Water flow

Flow from water supply

Known volume cylinder

(Backflow preventer)

Figure 3. Positive displacement pump injector. Source: Water and Nutrient Management for Greenhouses (NRAES-56)

Fertilizer concentrate

Figure 2B. The injector has an injection ratio of 1:16. For each gallon of concentrate taken up, it produces 16 gallons of liquid fertilizer solution. The injector draws 1 gallon of concentrate in about five minutes, and this will be mixed with approximately 12 to 18 parts of water through the hose (i.e., 5/8-inch instead of ½-inch produces a higher flow rate). Excessive backpressure will cause the siphon to stop drawing. The injector should not be more than 50 feet from the nozzle end to work properly. If more than 50 feet of hose is used, the injector can be connected between two sections of hose. (Photo credit: Hummert Int’l. Catalog 2003)

(over 100 gpm). For example, a 10-gpm flow rate will service only one ¾-inch pipe, and thus only one area of plants will be fertigated at a time. If this is all the water available from the water supply, a low flow rate injector is adequate. If the water flow is not a limiting factor, the amount of bench or ground area to be fertigated simultane- ously is the main factor.

This relates back to the size of the opera- tion. Factors to consider include:

• Minimum and maximum areas to be watered at one time.

• Fertilizer solution required for each square foot of ground area.

• Time needed to deliver the fertilizer solution.

Injection Rate Having an adjustable injection ratio is desir- able because a single fertilizer concentrate can be applied at different concentrations to different plants with varying nutrient requirements. Injection rates fall into four ranges: very low (1:4000 – 1:250), low (1:500 – 1:100), medium (1:200 – 1:100), high

(1:100 – 1:20) or highest (1:50 - 1:10). A low injection rate means that a small portion of concentrated solution will be injected into the irrigation line, while a high injection rate equates to a large portion of concentrat- ed solution being injected into the water. A low injection ratio also means a highly concentrated stock solution.

Stock-tank Size Stock-tank size should be based on propor- tion ratio and daily water usage. The stock tank should be sufficiently large enough to allow the entire fertilization job to be com- pleted with one batch of fertilizer concen- trate. A large stock tank is needed if a low proportion ratio is used and if the injector is used frequently. Also, if a constant liquid feed program is used, a larger stock tank size is beneficial.

Stock tanks need to be opaque. The chelat- ing agents in fertilizer (help make micronu- trients available to the plants) break down if exposed to light. Stock tanks should be covered to prevent algae and/or debris buildup, contamination or evaporation of stock solution. If debris buildup occurs, it may plug the injector intake and cause less than the required amount of stock solution to be taken up.

Water-soluble fertilizers tend to accumu- late in the bottom of stock tanks, which can result in large differences in fertilizer concentrations. If using a large stock tank, make sure the stock solution is mixed well before using it.

Installation Considerations

A permanently installed injector should be plumbed off the main water line, which will permit clean water to flow through the irrigation line to purge the line of fertilizer solution or supply water to crops where fertilizer is not needed. A bypass installation also allows easy removal of the unit in case of malfunction or the need for mainte- nance or replacement.

Verifying an Injector Is Working Properly

The dilution ratio should be known and ad- justed as needed. If large deviations (more than 5 percent) from the expected setting occur, contact the manufacturer for pos- sible repair, replacement or maintenance. Soluble fertilizers should be dissolved com- pletely; use hot water if necessary but allow the solution to cool before starting the injector. Constant agitation may be needed when applying wettable powders because they suspend but do not dissolve in water.

Regulations Concerning Water Supply Protection

Any irrigation system designed or used for the application of fertilizer, pesticide or chemicals must be equipped with an anti-siphon device adequate to protect against contamination of the water supply.

It is unlawful for any person to use any irrigation system designed or used for the application of fertilizer, pesticide or chem- icals if the system is not equipped with an anti-siphon device.

Craig Barrett currently serves as head of management at Barrett Engineered Pumps, including sales, engineering and service. Barrett belongs to and has been on some of

the governing boards of various industry organizations, including the Irrigation Association. 23 Nutrient solution Valves

Known volume cylinder


Fertilizer stock


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