Chapter 20

Fertigation in Greenhouse Production

Criteria for Selecting a Fertigation Injector

There are several important factors that need to be considered when choosing a fertilizer injector for a greenhouse irrigation system. Some of the more important factors are: water flow, injection ratio, types of chemicals, number of injection heads, water quality, mobility, and maintenance.

Water Flow

It is important to choose an injector carefully based on the water flow rate of the greenhouse irrigation system. Select the injector that matches the flow rate (gpm) that your irrigation system uses. Flow rate can be determinedby pipe size and water pressure, which is measured in PSI (pounds of pressureper square inch). The water 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). Knowing your irrigation flow rates is essential to effective operation of injectors.

Checking Water Flow Rate

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:

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. This increases the maximum flow rate by the sum of the connected injectors while maintaining the low minimum flow rate from one injector.

Injection 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. The injection ratio can be expressed as a ratio (1:50, 1:100, 1:200, etc.) or as a percentage (2%, 1%, 0.5%, etc.). The equivalence between those two are 1:50 = (1 unit/50 units) x 100 = 2%; 1:100 = (1 unit/100 units) x 100 = 1%; and 1:200 = (1 unit/200 units) x 100 = 0.5%. The ratio of 1:50 means that there is one unit of stock solution injected into fifty units of water.

Acid Injection

If the grower is injecting acid to reduce alkalinity and water pH, make sure the injector selected is equipped to handle acid. Use injectors with stainless steel parts or other options for injecting corrosive chemicals. The choice of an injector also should be made based on the type of acid used.

Water Quality

The amount of dissolved chemicals or particulate matter suspended in your water supply must also be considered before purchasing an injector. With the simpler venturi-type injectors water quality isn't as much of an issue but, with more sophisticated injectors, high amounts of sediment or very hard water can cause excessive wear of the pump mechanism.

Multiple Injection Heads

Commercial brands of soluble fertilizer can be mixed in a single stock solution tank so an injector with one head is adequate. However, when injecting acids to correct high water pH or when formulating custom fertigation solutions from stock chemicals, separate injector heads are necessary (See Figure 20.10). For example, calcium and sulfate cannot be mixed in the same stock solution tank because they form an insoluble precipitate (gypsum) that can plug up the injector or irrigation nozzles.

Portable 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.

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