A Reminder about Fertilizer Injector Maintenance

Spring is here, and we know you’re busy!  Here’s a quick but very important refresher on fertilizer injector maintenance.  Ensure they’re working properly on a regular basis to avoid extra headaches.

How do I know if my injector is working properly?

Hopefully damage to foliage wasn’t your first sign things were a little off! Some things to remember:

  • The dilution ratio needed for each application should be known and adjusted as needed.
  • Remember that tracing dye in water-soluble fertilizers should never be used as an indicator of the actual strength of a fertilizer mixture.
  • If you are using multiple products or chemistries ensure they are compatible for tank mixing before running them through the injector.
  • If deviations greater than 5% from the expected setting occur, contact an irrigation specialist for assistance or repair. (But first, double check that math!)

Fertilizer injectors come in all shapes and sizes in the greenhouse.

There are two common methods to check if your injector is working as expected. Regular calibration is needed to ensure that an injector is operating properly. You should complete both methods listed here to ensure accuracy.

  1. Input versus Output Volume Method

The Method: To determine the injection ratio, collect a known amount of fertilizer solution from the injector and then measure the quantity of concentrated fertilizer that was taken up by the injector.

Turn on the injector on to remove air bubbles and charge the system with the concentrated stock solution. Turn off the unit, remove the siphon tube from the stock tank and place it in an accurate measuring vessel. Fill the cylinder with a known volume of the concentrated stock solution. Turn on the injector and collect a known volume of diluted fertilizer solution.

An Example:  The injector is set for a 1:100 ratio – Therefore, 200 ml of concentrated stock should have been used to make 20L of diluted solution.

This method checks if the injector is delivering concentrated fertilizer at the correct ratio. It cannot determine if the final fertilizer concentration is correct, but the next method can.

  1. EC Measurement Method

The Method: This method is common for all water-soluble fertilizers. It’s quick, easy and doesn’t require a large batch of fertilizer. Fertilizer suppliers generally provide charts with EC readings of various concentrations of water-soluble fertilizer solutions either on their bags or on their websites.  Use this information to help with this method.  While you’re in the “calibration mode” you might want to also check that your scale and EC meters are in good working order.  If something doesn’t add up with the calculations below, check the EC meter – how long has it been since you calibrated it?

Collect a sample of clear irrigation water, preferably from a line that has been flushed or allowed to run for a few minutes before you grab the sample. Measure the EC of this irrigation water sample and the EC of the fertilizer solution.

An Example: The injector is set for an 1:100 ratio and a stock mix of 200 ppm Nitrogen is being used (100g/L of 20-8-20 fertilizer). The chart from Plant Products tells us that the EC fertilizer should be 1.25 mmhos/cm.  If the EC irrigation water alone is 0.3 mmhos/cm, the EC fertilizer solution should be 1.55 mmhos/cm.

This method checks if the final fertilizer concentration is correct, but it cannot determine if an incorrect final concentration is due to an error in mixing the stock solution or a malfunction in the injector. Both tests are necessary to determine if there are problems and which problem exists.

Sources:
https://ag.umass.edu/greenhouse-floriculture/fact-sheets/fertilizer-injectors-for-greenhouses
http://extension.uga.edu/publications/detail.cfm?number=B1237
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