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. There are clear arguments for injecting chemicals either upstream or downstream of the filter. The primary reason for injecting upstream of the filters is that any introduced contaminants or immediate chemical precipitations resulting from chemigation will be trapped by the filters, and thereby, the irrigation system will be saved.

Nutrient and Chemical Stock Tanks

The concentrated mixes of nutrients for the crop are stored in the nutrient stock tanks (Figure 20.7). Nutrients are premixed into concentrated solutions according to recipes formulated to provide crops with all their requirements for growth. These nutrients are mixed with fresh water to make the nutrient solution that supplies the crops with both water and nutrients. Fertigation systems generally have two nutrient stock tanks and one or two pH-regulating stock tanks. It is important to have separate nutrient stock tanks for calcium and phosphorus as sources, as these are likely to form an insoluble precipitate if mixed together. This will clog irrigation pipes and drippers.

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 are 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 mineral ions present in the nutrient solution.

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.

Backflow Prevention

Several safety measures are required for proper fertigation. These 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 the 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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