Chapter 17

Fertigation in Greenhouse Production

Chemical Injectors for Fertigation Systems

Fertilizer injectors are mechanical devices used to apply water-soluble fertilizers, pesticides, plant growth regulators, wetting agents, disinfectants, and mineral acids during crop production. They are a vital part of modern greenhouse operations. Since the introduction of injectors or proportioners, growers have enjoyed an easy, time- and labor-saving method of applying liquid chemical solutions to their crops. Injectors take a small amount of concentrated chemical solution, such as water soluble fertilizer dissolved in water in a stock tank, and introduce it into the irrigation line as a dilute solution at the proper concentration for delivery to plants. For instance, applying fertilizers in a liquid form with a fertilizer injector is more convenient than broadcasting or top-dressing with dry fertilizers. One part of stock solution is mixed with x parts of water in the irrigation line, where x is determined by the injector ratio. For example, for a ratio of 1:100, 1 part of stock solution combines with 99 parts of water for a total of 100 parts of final solution. Often, injector ratios are expressed as a percentage. Example, a 1:100 ratio equals a 1 percent solution. Chemical injectors can be divided into pressure differential methods and positive displacement pumps. Pressure differential methods such as differential pressure tanks and venture-type suction devices require some degree of system pressure and a pressure differential to operate. Positive displacement pumps must develop a pressure greater than that of the irrigation line at the injection point in order to introduce the fertilizer into the system. Positive displacement pumps are precise and operate on an external power source such as electricity, an internal combustion engine, or water power utilizing a portion of the irrigation water flow.

Pressure Differential Tanks

Pressure differential tanks, often referred to as “batch tanks,” are the simplest of the injection devices. Accurate measurement of each ingredient (including water) will ensure great accuracy. With a moderate tank mixing, it is relatively easy to achieve excellent uniformity. This process is known as batch mixing and it is arguably the simplest, safest, and most accurate way to achieve the final dilution strength. The mixing process is completely decoupled from the distribution process, allowing dilute solution to be removed from the tank at practically any flow rate without affecting the concentration accuracy.

Advantages and Disadvantages of Pressure Differential Tanks

Some of the advantages of pressure differential tanks include:

Some of the disadvantages of pressure differential tanks include:

Venturi Injectors

Chemicals can be injected into a pressurized pipe using the venturi principle. A venturi injector is a tapered constriction, which operates on the principle that a pressure drop results from the change in velocity of the water as it passes through the constriction. As water flows through the tapered venturi orifice, a rapid change in velocity occurs. This velocity change creates a reduced pressure (vacuum), which draws the liquid from the stock tank to be injected into the irrigation system, eliminating the need for a separate chemical injection pump. 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. The larger the pressure drop, the higher the injection rate, up to some maximum drop.

Advantages and Disadvantages of Venturi Injectors

Some of the advantages of venturi injectors include:

Some of the disadvantages of venturi injectors include:

Venturi Injector Configurations

There are a number of configurations used with venturi-type suction devices. First, the injector may be plumbed in parallel with a pressure-reducing valve as shown in Figure 17.2, which automatically creates a pressure differential across the injector, thereby allowing the injector to produce a vacuum and draw in material. This is a more efficient system because it eliminates the pressure drop in the mainline that occurs when a valve in the mainline is used to create a pressure differential.

Positive Displacement Pumps

Positive displacement pumps utilize the reciprocating action of a piston or diaphragm to inject fertilizer into the irrigation line. These pumps are installed inline and do not require a bypass. Water pressure, as it flows through the injector in the irrigation line, provides the energy needed to activate the injection process. Fertilizer solution is pulled into the irrigation line in precise volumes at a constant concentration and flow rate. Positive displacement pumps inject at a constant rate regardless of flow or pressure changes in the system. Positive displacement pumps provide greater injection ratios and require a smaller fertilizer stock tank.

Advantages and Disadvantages of Positive Displacement Pumps

Some of the advantages of positive displacement pumps include:

Some of the disadvantages of positive displacement pumps include:

Dosatron Injectors

Dosatron injectors operate without electricity, using water pressure as the power source. They are installed directly in the water supply line. Water drives the injector, which takes up the required amount of concentrate directly from a stock tank containing concentrate solution. The amount of concentrate is directly proportional to the volume of water entering the injector, irrespective of variations in water flow or pressure, which may occur in the main irrigation line.

DosMatic Injectors

DosMatic injectors operate without electricity and use water pressure as the power source. They are installed directly in the water supply line. Water drives the injector, which takes up the required amount of concentrate directly from a stock tank containing concentrate solution. The amount of concentrate is directly proportional to the volume of water entering the injector despite variations in water flow or pressure, which may occur in the main waterline.

Anderson Ratio:Feeder Injectors

Anderson ratio:feeder injectors feature positive displacement rather than siphoning to pump the correct amount of chemicals into the waterline and volume proportioning by a flow-metered pump. Maximum flow rates for these injectors range from 3⁄4 to 3,000 gallons per minute at pressures of 15 to 125 pounds per square inch with low pressure loss. Injection accuracy is maintained over a wide range of flow and pressure ranges. Injector ratios can be set from 1:80 to higher than 1:200 depending on the model.

Smith Measuremix Injectors

Smith Measuremix injectors rely on water passing through a water motor to provide the power to run the injector pump. The water motor actually meters the water running through the unit. The injector pump strokes once for every revolution of the water motor. The proportion of water to fertilizer solution remains the same, regardless of changes in water pressure. The Smith Measuremix is designed to dispense a variety of liquid chemicals. The water motor is made of bronze and stainless steel.

