Chapter 20

Fertigation in Greenhouse Production

Design and Management Considerations

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.

Injection Point

The injection point should be located so that the injected fertilizer and the irrigation water can become thoroughly mixed well upstream of any branching of the flow. To ensure that no contaminants are injected into the microirrigation system, a good quality screen or disk filter should be installed on the line between the chemical tank and the injector. The system should be allowed to fill and come up to full pressure before injection begins. Following injection, the system should be operated to flush the fertilizer from the lines.

Nutrient and Chemical 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.

Batch Method

The nutrient concentrates from the nutrient stock tanks are mixed with fresh water in the mixing tank to produce the nutrient solution. The concentrations of nutrients, and the pH are continuously monitored in the mixing tank and the amounts of nutrients added adjusted to ensure the nutrient solution is the correct pH and contains the correct concentration of nutrients. In hydroponics systems that use recycled runoff water this is also added back into the system through the mixing tank. Mixing tanks can be large tanks where a batch of nutrient solution is mixed prior to delivery to the crop, or a series of small tanks from which the nutrient solution is continually drawn, mixed, and adjusted as it is being delivered to the crop.

pH/EC Meters

Fertigation systems contain a pH meter to constantly measure the pH of the nutrient solution and adjust it to meet the requirements. The pH is important as the optimal uptake of nutrients by crops is pH dependent. Fertigation systems contain an EC meter in the mixing tank to constantly measure the EC of the nutrient solution and adjust it to meet the requirements. The EC is a measure of the total dissolved minerals salts in the nutrient solution. This is important as it determines the nutrients that are provided to the crop.

Automated Control Systems

When automated injection controls are used, it is possible to rapidly change the dilute solution concentration by adding stock materials relative to the water flow. This is useful if the feed strength needs to be adjusted throughout the day to match growing conditions or if the feed recipes or nutrient concentration need to be changed for various crops. In some instances, the actual fertilizer formulation can be changed by either switching to different sets of stock tanks or employing a single-element dosing design where the individual fertilizer constituents are separated into several stock tanks.

Backflow Prevention

Several safety measures are required for proper fertigation. These include can include interlock, low pressure drain, backflow valve, inspection port, check values, and chemical resistant hose and clamps. One of the most important safety components of chemigation systems is backflow prevention. Backflow is the movement of agrochemicals in the reverse direction towards the source water. Backflow can occur in two ways, back-siphonage and back-pressure.

Scheduling Fertigation

Two basic schedules for applying liquid fertilizers are constant and periodic. The application of a dilute fertilizer solution each time the crop is irrigated is known as constant fertilization, and the concentration of this applied fertilizer solution is exactly the nutrient concentration desired in the growing medium solution. Periodic fertilization consists of applying a more concentrated fertilizer solution according to some fixed schedule, such as once a week or every other irrigation.

Injection Duration

A minimum injection time of 45 to 60 minutes is recommended. This time is sufficient for uniform distribution of nutrients throughout the fertigation zone. Injection of “slugs” (highly concentrated solutions of fertilizer usually injected in much less than 45 minutes) is not recommended. They often result in nonuniform fertilizer applications and subsequent poor crop performance.

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