Chapter 20

Fertigation in Greenhouse Production

Fertilizer Delivery Systems

There are two ways to deliver fertilizer through the greenhouse irrigation system: (1) pre-mixing fertilizers at the final feed strength for the crop and storing in a tank, which is known as preferential differential injection or the batch method and (2) in-line injection. Pre-mixing is the simplest, safest and potentially most accurate way to achieve the final dilution strength. Since the mixing process is completely separated from distribution, the dilute solution can be removed from the tank at practically any flow rate without ever affecting the concentration accuracy. Although pre-mixing is the simplest method, the most popular method is in-line injection, which can range from simple mechanical injectors driven by water flow (using the venturi principle), to fully automatic nutrient control systems.

Pressure Differential Tank Injection

Pressure differential tank injection is the simplest of the injection devices (See Figure 20.1). 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 also 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. A pressure differential tank system is based on the principle a pressure differential is created by a partially closed valve, placed on the pipeline between the inlet and the outlet of the tank.

In-Line Injection

Most greenhouse operations use in-line injection (i.e., injectors) for fertigation. 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. Injectors are also used to apply pesticides, plant growth regulators, wetting agents, disinfectants, and mineral acids during crop production. In-line injection is generally more popular than dilute tank systems since they do not require large holding tanks and they offer more flexibility. With in-line systems, concentrated stock solutions are continuously injected into the water stream as irrigation events are underway. Mixing usually occurs in the downstream section of the irrigation pipe or in a small mixing tank. Injection volumes are based on system flow rates, pH/EC sensor feedback, or a combination of the two.

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