Gewa Injectors

The Gewa injector has no suction or pumping device. The concentrated fertilizer solution is forced into the waterline by the water pressure. The fertilizer concentrate is poured inside the injector into a plastic membrane suspended inside the tank, and the lid is sealed shut. When the water is turned on, it surrounds the membrane and applies pressure to the bag, which forces a calibrated amount of concentrate into the waterline.

Criteria for Selecting an Injector

There are several important factors when choosing an injector for an irrigation system. The choice of appropriate methods and equipment will depend on several factors. For injection of solid materials, agitation and mixing at pump site will be needed. Liquid fertilizers and agricultural chemicals, on the other hand, can be injected directly from their storage tanks. Some installations may require more than one injector because of vastly different flow rate requirements for the materials used.

Water Flow and Pressure

One of the first questions to consider when selecting an injector is the water flow rate range. The flow rate falls into three categories: low (0.05 to 12 gallons per minute), medium (12 to 40 gallons per minute), or high (more than 40 gallons per minute).

Injector Ratio

Fertilizer stock solutions are mixed according to the fertilizer injector ratio: each injector will deliver a certain amount of stock solution for each increment of irrigation water that passes through the injector. Injector ratios fall into four ranges: very low (1:4000 to 1:250, 0.025 to 0.4%), low (1:500 to 1:100, 0.2 to 1%), medium (1:200 to 1:100, 0.5 to 1%), high (1:100 to 1:20, 1 to 5%), or highest (1:50 to 1:10, 2 to 10%). A low injector ratio means a small portion of concentrated solution is injected into the irrigation line; a high injector ratio means a large portion of concentrated solution is injected into the line. Low injector ratio also means a highly-concentrated stock solution.

Types of Chemicals Injected

This is a particularly important consideration if acid injection is required. If you will be injecting acid to reduce alkalinity and water pH, make sure the injector selected is equipped to handle acid injection and be sure of the acid concentration to be used.

Multiple Injection Heads

Multiple injection heads (for incompatible chemicals) is desirable to have the ability to inject several fertilizers at the same time that cannot be mixed in the same stock tank. For example, if mixing calcium nitrate and ammonium phosphate are mixed in the same stock tank, the calcium will combine with phosphate and precipitate out of solution as calcium phosphate.

Multiple Parallel Injectors

In some situations, being able to fertilize a few plants at a low flow rate is just as important as being able to irrigate a large crop at a high flow rate. For these situations, more than one injector can be connected in parallel.

Portable of Injectors

With small greenhouse operations, it is convenient to be able to move the injector unit around, while with large operations where high volumes of fertilizer solution are regularly used at each fertilization, stationary injectors along with large stock tanks are more appropriate.

Repair Costs and Reliability

If the repair cost is equal to half or more of the cost of a new unit, replacement of an existing injector is advised. In the case of small injectors, keep a backup unit available to put into service in case the original unit malfunctions and must be sent to the manufacturer for repairs.

Installing an Injector

A permanently installed injector should be plumbed off the main waterline (water bypass), which will permit clean water to flow through the irrigation line to purge the line of fertilizer solution or supply water to crops when fertilizer is not needed. A bypass installation also allows easy removal of the unit in case of malfunction or the need for maintenance. Consider installation of additional equipment for optimal performance, including the following:

Stock Tanks

The stock tank should be large enough to allow the entire fertilization job to be completed with one batch of fertilizer concentrate. A large stock tank is needed if low injector ratios are used and if the injector is used frequently. A larger stock tank size is beneficial if a constant liquid fertilization program is practiced. Tank size should be large enough to contain the chemicals sufficient for at least one fertigation operation. The size of the stock tank can vary from as small as 5 gallons to as large as 2,000 gallons. Stock tanks are usually made of polyethylene or fiberglass because of their potential corrosion. Some growers locate the stock tanks within chemical containment structures in the event of a spill. 

Checking Water Flow Rate on Injectors

Water flow rate can be measured easily if a water flow meter is installed. Turn the irrigation system on full and read the meter at a noted time. Take a second reading after the water has run for several minutes. Convert the difference between the beginning and ending meter readings from cubic feet, the typical water meter unit, into gallons by using the following formula:

Calibrating an Injector

Periodic calibration is needed to ensure that an injector is operating properly. Use one of the following methods for a quick check of the final fertilizer concentration while it is in operation and then to distinguish a fertilizer mixing error from an injector malfunction.

Volume Ratio Method

One way to determine if an injector is delivering at the correct injector ratio is to directly measure the fertilizer concentrate input volume and dilute fertilizer output volume. The ratio of the input volume to the output volume should equal the injector ratio set on the injector. The following steps can be used to determine the actual injector ratio. 

Electrical Conductivity Measurement

This method measures electrical conductivity is commonly used for water-soluble fertilizers. Fertilizer suppliers generally provide charts with electrical conductivity readings of various concentrations of water-soluble fertilizer solutions. When measuring the required quantity of fertilizer (usually given as a weight amount), use a scale to ensure the proper amount. Only reliable, calibrated conductivity meters should be used to ensure accuracy of the readings. Standardized conductivity solutions are available from the measuring device manufacturer and should always be used to calibrate the meter. The following steps can be used to determine the to determine the electrical conductivity (EC) of the dilute fertilizer solution. 

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