be injected soon after preparation, keep in mind that solubilities decrease when the solutions are cool. If maximum amounts of fertilizer are dissolved in stock solutions and these solutions are cooled during the night (which occurs frequently in early spring and fall), some of the fertilizer may come out of solution. Therefore, it is generally not advised to dissolve maximum amounts of fertilizer in stock solutions that will be injected at some future time.

Sometimes growers dissolve two or more fertilizer formulations in the same stock solution. Keep in mind that the solubilities listed in Table 2 apply only when fertilizer is dissolved in pure water (water essentially free of minerals and other contaminants). Once fertilizer is dissolved in pure water, the purity of the water and the solubility of additional fertilizers in that solution are affected. The solubilities shown in Table 2 will not apply in such situations.

Table 2. Solubility of Selected Fertilizers in Pure Water.
Fertilizer Formulation Solubility (lb/gal)
Ammonium nitrate 9.8
Calcium nitrate 8.5
Potassium chloride 2.3
Potassium nitrate 1.1

If two or more fertili

Status and Revision History
Published on Feb 1, 2005
Re-published on Feb 27, 2009
Reviewed on Feb 15, 2012
Reviewed on Jan 30, 2017